THE GIFT OF

FLORENCE V. V. DICKEY

TO THE

UNIVERSITY OF CALIFORNIA
AT LOS ANGELES

THE DONALD R. DICKEY

LIBRARY
OF VERTEBRATE ZOOLOGY

A STUDY

OF THE

INCUBATION PERIODS

OF

BIRDS

WHAT DETERMINES THEIR
LENGTHS?

BY

W. H. BERGTOLD, M. D., M. Sc.

Member of the American Ornithologists' Union

THE KENDRICK-BELLAMY CO.

DENVER. COLORADO
1917

Copyright, 1917
By W. H. BERGTOLD

PRESS OF

CARSON-HARPER Co.
2019 Stout St
DENVER, COLO.

(2>9 %

PREFACE

In the course of certain studies in ornithology, more
particularly avian physiology, undertaken by the author
several years ago, it early became apparent that the factors
fixing or determining the length of the incubation period
with different birds was largely, if not wholly, unknown.

The following pages give the results of a prolonged and
detailed study of this phase of bird physiology.

The author regards all zoologic classification as a means,
not an end; the classification adopted in this work is merely
a means of facilitating the handling of a mass of data full
of contradictions and uncertainties, and the selection of this
particular bird classification was governed by a question of
expediency only; obviously the author in no way wishes to
be understood as believing this classification to be the best,
or the only one. The author believes, however, that this
classification is an up-to-date reflection of our present
knowledge of the relation of various birds to each other.

It is inevitable that mistakes of various sorts will be
found in this book ; in extenuation of such errors the author
trusts that his critics will recall that the labor involved in
the investigations reviewed in the following pages was one
of love, and carried on in the spare moments of a fairly busy
professional life.

A brief resume of pages 43 to 76 of this book was read
before the Annual Meeting of the American Ornithologists'
Union at Philadelphia, November 15, 1916.

It is a keen pleasure here to acknowledge, with many
thanks, my obligations to the following friends, who have
generously placed at my disposal incubation data, and cog-
nate information, all of which has been invaluable in the
preparation and prosecution of this study: C. W. Beebe,
B. Rhett Chamberlain, L. J. Cole, E. W. Collins, L. S. Cran-
dall, Louis Fuertes, J. D. Figgins, W. F. Kendrick, F. H.
Knowlton, D. E. Lantz, F. C. Lincoln, W. DeW. Miller, R. C.
Murphy, J. T. Nichols, R. J. Niedrach, Clyde Phillips,
W. S. Pickrell, Robert Ridgway, A. A. Saunders, Suther-
land Simpson, Witmer Stone, F. M. Watson, C. A. Watts
and A. P. Wilbur.

THE AUTHOR.

Denver, Colo.,
June 15, 1917.

49G542

SYNOPSIS

1 — Introduction: The Problem.

2 — Definitions ; Length of Incubation —

* How measured.

** Difficulties of fixing beginning and end.

* True or specific length of incubation.
**** False or apparent length of incubation.

Types of hatching —

* Successive.

** Simultaneous.

3— The Data—

* The amount.

* The sources.

'* Explanations of conflicts in.
**** Obvious errors in.

4 — Influences actually or seemingly altering the specific
length of incubation —

* Causing real variability.

** Causing apparent variability.

5 — What determines the true or specific length of incuba-
tion—
* Past Theories :

A — The parent, or the bird itself —

(a) Its size or weight.

(b) The age of the female.

(c) Condition of the parents.

(d) Faithfulness of the parents while incubating.

(e) Longevity of the parents.

(f ) Condition of young at hatching.

* Precocious.
** Altricial.

*** Completeness of development at hatching.
B— The Egg—

(a) Its size or weight.

(b) Its age or viability.

(c) Its shell.

(d) Its yolk size.

C — Telluric Conditions —

(a) Zone.

(b) Climate.

(c) Weather.

(d) Geographical location of species.

(e) Site of nest.

>*New Theories:

A — Body-weight : Egg- weight Index.
B — Temperature and Ascent Theory.

* Bird temperature.

** Rapidity of metabolism or "swiftness of life."
***Temperatures, incubation lengths, and taxo-

nomic position.
**** Collateral evidence.

6 — Present data and this new theory.
7 — Usefulness of new theory.

8 — Does correlation of temperature and incubation length
benefit species?

9 — Data need for further study of this problem.

10 — Conclusions —
A — Minor.
B — Major.

11 — List of incubation periods.
12 — Bibliography.

A STUDY OF THE INCUBATION
PERIODS OF BIRDS

What controls the length of incubation with birds? It
is the purpose of this study to find, if possible, a correct
answer to this question.

Introduction

The processes going on within an egg during incubation
are of fundamental and paramount importance to the species
and race, in no way less than the process of fertilization;
fertilization and incubation are co-equal and interdependent,
and through them the time space between generations is
bridged and the race perpetuated. Fertilization is governed
by definite limitations and conditions. Does it seem reason-
able to believe that the length of the equally important
period of time necessary to complete the marvelous steps of
development following fertilization is a matter of chance,
a "hit or miss" duration?

It is inconceivable that such can be true ; one arises from
a study of the embryology of the "chick" in amazement that
the wonderful and complex changes in an egg from a single
cell to a peeping chicken can be brought about in a brief
three weeks. Such perfection of detail, with all its potential
specific and racial conditions unfolded, must surely require
a fixed and definite period of time for its completion. Does
it not seem more reasonable that this period of time must
be relatively fixed for each species, and be controlled by
factors or conditions which collectively might be called a
law?

I believe that a knowledge of such factors, or such a
law, is not merely academic, but, on the contrary, is of de-
cided importance, and constitutes a block to be fitted in the
mosaic being slowly put together by ornithologists, each in
his day. Moreover, the writer has discovered, through his
study of the question, that it is fraught with fascinating
interest, and, too, opens up unexpected and wide fields for
original research.

The Problem

The problem in hand is to answer the question, Why
does a house finch's egg take fourteen days to hatch, an
ostrich's forty-two days, an emu's fifty-six days, or a hum-
mingbird's fourteen days? It is the work of this study to
analyze the published data concerning incubation periods,
and to examine the explanations heretofore given, as to
what governs the length of incubation, and to determine if

there be a law which controls the length of incubation, or
at least to detect indications of such a law, or to point out
lines of investigation which give promise of being helpful
in reaching a final solution of the problem.

Definitions

In this discussion the term incubation is held to mean
the period of time during which heat is applied more or
less continuously to a set of eggs, a period varying within
a wide range according to the species; by "incubation
period" is meant the whole time so involved, regardless of
its duration; and by "length of incubation" is meant the
number of days or weeks necessary to completely hatch the
young.

The records of incubation as given in the literature on
the subject embrace two varieties of lengths: (A) — the true
length (or specific length), and (B) — the false length (or
apparent length) ; the first, or true length, being the mini-
mum number of days, under opti?num conditions, necessary
to hatch a normal bird, while the second is the true length,
plus or minus the time added to, or subtracted from, it by
errors in observation, or through errors caused by the dif-
ferent types of hatching, types to be defined later on, or
plus the time added to it by such conditions as retard or
temporarily suspend embryonic development. With birds
which lay a considerable number of eggs in each set, and
only begin to incubate when the set is completed, it is not
difficult to fix the beginning of the incubation period, as
with eiders (137) ; when, however, several eggs are laid in
a set, and the female warms the first eggs more or less while
the other eggs of the set are being laid, it is impossible to
say exactly when the period of incubation begins, and the
incubation duration has to be determined for each egg by
marking it when laid.

It is almost impossible at times to decide when the
parent actually begins steadily to apply the heat necessary
to successful hatching. It is also extremely difficult to esti-
mate how much heating the first eggs receive while a whole
set is being laid, a fact necessitating one's defining the dif-
ferent types of hatching, so as to keep in mind the effects
of a parent's partially heating the first laid eggs. It has
been shown (92) that the domesticated pigeon's average in-
cubation period is seventeen days, the eggs (two in a set)
being laid on alternate days; the second egg usually hatches
in (almost) exactly seventeen days, while the first takes
eighteen and one-half days, measured from the day it is
laid. If it be assumed that the real incubation begins with
the laying of the second egg, it becomes manifest that the
first egg receives the equivalent of one-half a day incuba-

tion while the set of eggs is being laid. That this is true
cannot be questioned, since it has been found (92) by
"candling" pigeons' eggs that the first egg shows indubitable
evidences of beginning embryonic development by the time
the second egg is laid. It is also wise here to recall that all
eggs respond developmentally to lower degrees of tempera-
ture in the early parts of incubation than they do in the
later or last portions.

Hence, from the foregoing, it is evident that the diffi-
culty of fixing the real beginning of incubation must con-
tribute not a little to conflicts in the data on incubation
length, and has led to honest differences in the records made
on a given species by different observers. The correct and
exact method of measuring the length of incubation is to
mark each egg as laid and watch it daily until hatched.
This is often impossible because of psychic reasons, since a
bird may abandon a nest if too closely watched, or because
of physical reasons, as with birds nesting in holes. Some
errors have also been caused by lack of agreement as to
when the incubation terminates; thus some reports seem to
indicate that the observers date the end of incubation when
the egg is merely "pipped," while others fix the end when
the "chick" is completely hatched.

In cases where several eggs make a set for a single
hatching, all the eggs may hatch at the same (relatively)
time, in which event I propose to name it a "simultaneous
hatching," as is seen with the domestic hen, and when the
eggs hatch one after another at intervals of "a day or two,
a "successive hatching." The effects of these two types of
hatching on the estimation of incubation length will be
considered later on.

Conditions Necessary for Successful Incubation

The growth of a new bird individual really begins
directly after fertilization, which may occur a considerable
time before the egg is completed and extruded ; consequently,
the incubation period embraces only part of a bird's em-
brjTonic development, which part is, however, by far the
largest portion of the process of development.

Successful incubation depends on "keeping a fertile
egg * * for a sufficient period of time under certain con-
ditions of heat, moisture and position" (33)*.

This combination of heat, moisture and position is
achieved through the brooding of the parent (real or foster),
or by a mass of decaying vegetable matter, or by hot springs,
or through the care of the parents plus the sun's heat. It
may begin at once, after the first egg is laid, or after part

•Numbers in parentheses correspond to the number of the authority
quoted, and as listed in the bibliography. See Table No. 2.

of a full set of eggs is laid, or only after all of a full set
is laid.

I believe that the evidence permits one to hold that the
true length of incubation varies little, if at all, with the
species, however widely separated geographically, a view
substantiated by the Old and New World cuckoos, and the
small finches of the Northern Hemisphere and Australia.
However, the work of Cole and Kirkpatrick (92) seems to
show that at least with pigeons there may be a distinct,
though small, true variability in the length of incubation of
these birds. Whether this be a true variability, or one due
to retardation of development, is unknown to me ; an answer
to this point must probably be reserved until further light
is shed by future studies on the true lengths of incubation
in other species, and the possibility of such lengths being
really variable. The length of the incubation period must
be measured from the time the parent (or its substitute)
begins the steady application of heat to the eggs until the
young bird is fully released from its shell.

The Data

This study is based largely on the list of incubation
period lengths incorporated in this book. See Table No. 1,
which is made chiefly from lists published previously by
Evans (1 and 2) and by Burns (3), together with records
published singly by many others, plus those determined
and given to the writer by obliging friends.

The incubation length data include records from 625
species and sub-species, scattered amongst 84 families, and
representing every order of existing bird. The writer has
tried to give each record in the words of the original con-
tributor or compiler*, and a list of all reference is given in
the. bibliography.

It is necessary here to note that some, perhaps many,
of these records may be duplicates, an unfortunate state of
affairs, but unavoidable, because some of the previous papers
on the subject of this study have had no bibliographies. It
were better, I believe, to include some duplicates than to
exclude some original, unduplicated records, in an effort to
sort out and eliminate reduplications. The present list pre-
sented now by the writer probably contains records of in-
cubation periods of more different birds than have been
gathered together in any single previous publication, a com-
parative wealth of material giving the writer an excuse for

'Inasmuch as a great deal of the literature used in this study was
wholly inaccessible to the writer and had to be copied for him, he fears
that some errors incidental to such transcribing will have crept in, for
which he expresses his regret, however unavoidable on his part these
errors may have been.

10

trying to solve the riddle of the cause beneath the varying
lengths of incubation amongst birds.

Any attempt to draw conclusions from lists of incuba-
tion periods heretofore published seems, on preliminary
examination, to be hopeless; the evidence in places is highly
conflicting and inconsistent, so much so that one is at once
tempted to believe the length of the incubation period is a
matter of more or less chance, and controlled by no particu-
lar condition or set of conditions. Fortunately, however,
years of observations on domesticated birds and a vast
experience in the use of artificial incubators show that this
conception cannot be true, and also show that there is an
actual, or a relative, fixity of the length of incubation with
such species as have been so domesticated. Furthermore,
the evidence seems to show that there is no inherent or
known reason why a similar specific fixity should not apply
to all avian species.

It is regretted that the writer did not have personal
access to a larger mass of literature, for such would probably
have yielded many more records of incubation, additions
which would have greatly enhanced the value of these pres-
ent data, and would also have saved future students of the
same problem much drudgery in a search for such data. Of
the nineteen thousand or more (138) species and subspecies
of birds now known to ornithologists, the six hundred and
twenty-five species and subspecies given in this study form
but a small per cent., which may in fact be too small on
which to safely base final conclusions. A future larger and
more comprehensive study of the question will alone decide
this.

The conclusions in this study are based on the assump-
tion that artificial incubator records, and such other records
as show a substantial concordance, are correct, and hence
justifiably available as fundamental data.
Conflicts in the Data

It seems appropriate here to consider the fact that there
are many conflicts in the published records of incubation
lengths, and when I have been unable to examine the orig-
inal record, it has been accepted as quoted. When a record
has been secured by me from an original source, it has been
copied verbatim,, excepting in a few instances where it was
perfectly obvious from the context that the period had been
incorrectly estimated because of errors induced by "succes-
sive hatching,'' and in such a case the writer has tried, care-
fully and impartially, to make corrections for such errors,
and has listed the record as so corrected.

The conflicts in the records given for a single species
are often numerous, and are accountable, many times, by
errors brought about by the difficulty of measuring the in-

cubation length because of "successive hatching." With sets
of eggs with which the successive hatching type prevails,
it is impossible to determine how long it takes to hatch each
egg in such a set unless the eggs be marked. Let us see
what can (and evidently does) happen in determining the
length of incubation with the robin, a species which lays
one egg each succeeding day until four or six are in the nest.
Occasionally the parents do not incubate steadily until all
their eggs are laid, in which case it is found that all the
eggs take almost exactly fourteen days to hatch, counting
from the laying of the^last egg. If,' however, the period
be dated as beginning with the laying of the first egg, it
would have to be estimated as being eighteen days, a pal-
pable error of four^days. It is more common for this species
to have a set of eggs hatch irregularly ; it may be one on
the first day, two on the second day, and one on the third
day (of the hatching period), in which case no accurate
knowledge of the length of incubation could be gathered
without having marked the eggs for identification and in-
dividual study. Under these circumstances the first laid
eggs are partially incubated by the time the last ones are
deposited, causing the irregular hatching; and if the period
were counted as extending from the laying of the last egg
to the hour of the first hatching, the time elapsing would
probably be ten or eleven days, an estimate three or four
days too brief. I am convinced that many surprisingly
short incubation periods (as recorded in literature) are
much too brief, due to errors induced in the manner just
outlined. I am confident that the length of incubation of
the house finch is almost exactty fourteen days, but it could
be variously estimated as ten or eighteen days with different
sets of eggs if care were not taken to mark and carefully
identify the eggs as they are laid and hatched. Under such
conditions the larger the set of eggs, the longer or shorter
in days of error may be the estimate of the length of incu-
bation, errors (plus or minus) corresponding in- days to the
number of eggs in the set, or to the number of days between
the laying of the eggs. The effect of partial incubation
when a set of eggs is being deposited results in mixed types
of hatching, an added source of conflict in the incubation
length data.

On the other hand, it appears that some sets, embracing
several eggs, as with the flicker (69-70), may have all the
eggs hatch at once (relatively), even though the early eggs
are apparently subjected to partial incubation. This may
be due to the possibility that the fresher eggs (later ones
laid) develop comparatively more swiftly (at the normal
rate) than do the older eggs (those laid first), resulting in
all breaking out at nearly the same time. The possibilities

of observation error are much smaller with eggs hatching
simultaneously. The eider (137) lays six to ten eggs before
starting its incubating, and all hatch on the same day ; yet,
if it were not known that the steady application of heat to
the eggs does not begin with this duck until all the eggs
are laid, six to ten days could be erroneously added to the
period of incubation.

Another fruitful source of error in computing the days
of incubation is brought about by influences which retard
or temporarily suspend the embryonic development, i. e.,
cooling or neglecting the eggs after they have been incubated
for a while, a combination often adding several days to the
true length of incubation, exemplified by the records of the
ostrich and the albatross. It is also to be noted that many
observers seem to date the end of incubation when the eggs
are "pipped," while others report it as ended when the bird
is fully hatched. Many periods are given approximately
only, indicating, perhaps, that" the observer was unable to
keep daily watch of the nest, or did not deem it necessary
or important to determine the length of the period with
exactitude, this latter possibility being the source of much
discord in the records, and giving birth to such statements
as "about seventeen days" or "twenty to twenty-eight days"
or "after a few weeks the young are hatched" (64).

The evidence, it seems to me, points very strongly to the
existence of a true (or specific) incubation period, which
under optimum conditions varies little with each species or
subspecies; hence, if there be marked discrepancies or dif-
ferences in the records of such species and subspecies, it
seems reasonable to believe that the records (at least in
part) in such cases are inaccurate. The records of hum-
mingbirds and wrens are good examples of conflicts, prob-
ably to be explained on the score of inaccuracy, or error in
estimating the start of incubation, for it seems highly im-
probable that there is a difference of four days in the period
of the Ruby-throated Hummingbird and the Black-throated
Hummingbird, even though they are specifically distinct,
while it is more improbable that there is a difference of two
days in the incubation of the Carolina and the Florida
Wrens, which are but geographical races of the same species,
and these remarks apply equally well to the case of the
Loggerhead and Migrant Shrikes, and the Western and
Eastern Meadowlarks. The Cedar Bird is a good example
of how wide a difference can be found in the incubation
records of a given species; the internal evidence in this
instance convinces me that sixteen days is probably correct.

There are clear indications that other errors or conflicts
have crept in because of typographical (or clerical) mis-

13

takes, as, for example, when it is recorded by one writer that
the Hummingbird's incubation period is "eighty days."

There is a singular disagreement in the recorded lengths
of incubation of species which might be called semi-domesti-
cated, as the white stork, a lack of agreement inexplicable
to the writer, unless it be due to faulty observation.

It is necessary, when analyzing these data, to remember
that an agreement in the records quoted by two or more
different authorities does not thereby mean that the records
are conclusively correct because of such agreement, for one
writer may have (and evidently has) copied from another
without indicating or crediting such fact.

Conflicts and lack of consonance in the records of the
length of incubation, and inconsistencies of testimony on
the same, are not surprising; the writer's slight personal
experience in trying to determine accurately the incubation
period of a few species has shown him the many difficulties
to be encountered and overcome in such a line of work.
Many birds abandon a nest (and its eggs) if it be too closely
watched, or if the nest be disturbed in the least; and to
overcome this difficulty calls for limitless care and patience
while observing the nest. It is a time-robbing task to visit
a nest daily, it may be hourly, to ascertain when the eggs
are laid, to mark them as laid, and to watch when they
hatch, all of which must be done with some species if one
is to succeed in making an accurate determination. Newton
(25) long ago deplored the scantiness and inaccuracy of the
then existing data on incubation, because (he said) correct
data were greatly needed to check up and compare the em-
bryology of different bird species at relatively the same-
stages, as an aid to put taxonomy on a sound basis. It is
obvious that every ornithologist will concur in this, and the
writer hopes to show later on that a more extensive and
accurate knowledge of the true length of incubation of dif-
ferent species may help the taxonomer otherwise than
through embryology. In justice to the multitude of bird
students who have contributed indirectly and directly to the
present list of incubation periods, one must recall that here-
tofore there has been no apparent indication of a need for
exactitude in measuring the length of this period. Notwith-
standing the unavoidable errors and discrepancies probably
embodied in this list, it is a splendid commentary on the
enthusiasm, care, patience and self-denial of ornithologists
the world over that so many records have been made, many
of which are patently of great, accuracy.

From this brief survey of the conflicts in the data, it
is evident that this list of incubation periods is made up
of both true and apparent lengths of incubation, the latter
probably being in the majority, and that it needs more

14

time and extended observations, with the idea of learning
the true period of incubation, to successfully sift apart these
two kinds of records.

Influence Altering the Incubation Length

In this study it is assumed that the true length of incu-
bation is a blastogenic characteristic, fixed for, and as such
inherited by, each species; that it is comparatively inelastic,
and yields exceedingly slowly to change; that with each
species it embraces, under optimum conditions of "tempera-
ture, moisture and position," a fixed minimum number of
days, just sufficient (and no more) to bring about the com-
plete development and hatching of a normal bird. A strik-
ing proof of its inelasticity and prepotent inheritability is
seen with domesticated birds, more particularly pigeons and
chickens ; for were this period plastic, under man's selection,
as are the tissues, functions and habits of these birds, one
would expect to find such plasticity showing itself in a
patent variation of the incubation period of such domesti-
cated birds. Man can, and has been able to cause, or fix,
most extraordinary changes in his domesticated birds ; with
pigeons, not only an increase in the number of tail feathers,
but even a lessening of the number of ribs (138), and with
chickens, not only the almost unbelievable alteration in
sizes from that of a bantam to a huge Cochin-China, but
also an increase in the number of toes (the five-toed Dork-
ings). Man has domesticated many other birds, and if with
them the period of incubation were not fixed, it seems rea-
sonable to believe that it should have exhibited variations
comparable to those variations of body, etc., mentioned
above, in pigeons and chickens. Yet, if I read aright, there
is not the slightest indication of any alteration in the incu-
bation period of any of man's domesticated birds; on the
contrary, all seem to adhere rightly to the ancestral char-
acteristic as shown in congeners still wild, or in wild species
most closely related. It has been definitely determined
from the experience of hundreds of poultry raisers, in
natural and artificial incubation, that the incubation period
of the domestic hen is almost exactly and almost invariably
twenty-one days: it matters not what breed, bantam or
brahma, nor however remote from, or near to, the ancestral
jungle fowl, the period of incubation remains the same as
that of the jungle fowl, viz., twenty-one days. The same
may be said, in effect, of the turkey, quail, pheasant, canary,
pigeon, duck, goose and, so far as the writer can learn, all
other domesticated birds.

Furthermore, birds belonging to families having a
fairly similar incubation period, i. e., finches, all exhibit this
uniformity, even though separated by large geographic

15

spaces, and even possibly by geologic time-spaces; thus, the
swallow and the finches of Australia still cleave to the an-
cestral period of their cousins of the North, and the English
and New Zealand gannets have identical incubation periods.

Certainly the incubation length seems far more change-
less, persistent and deep-rooted in nature than are other
characteristics of birds, as, for example, the proventriculus
mucosa of a gull (25). Furthermore, as has been suggested.
it is not subject to selection by man, as are other character-
istics, for while he has, in effect, changed a jungle fowl into a
five-toed dorking, and a rock-pigeon into a fan-tail, still
both of these species have retained their original lengths of
incubation.

If the structure of the egg shell persistently tend to be
characteristic, and remain the same with groups of birds, or
with the species (110), why should not the far more im-
portant process of embryonic development, and its length,
do likewise? While the immediately foregoing would seem
to show the length of the period of incubation as a fixed
characteristic, yet a superficial examination of a list of such
periods leads one to believe that there is, in fact, considerable
variability in the length of incubation of a given species,
and unless one can learn if this be true or false, it were
absolutely useless to attempt to draw conclusions from the
facts published on incubation lengths, because, viewing these
facts as evidence, they are in many parts hopelessly con-
flicting. I am convinced that most of such conflicts are to
be explained by a careful study of the influences apparently
affecting the true length of incubation, i. e., influences caus-
ing apparent variability in the true length of incubation.

Variability in the length of incubation may be true,
that is, permanently lengthening, or actually shortening, the
minimum number of days for successful hatching under
optimum conditions, and apparent, shortening or lengthen-
ing by slowing of the embryonic development by errors of
faulty time measurements.

True variability — The writer questions very much
whether there be any decided ti"ue variability, i. e., a vari-
ability occurring when all necessary conditions are optimum.
There is a small amount of experimental evidence at hand
which shows conclusively that with the domestic hen it is
possible, by suitable regulation of temperature conditions
in an artificial incubator (33), to shorten the length of in-
cubation a few hours only. This is well known to poultry
raisers, who know also that the dividing line between suc-
cessfully shortening the period and killing the embryo is
exceedingly difficult to maintain, even impossible at times.
All secondary influences which tend to induce this subtle
influence of temperature increase, especially towards the

1C

end of incubation, may be considered as coming under this
caption. Such conditions as are quoted in nearly every
poultry raiser's manual can be mentioned here ; an attentive,
faithful setting hen may bring out a hatching of chicks in
twenty days (33), and favorable weather and a suitable nest
site (104) also tend to maintain optimum temperature con-
ditions, with possibly a true, but very slight, shortening of
the incubation period.

This true variability towards the side of shortening the
period of incubation is probably in progress now, the world
over, with many different species, especially the higher
birds, and it will be taken up in greater detail later on.

Apparent variability — By apparent variability I would
have understood all lengthening of the specific incubation
period which is merely an addition to it of days of pro-
longed and retarded embryonic development caused by the
various factors mentioned in this discussion, or seeming
shortening or lengthening due to error of determination.

Apparent or false variability is, in the greatest number
of cases, merely the result of cooling the egg during incu-
bation, which slows down the developmental pace, or it may
actually suspend it for a while; in fact, the developmental
process "can be suspended and held in check for several days
without destruction of the germ" (33). There can be no
question as to the effect of cooling the eggs while they are
being incubated, because it has been proven many times
(by accident or design) with the domestic hen (33) that its
period of incubation can, by such cooling, be extended to
twenty-three or even twenty-four days. It is probable that
eggs of the lower birds can be chilled a much longer period
than can those of the higher species, without killing the
embryo, a fact which probably helps to explain the seem-
ingly great variability shown in the incubation records of,
for example, the emu.

There are many ways by which this qooling action
occurs : a restless, inattentive hen, a cold site for a nest, con-
ditions preventing the eggs from receiving the necessary
heat properly, as too thick shells, or which permit too rapid
radiation, as too thin shells, or a poorly constructed nest;
eggs which are too small radiate their lieat too quickly, on
exposure, since they are relatively of larger surface area
than are larger eggs; parents in ill health or badly nour-
ished will not produce optimum temperature conditions.

It appears to me that all of the conditions which are
outlined in this study, which apparently modify the true
or specific length of incubation, should be taken into account
in the future in all field, incubator and zoologic park work,
in order that their effects may be eliminated in an endeavor
to determine with the greatest possible accuracy the true

17

incubation period of our living bird species, especially since
the opportunities for such study will grow, as time passes,
fewer and fewer before the devastating onrush of civil-
ization.

Some of these conditions as outlined above may never
occur in nature, yet it is wise to bear them in mind, and to
eliminate their effects when engaged in studying the incu-
bation of any bird.

It seems clear that a definite and correct explanation
of the factor or condition which determine the true length
of incubation would long since have been reached, had the
data been more abundant and more amenable to study and
analysis.

Because of the several distorting influences outlined
above, there have arisen a number of explanations as to
what determines or fixes the length of the incubation period,
explanations which are now in order for detailed considera-
tion.

Size of Bird

Before reviewing the information bearing on this ex-
planation of the controlling factor of incubation duration,
it becomes necessary to define "size of bird." An examina-
tion of the different uses of this expression by various
authors sheds no light on exactly what it means with them,
and also gives no indication that all such writers mean by
it the same thing. One finds the following expressions in
current use: "(length of incubation) in a general way is
proportionate to the bird's size" (15) ; "(length of incuba-
tion) varies with the size and vitality of the bird" (12) :
"duration of incubation in general depends on height* of
bird" (38) ; "according to the size of the bird the incuba-
tion period varies, short or long, with hummingbirds ten
days, with ostrich fifty days" (9). "Size of bird" may mean
its dimensions, or its bulk, or, by implication, its weight.
It does not seem possible that past writers on this subject
could have meant size as indicated by the usual linear meas-
urements given in describing a bird, for these would lead
into a maze of characters, i. e., length of neck, total length,
length of bill, tail, legs or body, or the standing height,
which are extremely plastic and subject to such wide varia-
tion as to make it inconceivable that such shifting characters
could directly influence so deep-rooted and inelastic a char-
acter as the duration of incubation; in other words, that
these variable characters could be paralleled by variations
in the almost changeless true length of incubation.

As for bulk in a bird, one must first recall that it and
the usual measurements do not necessarily go hand in hand.

"Meaning its stature.

18

A good example that measurements are not of necessity in-
dices of bulk is given by a comparison between the black
vulture and the turkey buzzard, for the latter's measure-
ments are greater than the former's, yet its bulk is less (100).

This point is mentioned as illustrating again how con-
fusing would be bulk or measurements if considered as in-
timately related to, or strongly influencing, the length of
incubation.

It seems to the writer that bodily bulk must have been
meant when the expression "size of bird" was used; bodily
bulk and weight are closely related, but both vary more or
less according to the bird's age, the sex, the season of the
year and the abundance of food, etc., etc.

The sequence of the steps of development in bird em-
bryos being practically the same in all birds, it would seem
reasonable to believe that the larger the bulk to be grown,
the longer it should take to complete its evolution. How-
ever, it is not alone the differing bulk to be produced which
may help to bring about varying incubation periods, but
also the fact that many of these steps of development in
different species are greatly abbreviated or jumped almost
completely; hence, one can say that it is the speed and
duration of the different steps in embryonic development
which produces differences in the lengths of incubation.

It is quite patent that, in a general way, there is a rela-
tion between the size (or bulk) of a bird and the duration
of its incubation period, yet there are so many striking ex-
ceptions that one, at best, must hold it to be only a loose
and rather indefinite correlation, and probably not a rela-
tion of cause and effect, but rather an example of two effects
influenced by a single underlying cause, a suggestion to be
more fully elaborated later on.

Using the words "size of bird" in the rather indefinite
way found in most writings, one finds some interesting con-
ditions in its relation to the length of incubation, having
in mind the prevailing notion that the larger is the bird,
the longer is its period of incubation.

Notice the difference in the sizes (or bulks) of the fol-
lowing pairs of species, each pair being recorded as having
similar incubation periods: great-tailed grackle and tree
creeper, chipping sparrow and evening grosbeak, golden
eagle and puffin, ostrich and kiwi. If bulk or size alone
counted with these species as the controlling factor deter-
mining the length of incubation, there should be noticeable
differences in the incubation periods of these birds, rather
than a definite similarity, as is the case. On the other hand,
it is not uncommon to find a decidedly smaller bird having
a longer incubation than is found with a decidedly larger

it)

bird, as, for example, is found with the lapwing and the
domestic hen, or the killdeer and the hen.

If size or bulk controlled the length of incubation, the
hen should have a much longer period of incubation than
that of the lapwing or killdeer, which, however, is not the
case. The domestic hen is much larger than any of the fol-
lowing species, lorikeet, pied-billed grebe and common tern,
yet all have almost identical incubation periods.

Many birds are quite alike in size, yet exhibit marked
differences in their respective incubation lengths, i. e.,
meadowlark and upland plover, kiwi and domestic hen. The
swift and the raven are recorded as having practically the
same incubation period, yet how great is the disparity of
their sizes ! This loose relation of size of bird and length
of incubation is more noticeable within the confines of
natural groups (families), the Buteonidae for example, a
fact which was pointed out by H. Milne-Edwards (38) as
long ago as 1863, and recently Cole and Kirkpatrick (92)
intimated their belief in such a relation in the pigeon family.

While it is true that this connection between bulk and
incubation length is strikingly evident in some families, the
contrary obtains in many others, so that one can hardly con-
sider it a law. The lapwing is smaller than the wood-cock,
but has a longer incubation period, and with the Laridae it is
found that the sooty tern and the herring gull have similar
incubation periods, yet are markedly different in size.

The body of at least one species of the Megapodidae is
about the size of a domestic hen, yet this species' incubation
period is twice as long as that of the hen. What shall one
say of the several races of song sparrows, with their marked
variations in size, and the incubation period of this species
(including the subspecies) ? It is highly improbable that
the differences in these sizes are paralleled by differences in
the incubation lengths.

It is impossible to explain or understand the situation
which arises in considering the record of the lammergeier,
the incubation period of which is given as twenty days ; this
species is larger than the golden eagle (159), and its short
incubation period (as recorded) is inexplicable under any
given theory (past or present) , and the writer believes the
record is incorrect.

It would seem from the above examples that there is
too much lack of concordance between the bird's bulk and
its incubation period to admit a controlling relation of the
first over the second, even admitting that there is a loose
relation between the two.

If now one assumes that by size of bird is meant weight,
one has a more stable standard to go by, especially if one
assume certain criteria as necessary in taking the weight.

First, shall it be the male's or the female's weight? This
is decidedly important, for there is a great difference in the
weights of the two sexes with many birds — it may be 20%
in the ostrich, and the male prairie falcon's weight is but
331-3% as much as its mate; and the female European
sparrow hawk (Accipiter nisus) is said to weigh, at times,
twice as much as its mate (181). Secondly, it is necessary
to take a season which may possibly have the greatest in-
fluence, if any, over the length of incubation, i. e., the breed-
ing season, for it will probably be found that a bird's weight
follows most closely a normal at this time of the bird's
physiological year, and hence be less fluctuating and more
influential.

If now one tries to study a comparison between birds'
weights and the duration of their incubation periods, there
arises at once an awkward obstacle, namely, a ridiculous
paucity of data on bird weights. The following list gives
all the records of bird weights which have come to light in
the prosecution of this study, plus those determined by the
writer.

TABLE NO. 3

Weights of Birds in Ounces Avoirdupois

Weight Authority

Ostrich 9 avg. 4000.00 141

Kiwi 64.00 42

" 60.00 168

Emperor Penguin 1440.00 20

Adelie Penguin 42.50 20

Albatross 224-288.00 185

Yellow-billed Tropic Bird 14.00 26

Pelican (Pelicanus mitratus) 512.00 181

White Pelican $ , Juv. October 240.00 78

Great Blue Heron 96-128 180

Heron (Ardea cinerea) 64.00 181

Wood Ibis 144-192 180

Domestic Duck (Rouen) 9 128.00 34

Shoveller Duck— $ —May 11 17.25 78

Grey Wild Geese 9 160.00 34

Greater Snow Goose 80-104.00 180

Canada Goose 128-224.00 180

Brant • 64.00 180

Whistling Swan 192-304 180

California Vulture avg. 320.00 100

Gyrfalcon (Falco rusticola gyrfalco) 84.00 180

Prairie Falcon 9 72.00 145

" " $ Dec. 3 -22.30 78

« " $ 24.00 145

Western Sparrow Hawk 9 July 4.76 78

21

Weight Authority

European Sparrow Hawk 5.00-6.00 181

Eastern Sparrow Hawk 4.00 180

Honey Eater (Parnis apivorus) 32.00 181

American Goshawk 47.00 157

Hen Harrier (Circus cyaneus) 12.00-13.00 181

Western Eed-tail Hawk 5 64.00 145

" " " $ 48.00 145

Buzzard (Buteo asiaticus) 32.00-40.00 181

Western Red-tail Hawk, June 39.50 78

Red-shouldered Hawk 32-48.00 180

Swainson's Hawk 9 56.00 145

" " 25.90-56.00 180

" $ 40.00 145

" 9 April. 39.00 78

American Rough-leg Hawk, Jan 30.37 175

" " " Dec 33.65 78

Golden Eagle 184.00 60

" " 160-192.00 180

Bald Eagle 184.00 60

" " 128-192.00 180

Globose Currasow 9 Feb 114.00 169

Bobwhite 5.50-6.50 46

Scaled Quail 9 Jan 7.00 78

" $ "....: 7.50 78

Grey Partridge 12.00-13.00 46

Capercaille $ 184.00 135

Dusky Grouse 40.00-56.00 48

Ruffed Grouse 30.00-40.00 46

" " (Bonasa umbellus

umbellus) 18.00-24.00 180

Sage Grouse $ 128.00 48

Wild Turkey 9 avg. 160.00 78

• " 160.00-288.00 49

Guinea Fowl 9 avg. 56.00 78

Ring-neck Pheasant 9 avg. 3. 36.00 108

Golden Pheasant 9 20.00-24.00 108

Domestic Hen (mixed breed) 64.00-80.00 78

American Coot (Fulica americana) . . . 16.00-20.00 180

Great Bustard 480.00 10

Wilson Phalarope 9 May 11 avg. 3, 2.34 78

European Woodcock

(Scolopax rusticola) 9 8.00-27.00 181

American Woodcock 6.00 95

" " $ 5.00-6.00 180

" " 9 6.00-9.00 180

Wilson's Snipe 4.00-5.00 180

Common Snipe (Gallinago caelestis) 3.00-8.00 181

Solitary Snipe (Gallinago solitaria) ... . 7.50-10.00 181

Weight Authority

Swinhoe's Snipe (Gallinago megala) 6.00-8.00 181

Jack Snipe (Gallinago gallinula) 2.00 181

Greater Yellow-legs 6.00-10.00 180

Lesser " " 3.50-5.00 180

Spotted Sandpiper (Actitis macularia) 9 May.. 1.53 78

Curlew (Numenius lineatus) 12.00-14.00 181

Killdeer 9 July 3.10 78

Kumlien Gull '(Larus kumlieni) 21.00 180

Lesser Tern (Sterna minuta) 2.00 181

Band-tail Pigeon avg. 12.00 127

Domestic Pigeon 10.00 78

Passenger " (Estopistis migratorius) . . . 12.00 180

Mourning Dove avg. 4.50 78

" " 5.00-6.00 180

Roadrimner $ in October 11.00 78

Cockatoo Parakeet 2.88 152

White Cockatoo 21.25 152

Great Sulphur Crested Cockatoo 26.62-43.62 152

Lesser Sulphur Crested Cockatoo 12.25 152

Bare-eyed Cockatoo (Cocatua gymnopis) . . . 19.25 152

Molucca Cockatoo 34.12 152

Leadbeater's Cockatoo 14.50 152

Rose-breasted Cockatoo 18.75 152

Blue and Yellow Macaw '. 37.00 152

Great Blue and Yellow Macaw 51.75 152

Midget Macaw (Ara severa) 10.12 152

Elegant Grass Parrakeet 3.62 152

Alexandrine Parrakeet (Paliornis torquata) . . 8.50 152

Belted Kingfisher 5.00-6.00 180

Long-eared Owl, April 11.28 78

Barred Owl 20.00-32.00 180

Screech Owl (Otus asio asio) 4.00-6.00 180

Eagle Owl (Bubo maximus) 112.00 181

Great Horned Owl 56.00 145

" (Bubo virginianus

virginianus) 48.00-72.00 180

Burrowing Owl, May 5.84 78

Western Night-hawk $ June 2.75 78

Sennett's Nighthawk 9 2.25-3.35 177

Pacific Nighthawk 9 4.00 177

Broad-tailed Hummingbird $ July 10 78

Rocky Mountain Hairy Woodpecker $ 2.25 78

Hairy Woodpecker (Dryobates villosus

villosus) 3.00 180

Downey Woodpecker (Dryobates pubescens

medianus) 1.50 180

Williamson's Sapsucker 9 April 8th 1.62 78

Red-headed Woodpecker 9 July 2.80 78

23

Weight Authority

Ant-eating Woodpecker $ in October 2.75 78

Lewis Woodpecker $ Aug 3.81 78

Western Flicker $ Aug 4.30 78

Kingbird $ July 1.60 78

Arkansas Kingbird 9 July 1.60 78

Say's Phoebe $ June ". 91 78

Hammond's Flycatcher $ July 40 78

Horned Lark (Desert) , Nov 1.18 78

Townsand's Solitaire, July 1.40 78

Western Robin 9 June 3.35 78

Catbird 9 July 1.40 78

Water Ousle, July 2.30 78

Bohemian Waxwing 9 avg. 11, Feb 2.21 175

$ avg. 11, Feb 2.22 175

Rock Wren $ May 60 78

Western House Wren 9 June 50 78

White-rumped Shrike $ June 2.03 78

Cassin's Vireo, July 60 78

Warbling Vireo, July 46 78

Rocky Mountain Nuthatch 9 Aug 65 78

Pigmy Nuthatch 9 Aug 38 78

Long-tailed Ch'ickadee $ July 40 78

Magpie, May 5.34 78

Long-crested Jay, July 4.00 78

Rocky Mountain Creeper 9 July 20 78

Orange-crowned Warbler $ May 37 182

Yellow Warbler $ May 35 '78

Myrtle Warbler $ May 50 182

" " $ May 42 182

Audubon's Warbler $ May 40 182

Macgillivary's Warbler $ July 40 78

Western Tanager $ July 1.10 78

Red-winged Blackbird 9 (Thick-bill), June. . 1.60 78
Red-wing Blackbird (Agelaius phoaniceus

phoeniceus) 2.50-3.00 180

Meadow Lark (Sturnella magna magna) 4.00-5.00 180
" (Western) (Sturnella neglecta),

June 3.97 78

Rusty Blackbird 2.00-2.50 180

Brewer's Blackbird 9 June 2.60 78

Bronzed Grackle, May 10 3.87 182

House Finch 9 May 66 78

Arkansas Goldfinch 9 July 47 78

Pine Siskin $ July 43 78

English Sparrow 9 avg. 1.05 78

Western Vesper Sparrow $ Oct 85 78

" $ Apr. 24 1.00 78

Lark Sparrow 9 95 78

24

Weight Authority

Red-backed Junco $ July 70 78

Cassin's Sparrow $ July .70 78

Western Song Sparrow $ Feb. 28 88 175

" Tree " 9 avg. 2, Feb. 28 65 175

$ avg. 3, Feb. 28.... .71 175

Spurred Towhee 9 June 1.50 78

Black-headed Grosbeak $ June 1.30 78

Lark Bunting $ June 1.50 78

It will be necessary here to consider but a few compari-
sons of different species and their weights and lengths of
incubation; the ostrich, kiwi and emperor penguin have
identical incubation periods, yet their weights are two hun-
dred fifty, four, and ninety pounds, respectively; the do-
mestic goose and the sparrow hawk have nearly similar in-
cubation lengths, yet the first weighs ten pounds, while the
second but five (more or less) ounces; the ruby -throated
hummingbird and Cassin's vireo incubate their eggs almost
exactly the same length of time, yet one weighs one-tenth
of an ounce and the other six-tenths of an ounce. These
examples form comparisons between species of different
families, where one can expect such lack of parallelism be-
tween weights and incubation periods. With species within
the confines of a single family, however, equally sharp lack
of correlation of weights and incubation lengths occur; the
bobwhite and the grey partridge have similar incubation
periods, yet their weights are as five and one-half is to
twelve, and there are several species in Fringilline birds
which have identical incubation lengths, but differ markedly
in weight.

While the available data on bird weights is, deplorably
insignificant, when the species involved are compared to
the total number of known birds, yet it would seem reason-
able to expect more indications of a relation of weight to
incubation length, if the weight fixed this length, than one
finds in the data at hand. I feel that whatever relationship
appears in these curves of weight is not one of cause and
effect, but, as has been said before, a correlation of two effects
to a third factor as an underlying single cause. The singu-
lar and suggestive fact in this phase of our question is that
with man's domesticated birds with which one can see a
variation of. at times, several hundred per cent, in weight,
there should be no corresponding change in incubation.

It is highly desirable to have more recorded weights of
birds, especially of the breeding female, since biologic char-
acters of birds will be more and more called upon to aid in
the future in solving many riddles in avian physiology.
Until a much larger mass of data along these particular

25

lines has been accumulated, one must suspend final judgment
on the question of how much does a bird's weight or size
influence its incubation length.

Age of Female

It is well known that pullets of our barnyard fowl lay
eggs averaging smaller in size than does the mature hen,
and this condition also holds good with pheasant pullets;
poultry men know that eggs of mean size and weight from
each race of our domestic hen hatch more successfully than
do too large or too small eggs, whence it might be held that
the age of the female may affect the length of incubation,
since it has been shown that the mature hen is apt to lay
eggs near the normal for her race, which are more uniformly
successful in hatching. Whether or not the larger or the
smaller than normal eggs really hatch later or earlier than
the average I do not know; in the absence of definite data
in answer to this, the question must be left open and un-
decided. Possibly the age of the female really affects the
fertility or viability of the egg, and not the incubation
length. It is also possible that very old females may exhibit
a tendency to a slowing of metabolic intensity, which would
unfavorably affect the incubating temperature.

Condition of Parents

It is not possible to say, owing to the lack of exact in-
formation, if the physical condition of the male has any
influence on the length of incubation with the species with
which the female does all of the incubating; nevertheless,
it is conceivable that old, or immature, or weak males may
give the new individual in the egg a poor start, entailing
perhaps a slower rate of development, resulting in a longer
period of incubation.

There is no doubt in my mind but that poor health in
either parent (when both incubate) would result in what
amounts to a cooling of the eggs during incubation, and a
resulting apparent lengthening of this period, through slug-
gish embryonic development, all because of the setting bird's
temperature being lower than normal. While several writers
mention the physical condition of the parents as being a
factor in affecting the length of incubation, none has given
any data, experimental or otherwise, in support of the sug-
gestion. It must be left open and undecided.

Conduct of Parents

The assiduity (or neglect) of the incubating parents
in covering theip eggs unquestionably results in the eggs
being hatched "on time," or "late," or "not at all." In other
words, the length of incubation is unquestionably affected
by the incubating bird being frightened from its nest too

frequently, or kept from its nest too long, or through the
parent birds being inattentive.

This effect on incubation has long been known to poul-
try raisers; it is, however, an effect not altering the true
length of incubation, but merely one of cooling and retarda-
tion of embryonic development. Such conduct of parents
does not affect the true or specific incubation length.

Longevity

In a valuable paper on longevity in birds, Gurney (132)
suggested that there might be some relation between bird
longevity and the length of incubation.

It is nearly impossible to reach any definite conclusion
on this suggestion since veiy little is known on that variety
of longevity which is most likely the only one which affects
the fluctuations of bird population and their correlated
biologic results. There are nearly a hundred records relat-
ing to the ages to which various species of birds live in
captivity or when domesticated, but this is potential
longevity. What is lacking, however, is information on
the mean or average longevity, the length of life which birds
attain in nature, under normal conditions of life's pressure
for and against them. Brehm (132) thought that longevity
was more or less correlated with size, and there are some
indications that within the Class this is true, but it fails to
hold good when comparing species of differing Classes.

A curve was plotted from the longevity data given by
Gurnej7 and gathered by the writer from other sources and
placed in juxtaposition with the curve of incubation lengths
of the same species; it showed no correlation between the
two. It is safe, from the present data, to hold that length
of incubation and longevity have no relation in fact, a con-
clusion which H. Milne-Edwards (133) reached many year»
ago.

State of Young at Hatching
Precocious,
Altricial,
Completeness of development.

In these three conditions, given as determining factors
affecting the length of incubation, there are more or .less
confused, it seems to me, two distinct ideas; the first is that
of precocity in its usual sense, i. e., the state of self -helpful
activity and semi-independence of young birds at hatching ;
and the second is that of the condition of the young being
well on towards completeness of development at hatching.
The first conception may or may not include the second,
while the second always includes the first — a newly hatched
duckling is typical of the first, while a newly hatched mega-
pod bird is typical of the second. In considering precocity

27

it is easiest to handle the idea at the same time with altricial
conditions of the young.

Precocity and altricial characters are antipodal, and if
precocity confers (or engenders) a long incubation period,
it seems reasonable to expect that an altricial species should
have a short period. This expectation is realized in a num-
ber of instances, the ostrich and the English sparrow being
good examples. There are, however, many striking excep-
tions; the domestic hen and many parrots have identical
incubation periods ; one is typically precocious and the other
is highly altricial; and yet under this explanation the first
should have a long period and the second a shorter one.
Most, if not all, of the Charadriidae are precocious, which
ought to bring about with these species uniformly long incu-
bation periods, yet the records clearly show a great variety
of lengths of incubation with this family, just about as one
would find it in any other fairly large and diversified
natural group, i. e., from sixteen to thirty days. Burns (3)
justly calls attention to the lack of definite relation between
precocity and long incubations, and altricial characters and
short incubations, comparing with this in mind, ducks and
large hawks, chats and sandpipers, tropic birds and gulls,
all examples which in his belief disprove the correctness of
the suggestion of a causal relation between precocious and
altricial characters and the duration of incubation. If
precociousness engendered long incubation periods, a
majority of the so-called precoces should have this
type of incubation length. Now, if one examine the
records of the lengths of incubation amongst the pre-
cocious Katitae, Crypturidae, Phasianidae, Anatidae and
others, one finds the incubation length varying from
fourteen to fifty-eight days. In other words, with the so-
called precoces one finds a wide range of variation in the
length of incubation, just as one would find with almost any
other group of orders and families indiscriminately mixed
together. The supposed correlation of precocity and long
incubation may have arisen through a belief that the pre-
cocious birds laid large eggs, and that large eggs presuppose
long incubations, but large eggs are by no means the rule
with the prococes, as is witnessed with the quail, hen,
grouse, etc. Furthermore, this does not help this assumed
explanation since, it will be shown later, the length of in-
cubation is not closely correlated to the size of the egg.

More or less relevant to this phase of our problem is
Gadow's (150) belief that there is a direct relation between
the length of incubation and the nesting period. He assumes
that the developmental period is made of two portions,
embryonic and post-embryonic, and that the nest period
covers the post-embryonic developmental stage, which is by

no means a safe assumption, since many altricial birds, and
practically all the prococes, continue post-embryonic de-
velopment for a good while after leaving the nest.

Gadow's conclusion is that a long nest period is pre-
ceded by a short incubation period and (inferentially) vice
versa.

While the incubation and nesting periods of some birds
support this view, there is also much evidence against it.
The screamer and the noddy tern do not bear support to
the idea. The secretary bird is reported to incubate forty-
two daysr yet its young do not leave the nest for six months
after hatching (138), and the condor incubates fifty-six
days, while its young are reported to live seven months in
the nest after being hatched (154). This question can be
examined in another way by making a ratio of incubation
length and nest life, counting the latter with precocious
birds as zero. The following list gives this ratio with a
few species : Domestic hen, 21 ;0 ; house-finch, 14 ;14 ; sparrow
hawk, 29 ;29 ; golden eagle, 30 ;35 ; yellow-headed tropic bird,
28 ;62. It seems to me that while there is much color of truth
in this suggestion made by Gadow, whose eminence in zoo-
logic work compels attention to his ideas, there is so much
against the theory that judgment must still be withheld on
its finality.

Under the second way of putting the explanation
comes Arrigoni's (12) statement, which voices also that
of Newton (25), Evans (1-2), and Claus (10). Arrigoni
writes: "The period of incubation varies, and is in relation
with the state of perfection in which the young are born."
It is true that the young of the precoces are physiologically
more perfect than are the young of altricial birds, but both
are far from being morphologically perfect, and all have a
long way to go before becoming so. An English sparrow's
nestling is typically the opposite of precocious, yet it hatches
out in fourteen days, and spends but fourteen days in the
nest. I doubt if a young killdeer reaches an equal level of
development in fourteen days after hatching. As a matter
of fact, it takes three weeks (after hatching) for young kill-
deers to reach a stage of growth permitting them to follow
the parents on the wing; it is ten days (after hatching)
before they are able to lift their bodies off the ground with
their wings alone (183).

The only birds known to the writer whose young are
hatched in a condition approaching "perfection" are the
megapodes, and, the writer hopes to show later on, the length
of incubation with these birds is not correlated with the state
of perfection at hatching alone, but rather with quite an-
other characteristic.

Precocity seems to me to be an acquired character of
expediency, found in a heterogeneous mixture of species.
To many, precocity is synonymous with "lowness," and is
said to be a retained reptilian characteristic, the nearer a
bird to its proto-reptilian ancestor, the greater its precocity.
By this token the low birds should have both long incuba-
tion periods and noticeable precocity. However, there are
quite a few "low" birds which are the reverse of precocious ;
they are definitely altricial, e. g., pelicans, water turkeys
and cormorants (65). Pycraft (115) puts the facts much
more clearly, and with greater truth, when he says, "When
the nestling is active from the moment of hatching, the eggs
have a relatively longer incubation period than in cases
where the nestlings are for a long time helpless."

The writer feels justified in holding with Burns (3)
that the possession of precocious or altricial characters does
not confer thereby long or short incubation periods, and that
they are not correlated to the length of incubation as causes
to effects.

Size of Egg

Glaus (10), Fiirbinger (102), and Chapman (65) all
state that the incubation period varies with the size of the
egg. Burns (3) says it "seems to depend almost altogether
on mere size or bulk" of egg, while Evans (2) in his second
conclusion feels willing to hold "that within each group,
the larger the egg, the longer the period."

What is meant by "size of egg" ? It does not seem pos-
sible that these (and other) writers mean mere size as ex-
pressed by length and breadth, for these two dimensions
cannot possibly account for, and produce, the endless varia-
tions in the shape of eggs, variations of shape which produce
corresponding differences in bulk, nor does it seem possible
that they believe the infinity of bulks produced by these dif-
ferences in shape would be paralleled by corresponding
alterations in the incubation period. If there be any rela-
tion between the size of the egg and the duration of its
incubation, the writer feels that the term "size" should be
taken to mean weight, for after all is said, these differences
in measurements and shape result in corresponding varia-
tions in egg weight. If all birds' eggs were of the same
specific gravity, and if there were a fixed relation between
the length-breadth index and the weight, one could easily
ascertain the weights of a large number of different birds'
eggs, since there is an enormous accumulation of length-
breadth measurements at hand, collected with infinite care,
toil and patience for years by ornithologists all over the
world. Unfortunately I was able to find no data available
from which one can learn if all birds' eggs have similar
specific gravities, though Spohn and Riddle (173) make

30

statements which seem to indicate that the specific gravities
of eggs from differing species are dissimilar; and unfortu-
nately, also, the length-breadth index bears no relation to
the egg weight (140). The writer studied this question for
some time, seeking to evolve a formula whence one could get
the egg weight from its length-breadth index, but found it
impossible of achievement, in which finding he is supported
by Curtis (140), who says, "The shape of the egg (hen's),
as measured by the length-breadth index, is negatively cor-
related with the weight of the egg and with the weight of
each of the egg parts."

It therefore seems to me that the only datum to be used
in considering this phase of the problem in hand is, perforce,
the weight of the fresh egg. and the relation of the egg
weight to the length of incubation will be taken up a little
later on in this section, while it will be expedient now to
consider, more or less carefully, the relation of mere bigness
to the incubation length.

It is quite apparent, after a careful review of the facts,
that there is a loose relation between the size of the bird
and its egg; the smaller the bird, the smaller the eg<r, glaring
exceptions noted. This parallelism is more noticeable within
the confines of natural groups (families), and within these
groups there is also a loose relation between the size of the
egg and the duration of incubation, i. e., the larger the egg
(and the bird), the longer the incubation, a relation well
illustrated by the Buteonidae. There are, however, families
wherein this relation does not hold good, or wherein the
exceptions are too noticeable to be disregarded. Thus the
egg of the guinea hen is decidedly smaller than that of the
domestic hen (51), yet the guinea hen has the longer incuba-
tion period; on the other hand an example is found, in the
same family, where the larger egg is accompanied by the
longer incubation period, i.e., with the Mikado pheasant as
compared with other pheasants. There is, relatively, an
enormous variation in the sizes of the eggs of fringilline
birds, but the great majority of this family conform more
or less closely to the fourteen-day period. There are so
many contradicting facts in relation to this theory that one
seems impelled to believe that egg size is n*)t a factor In
determining the true length of incubation; the size of eggs
varies often to a great degree in a single set. and eggs in
different sets from individuals of one species differ much in
size; the average measurements of eggs from the same
species in different regions vary, i. e., those of the Mikado
pheasant (68). Nevertheless, all these variations are un-
accompanied by corresponding differences in the lengths of
incubation. A fact of great weight in this discussion of egg
size and incubation is that, notwithstanding the remarkable

31

variations in the size of the eggs of the domestic hen and
its different races, there is no corresponding variation in the
incubation period, be the egg from a bantam or a shanghai ;
in other words, the jungle fowl and its bizarre descendants
all retain the ancestral length of incubation, viz., twenty-one
days. It seems to the writer that if this length were plastic,
and altered hand in hand with changes in the size of the
egg, it should be strikingly apparent with the domesticated
bird, especially the common barnyard fowl, yet, as a matter
of fact, there is, to reiterate, not only no change to be noticed,
but a tenacious retention of the primitive ancestral duration
6f incubation.

Poultry raisers (103) have long recognized several dif-
ferent factors which produce large variations in the size of
hens' eggs, and these factors have been given by Curtis
(140), as follows: Age of hen, season of laying, state of
health of hen, position of egg in the egg laying period
(i, e., the first -or the last egg laid), the rate of egg produc-
tion, and lastly the food consumption of the hen. It is
possible that some of these factors can have no existence
with birds in the wild state, yet, on the other hand, some
appear to exist with birds in natural conditions, and still
seem unaccompanied by parallel changes in the length of
incubation.

If egg size (indicated by measurements) be examined
as to its relation to the length of incubation, one finds a
complicated condition of affairs. Newton (25) says that
the eggs of the guillemot (A. troile) are ten times the size
of those of the raven, yet the incubation periods of these
birds are but three to one, wrhile the eggs of the eagle (sp ? )
and those of the guillemot are recorded (25) as being almost
the same in size, but there is a difference of four or six days
in the lengths of incubation of these two species. The incu-
bation periods of the elf owl and of several hummingbirds
are similar, yet there is a large difference in the sizes of
their respective eggs. The eggs of the flamingo and some
megapods are nearly alike in length and breadth, neverthe-
less the first incubates twenty-eight days and the second
forty-two days, rather than being identical, as would happen
were this egg-size control theory correct.

The above are all examples from species belonging to
different families, and as Evans has stated, there is not much
relation between egg size and the length of incubation out-
side of the boundaries of natural groups (families). There
are, however, numerous exceptions also to be found within
families.

The eggs of the white and the brown pelican are recorded
as differing noticeably in size, but both species have the
same duration of incubation. Furthermore, eggs laid by

32

white herons (138) in Europe are much larger than eggs
laid by the same species in India, without, so far as the
writer can learn, any difference in the incubation periods
of the European and the Indian birds.

It is self-evident that size, when determined by linear
measurements, is an exceedingly shifting datum on which to
base comparisons of different incubation periods. If. there
be any relation between these periods and the egg size or
bulk, it seems important to secure a better datum than the
above, and in a fresh egg's weight, the writer believes, one
has a datum beyond criticism, as to its meaning, and one
which is unaccompanied by any questionable secondary con-
siderations, and hence furnishes much more secure ground
on which to build comparisons, and from which to draw
conclusions.

Unfortunately, the number of recorded egg weights
from different species, gathered by the wrriter for this study,
is very small, though there are, doubtless, many others
tucked aAvay in inaccessible publications; the following list
(Table No. 4) gives all those found in literature, by the
writer, plus a few which were determined by him person-
ally. An additional list (Table No. 5), is given of the
weights of individual eggs occurring in sets. All weights
are in ounces and pounds (avoirdupois), and, so far as
known, are of fresh eggs, unless otherwise specified :

TABLE NO. 4

Weights of Bird Eggs in Ounces Avoirdupois

Weight Authority

Ostrich 48.00-60.00 10"

Ostrich avg. 60.00 141

Emu 20.00 10

Kiwi 14.00-20.00 10

Kiwi 14.00-15.00 42

Kiwi 14.00 168

Emperor Penguin 16.00 20

Adelie Penguin 4.80 136

Adelie Penguin 4.56 20

Yellow-headed Tropic Bird 1.42 26

Loon (Sp?) avg. 4, 5.67 178

Black-crown Night Heron avg. 4 sets, 1.25 .178

Mallard Duck (domesticated) avg. 2.83 78

Pekin Duck (domesticated) 2.38-3.00 78

Goose (toulouse) avg. 6.00 78

California Vulture (estimated by W. H. B.) 10.90 10

Sparrow Hawk 50 56

Western Red-tail Hawk 2.10 78

Mallee Fowl (Lipoa ocellata) 6.50 186

Globose Curassow 8.00 169

Domestic Turkey avg. 3.23 78

Weight Authority

Guinea Hen 1.40 78

Guinea Hen • 1.40 51

Silver Pheasant avg. 1.50 109

Ring-neck Pheasant avg. 1.20 109

Golden Pheasant 1.00 109

Reeves Pheasant 98 109

Domestic Hen 1.90 78

Domestic Hen 2.00 51

Killdeer 40 78

Mountain Plover avg. 3, .52 182

Common Tern avg. 4 sets, .64 178

Domestic Pigeon 50 78

Mourning Dove 40 78

Kingfisher (Ceryle alcyon) avg. 3, .45 178

Long-eared Owl '. avg. of 2, .82 78

Screech Owl 58 67

Western Nighthawk. 35 78

Broad-tailed Hummingbird avg. .02 78

Orange-shafted Flicker avg. .25 70

Kingbird avg. 4, .15 178

Arkansas Kingbird 14 178

Veery avg. 4, .10 178 .

Eastern Robin avg. 4, .06 178

Western Robin avg. 3, .23 78

Catbird avg. 4, .06 178

Barn Swallow avg. of 2 sets, .05 178

Bank Swallow avg. of 4 sets, .05 178

Tree Swallow avg. 4, .06 178

Cliff Swallow avg. of 3 sets, .07 178

Red-eye Vireo avg. of 2 sets, .07 178

Magpie avg. 17, .34 78

Crow (Corvus brachyrhynchos brachyrhynchos

avg. 6, .62 78

Yellow Warbler avg. of 2 sets, .04 178

Ovenbird avg. 4, .09 178

Redstart avg. 8, .05 178

Bobolink avg. 5, .10 178

Western Meadow Lark avg. 3, .20 78

Brewer's Blackbird avg. 6, .18 78

House Finch . .avg. 8, .08 78

English Sparrow avg. 10, .09 78

English Sparrow avg. 18, .09 178

Western Vesper Sparrow avg. 4, .09 78

Chipping Sparrow avg. of 2 sets, .05 178

Song Sparrow (subsp ? ) avg. 14, .07 178

Black-head Grosbeak avg. 3, .10 78

Lazuli Bunting 07 78

Lark Bunting avg. 4, .10 78

34

TABLE NO.

Weights of Eggs in Sets, in Grains

Set

Egg Number

1

2

3

4

5

6

7

Killdeer . 1

175
221

190
214

186
215

198
229

2

Nighthawk (Western) (Sup sp?)..

140

128

Broad-tail Hummingbird 1

2

8

8

9

59

70

60

66

109

105

109

140
132

140
140

144
144

140
152

144
155

144
152

152

2

Western Meadow Lark

83

87

87

Brewer's Blackbird

79

79

83

75

83

English Sparrow 1

46
36

41
40

42
40

43
38

34
44

38

38

34

42

Black-headed Grosbeak

62

60

58

56

56

56

52

If, now, one examined these various egg weights some
interesting conditions appear: First as to isolated cases of
comparisons, one notices that the ostrich and kiwi have equal
lengths of incubation, yet the ostrich egg weighs three and
three-quarters pounds, while that of the kiwi is less than a
pound ; a pigeon's egg weighs five times more than an Eng-
lish sparrow's (in fact the pigeon's egg is heavier than the
sparrow itself) but there is only three or four days' differ-
ence in the incubation periods of these two species ; the eggs
of the yellow-billed tropic bird and those of the guinea hen
are almost identical in weight, nevertheless these two species
have incubation periods differing in length at least four
days. If one make a curve of the incubation periods, ar-
ranging them according to the weights of the eggs, the
heaviest first, it can be noticed that again there is a general
tendency for the incubation length to shorten as the egg is
lighter, but there are so many sharp deviations from the ex-
pected regularity that one suspects that there is another
factor, not influenced by the egg weight, which is at work
in fixing the length of incubation or at least such a second
factor is working in conjunction with the effect of egg
weight. This possible second factor will be discussed in the
later portions of this study. Suffice it here to say, that,
while this list of egg weights is one altogether too short on
which to base a final judgment, which must be held for the
time in abeyance, these curves and such other considerations
as just outlined compel me to believe that egg weights are

35

only loosely related to the lengths of incubation, and both
are effects of another single underlying character.

The more consideration I have given to this phase of
our problem, the more incredible it seems to me that condi-
tions as variable and elastic as the size and weight of eggs
have been proven to be, can have much, if any, influence on
the true duration of incubation, an epoch in the individual's
life of the utmost importance to it and its race.

Age of the Egg

Any effect that the age of an egg may have on its in-
cubation duration is probably correlated with the length of
viability of such egg, by which term the writer would have
understood the possibility of the embryo's remaining alive
without the egg being incubated. Viability undoubtedly
varies with the species, and too, it must be related to the

number of eggs in a set, and to the time interval between

of the eggs
often, with the ruffed grouse, the first eggs must remain

the laying of the eggs in such a set. It is self-evident that,

viable at least fourteen days. It appears from Crandall's
observations (94) on an emu in captivity that this particular
bird laid six eggs at intervals of five days each ; if this were
to occur in nature and all the eggs were to hatch it would
show an amazingly tenacious viability for this species. Pick-
nell (160) states that in captivity the ostrich lays twelve
to sixteen eggs during a period of thirty days, and at the
end of that time begins to incubate, which is another ex-
ample of long viability.

I believe that eggs of the lower birds will be found to
remain viable a much longer period than those of the higher
birds ; I doubt very much that a robin's or a hummingbird's
egg would remain viable, even with extraordinary care, three
or four weeks after being laid. The grounds for this belief
are that long viability is probably a reptilian character,
and is related to a particular phase of a bird's physiology, to
be discussed later on. It is highly desirable to have this
question of the duration of viability of birds' eggs thor-
oughly investigated. All that I could find in literature on
this question was that hens' eggs remained viable up to the
end of the eighth or ninth day, and those of pheasants, up
to the fifteenth or twenty-first day. Inside of the limits of
viability, the age of an egg (at least a hen's egg) unques-
tionably seems to influence the apparent length of incuba-
tion, for if fresh and old eggs be placed together in an
incubator (or under a hen, it is said) (162), the fresher
eggs hatch first. It also appears that the nearer the egg is
to its limit of viability when placed under incubation con-
ditions, the longer it takes to hatch.

Heinroth (162) says that some ducks sit upon their
eggs at night during severe cold, to protect them until the

36

full set is laid, and yet that all of the set hatch on or about
the same time. This statement uncovers a possibility which
is of interest, and it is also one needing further investiga-
tion; it is quite possible that the first eggs laid in such a
set undergo a slow but appreciable development, a speed
of embryonic growth which is perhaps never quite equal to
that of the later fresher eggs of the set, yet the amount of
development in the first eggs make up for the lack of
developmental speed and all the eggs in such a set mature at
the same time. One must not forget that there is an ap-
preciable degree of development of the embryo before an
egg is laid, and that it is probable that the speed of this
"uterine" development is continued at the same, or at an
accelerated speed if the egg be incubated at once after being
laid, and, contrariwise, if this initial development be sus-
pended for a considerable time after the egg is laid, the
developmental process is slow in speeding up and in getting
under way again, all of which apparently elongates the true
incubation length. It is said (49) that perfectly fresh turkey
eggs, if incubated at once, hatch a "few hours earlier" than
older eggs. This whole question of the behavior of
fresh and older eggs, especially of wild birds, under incu-
bation conditions needs attentive and thorough investiga-
tion, especially through the channels of experimental
methods.

Influence of the Shell

How much difference, if any, variations in the egg shell
make in the duration of incubation of birds' eggs under
natural conditions is unknown to me, and it seems, a priori,
that this question would be exceedingly difficult to settle.
It is a well-recognized factor producing variations in the
success of artificial incubation of hens' eggs, since all poultry
raisers advise against mixing the eggs of different poultry
breeds in the same incubator, alleging deciding want of suc-
cess with such practice, and ascribing this to different de-
gress of thickness, hardness, coarseness, porosity, etc., of the
shell of different breeds; this assumption probably has some
truth, for these differing conditions of the shell might affect
the readiness with which such eggs respond to the heat of
the incubator. It is probable that shePs thinner than normal
would permit of too rapid radiation when uncovered, which,
if prolonged, would result in the embryonic growth being
slowed down, only adding hours or days to the true length of
incubation, and not causing a permanent modification of this
specific length. The proof that the shell of eggs of wild
birds varies is too clear to be overlooked, and its possible
modifying effect on the length of incubation cannot be dis-
regarded. However, Meyer (178) has shown that the egg
shell of the song sparrow can vary 46% in the weights of

37

the extremes of different eggs; there is no evidence of any
similar variation in the incubation period of this species.

Further study on this point is to be made before one can
render a verdict for or against it.

Size of Yolk

It has been suggested by Py craft (138) that an abbre-
viation of the incubation period with many birds has been
brought about by a gradual lessening of the "food yolk,"
presumably meaning that part of the egg commonly termed
the yolk.

Enunciated by so keen a student and investigator, and
such an original thinker, this suggestion must be considered
with care and attention.

It is extremely difficult to discuss this idea in any way,
both because Pycraft produces no statistical or experimental
evidence in support of his theory, and because there seems
to be no information available concerning the size of the
yolk in various birds' eggs.

The only publication coming to the writer's hands,
which throws any light on this theory is a paper by Curtis
(140), dealing with the factors which influence the size,
shape, and physical constituents of the egg of the common
hen. In this study (a model of its kind) the yolk is shown
to be the second most variable component of the egg, and
that of two eggs, the yolk is relatively larger in the smaller
egg; also that the weight of the yolk is not determined, for
example, by the hen's weight alone but that it is markedly
modified by her hereditary constitution, physical condition
(state of health), stage of development (age), the season
of the year, and the position of the egg in the series of eggs
laid at the time. If these deductions by Curtis apply equally
well to other birds' eggs, and, until shown otherwise, it is
fair to hold that they do, it is difficult to see how Pycraft's
suggestion can be tenable, since, in general, the shorter in-
cubation periods are characteristic of birds laying small
eggs, which, as has just been shown, have relatively larger
yolks, in place of smaller yolks, as his hypothesis requires.

Pycraft mentions specifically, as having reduced yolk-
size (or weight?) "razorbills, guillemots, (and) many gulls,"
but it is not possible to resolve this problem as it may apply
to these and other species, until the relative sizes of their
egg yolks be determined and the periods of incubation of
these species be compared and viewed as to their relation to
such yolk sizes. While judgment must be suspended also
on this point, at least until a considerable mass of informa-
tion relating to the size of the yolk in the eggs of different
species of birds be secured, still it seems to me that this
hypothesis is rather dubious.

Telluric Influences

Under this caption are to be included the supposed ef-
fects of :

A. Geographical zone

B. Climate

C. Weather

D. Geographical residence of species

E. Site of nest.

All of these have been mentioned by various writers as
being conditions controlling or determining the incubation
length ; I am convinced that such influence any of these
may have on the length of incubation is merely that
of cooling the eggs and retarding the embryonic devel-
opment, merely adding hours or days to the true length of
incubation, which apparent length returns to the original if
the retarding influence be removed.

It has been abundantly demonstrated with eggs in ar-
tificial incubation, that the duration of incubation can be
extended by subjecting the eggs to temperature lower than
the optimum, and that the degree and duration of the lower-
ing of the temperature nave limits, beyond which the em-
bryo dies. Yet it is astonishing how much cooling or chill-
ing a set of eggs will successfully withstand in incubation.

The writer has known of a set of house finches' eggs
being left uncovered all night during a spell of cold weather,
and yet producing a full number of normal and vigorous
nestlings. The structure of the egg lends itself to resistance
to too rapid cooling, for the shell, with its small "pores"
full of air, and the shell membrane, together, make a good
insulating medium. Brehm (quoted by Ingersoll. 110) , says
it requires one hour and forty-five minutes at fifteen degrees,
Fahrenheit, to freeze a living egg; this means that it takes
seventeen degrees, Fahrenheit, of frost for nearly two hours'
exposure, to kill the developing embryo, and it is apparent
that this time may vary proportionately with the size of the
egg. It is well known that eggs in the later stages of in-
cubation cannot resist successfully such low, or prolonged
low, temperature as just mentioned, but that they do suc-
cessfully withstand milder degrees of frost for much longer
periods, especially during the early days of incubatipn, is
equally well known.

It is also known that the incubation period of the do-
mestic hen can be prolonged to the twenty-third or twenty-
fourth day by judicious cooling during incubation. That
this effect of cooling obtains during the incubation of
various other birds at large is undisputed.
Zone

The writer has been unable to find any published infor-
mation bearing on the possible effect of geographical zone on

39

the length of the incubation period, though one or two gen-
eral statements have been encountered, given, however, with-
out any detailed facts in support of the same. Newton (25)
says that there have been "no observations made on the ques-
tion if there be a difference in the length of incubation be-
tween polar and tropical individuals of the same species."
Arrigoni (12) is the only writer whose statement on this point
is definite, though it is unaccompanied by substantiating
facts: he says "it seems, however, that birds of the same
species which nest near the pole, and near the tropics, have
equal periods of incubation" (sembra pero che gli nidificono
e presso al polo e presso i tropici abbiano equale periodo de
incubazione). It is highly probable that the bodily temper-
atures of birds (as is the case with Homo) vary little, if at
all, with changes of zone, being, most likely, the same at the
polar region as at the tropics; there is, however, some evi-
dence which points to the possibility that atmospheric con-
ditions may influence a bird's temperature, but the question
has not yet been thoroughly studied and worked out. Never-
theless, I believe that birds, while incubating, would be able
to maintain the necessary optimum incubating temperature
equally well in the cold zones as in the hot zones. In this
connection, one must not forget that a large number of birds'
nests are constructed of such material and in such a manner,
as to retain to the best advantage the heat applied by the
parents to the eggs, facts which bear especially on the
question of incubation at the polar regions, the eiders being
marked examples of birds with nests built to retain heat.
The huge, bulky magpie nest has a central bowl of non-
conducting clay or mud, and in many nests are incorporated
most excellent non-conductors of vegetable or animal matter.
The chipping sparrow uses horse-hair, the writer has found
a nest of the Arkansas kingbird lined with rabbit's hair and
a house finch's nest lined with sheep-wool; and what better
non-conductor can be found than the cotton-like mass of
the hummingbird's nest?

Far be it from the writer wishing to be understood as
holding that these materials are consciously selected for the
purpose by the nest-builders; attention is merely called to
the fact that provision to retain the applied heat is to be
found in many differing nests, and the nests of the polar
regions are no exceptions.

Many birds successfully go through the duties of incu-
bation during the winter, when the forests are deep with
snow, and akin to the arctic regions, yet, so far as I know,
this does not seem to change their incubation period in com-
parison with their relatives nesting under less rigorous con-
ditions. If the foregoing be true, it seems to me that geo-
graphical zone has no effect on the true length of incuba-

40

tion. It is stated by Chapman (65) that individuals of the
same species living in the Tropics lay fewer eggs than do
those in Northern regions, and it would be of value and help
if one knew if this difference in the number of eggs is ac-
companied by a change in the incubation length, all of which
still remains to be investigated. At the same time, it is nec-
essary to call attention to the fact that the snow bunting's
length of incubation is much out of relation to the periods
of other fringilline birds, a fact possibly due to zone, or to
errors of observation, and that Clark's crow has an incuba-
tion period which is rather long in comparison with others
of its family, or its taxonomic position. One of its con-
geners (the Canada jay) nests during the snows of winter,
and at high altitudes, too, without having an unusual length
of incubation.

Some records of incubation lengths state that the eider
ducks (137) have a shorter period than does the domesti-
cated duck in warmer climate. Such statements remain to
be substantiated, but still add force to the belief that, in the
future, incubation lengths must be studied with an eye to
eliminating such influences as slow down and prolong em-
bryonic development, and thereby distort the true incubation
period.

Heinroth (162) believes that the short periods of
Mereca penelope, Chen rossi and Dafila acuta (which he
gives as twenty- two to twenty-three days) is due to the short
northern summer; granting that these periods are correct,
why is the period of the teal in more southern latitudes no
longer ? Is the period of the Emperor penguin nearly fifty
days, because it occurs in the dark, winter months? I think
not.

Climate

Information on the possible effect of climate on the in-
cubation length is absolutely lacking, so far as the writer
has been able to determine. It is conceivable that a species
living in a moist climate might be unsuccessful in incuba-
tion, if suddenly made to live in an arid region (or vice
versa), because of its egg's structure having been adjusted
for generations to a given average humidity, which adjust-
ment fails in the new locality. While such a possibility is
extremely remote under natural conditions, a similar result
does occur with hens' eggs in artificial incubators in Colo-
rado (a semi-arid region), the writer having been given to
understand that failures to hatch in incubators, in Colorado,
are frequently due to egg dessication. It would shed some
light on this question if one knew if there were any differ-
ence in the lengths of incubation of white heron in India
and in Europe, because of the two-fold effect of differing
egg size, and climate.

41

Climate alters the size, color, shape and many other
characteristics of birds, but does it change the incubation
length ? This remains to be demonstrated. If it does, it is
probably exceedingly slow in effect, and only through
minute increments slowly accumulated, and probably always
producing alterations on the side of a shorter true length of
the incubation period.

Weather

It is said that varying weather conditions change the
incubation period, and while the writer has found in his
study of the house finch (77) that, short of actual freezing
or prolonged chilling, the weather has no influence whatso-
ever on this species' incubation period, and Macdonald's
(146) report on the incubation of the horned lark seems to
point the same way, yet Job (95) states definitely that the
incubation period " of the quail is one day longer if the
weather be very cool or wet, and Knight (105) states that
he has recognized a prolongation of two to four days in the
incubation periods of certain species of Lanius and of Geo-
thlypis because of varying weather conditions. It is possible
that prolonged droughts might also prolong, or, in fact,
render unsuccessful natural incubation because of a tendency
to egg dessication or to over-heating if the parent were
compelled to leave a nest uncovered too long. I believe that
if there be any difference in the length of incubation seem-
ingly referable to weather, that future careful study of such
effects will show that the true length is not altered, but
that the change is merely one of prolongation due to cooling.

Locality

Geographical locality has been mentioned as a cause of
differing lengths of incubation in similar, and in unrelated,
species. I know of no indubitable support for this theory.
There is no evidence that a robin's period is longer or shorter
in New York than in Georgia, or Colorado, or Canada; on
the contrary, all the facts seem to support the opposite idea
that there is no change in the period, whatever the locality.

Site of Nest

Casey Wood (104) says that the English sparrow's in-
cubation period "varies slightly between twelve and thirteen
days, depending on the weather, the locality of the nest*,
and the amount of time the bird is on the nest," a variation,
so far as the nest site is concerned, patently due to the vary-
ing temperatures produced by a well-sheltered or an exposed
nest, the sheltered nest promoting optimum conditions and
a resulting true length of incubation.

However, one writer (162) states definitely that site of

•Italics by W. H. B.

42

nest produces, not variability of the incubation period, but
differing lengths in different species. Thus, by this explana-
tion, the tree-nesting ducks and the European vulture (Vul-
ture monachus) have long incubation periods because of
their nesting places being so located as to have security from
ordinary foes.

This, however, does not explain the long incubation
periods of ground-nesting Anatidae, such as a goose or a
swan, nor can it be reconciled with the short, incubation
period of a flicker, with which species the nesting place is
reasonably free from intrusion by foes, or the short periods
of cliff-nesting swallows.

The writer believes that all of these supposed influences
on the true length of incubation discussed above really bring
about only a retardation or a suspension of embryonic de-
velopment, a suspended animation in ovo, as it were, and
that in no way do they affect the true length of the incuba-
tion period, which, when distorted, is resumed with the next
incubation or the following generation, granting the en-
vironmental conditions be normal. From the point of view
of this study, one must carefully differentiate between a
change of incubation length which is temporary, and affect-
ing only one set of eggs, and one which is permanent, and
passed on to succeeding generations.

Assuming, as one must, that chilling the eggs during
incubation does produce such effects, the question must be
interjected, "Do the eggs of all birds withstand equally well
similar degrees of chilling?" It must be considered because,
if they do not, those birds whose eggs endure successfully
the longer periods and greater degrees of cooling will ex-
hibit a greater variability in their incubation periods, the
same being a source of perplexity in studying the problem
in hand.

As has been intimated before, I believe that it will be
found by future investigations (if it has not already been
so determined) that eggs of the more primitive birds will
remain viable under adverse conditions much longer than
will those of the higher birds under like conditions ; in other

swallows' eggs would hatch if subjected to a course of treat-
ment as was given to some eggs of "shore birds" by Beebe
(114). This observer gathered a number of eggs of terns,
skimmers, gulls, green herons (all the eggs being well on
in the incubation stage) for purposes of embryologic study,
which were carried about for several days without any sus-
picion of their remaining viable; yet several hatched on
being placed in an incubator, and produced normal "chicks,"

which grew to maturity. This experience discloses but one
more of the many interesting points embraced in this prob-
lem, which are yet to be thoroughly studied and settled.

New Explanations
Body-weight: egg-white index, and incubation length.

Newton (25) says that "the size of the egg is generally,
but not at all constantly, in proportion to that of the
parent," attributing this relation to the necessity of the
parent being able to completely cover and incubate all the
eggs it lays in a set. This relation between the size of the
body and of the egg is only general. It is also true, in a
loose way, that the smaller is the bird, the smaller is its egg.
There are many notable exceptions to be found on either
side of the equation; the kiwi (168), megapods (16), some
gallinaceous birds, and the barbary duck (161) lay eggs
very large in proportion to their bodies, while with storks,
cuckoos and several other species the reverse holds true.

These facts, and my own study of the weights of birds
and their eggs, suggested to me that there might be some
relation between the ratio of these two weights and the
length of incubation of the egg. This possibility was in-
vestigated far enough to demonstrate that there is not the
least relation discernible between the two, hence it is men-
tioned here, only to be dismissed.

A New Explanation Based on Physiologic Grounds

Most of the explanations previously propounded seem
inadequate, and more or less illogical, because they have
been based on passive conditions, such as an anatomical
character (size of body), or on a histologic character (size
of egg), or have been built on effects which merely retard
or suspend embryonic development, effects which do not
alter the true or specific length of incubation. None has
been directly correlated with an active biologic or a physi-
ologic process or condition, which alone, it seems to me, can
directly modify so vital a span as is the true duration of
incubation.

Only two of these past theories have been founded on
physiologic grounds — longevity and the health of the par-
ents. The first cannot at present be said to have any bear-
ing at all on the duration of incubation, and the second is
inseparably related to another which seems to have an all-
important influence on the true length of incubation, a con-
dition now to be taken up and discussed.

It is hard to realize that a fertilized egg is, during
incubation, not an organ or a detached part of a preceding
bird, but that it is in reality a growing new individual,
and as such must respond, or be subject, to control by special

44

biologic and environmental conditions, which probably are
peculiar for each family, or perhaps for each species.

This is not an idle fancy, for it has been shown by
experimental investigations (156) on the temperature of
developing bird embryos that "the embryo of a chick must
be looked upon as a cold-blooded animal" during the early
days of incubation, and that later it shows more and more
characters of a warm-blooded animal. If this contention
be correct, then the fertilized egg, as such a new individual,
must be, in its growth, fitted, and respond, to conditions
peculiar to it and its immediate ancestors. In other words,
it must be in consonance with its environment, and react to,
and be affected by, it as in any other individual. The physi-
ology of the embryo is determined both by its heredity and
its environment. Would it not be logical to look for the fac-
tor or factors, condition or conditions, which fix the true
length of incubation amongst the environmental conditions
which make for or against the health of the developing new
bird? Would it not be wiser to seek for an answer to our
problem in the domain of the bird's physiology?

It has already been stated that, for an egg to be success-
fully incubated, it must be subject, during incubation (a) to
a correct position, (b) to an atmosphere of proper moisture
content, and (c) to a certain degree of temperature.

The attainment of a correct position for the egg during
incubation is apparently automatically brought about by
the movements of the incubating parent with all birds ex-
cept the megapods. It seems undecided whether these sin-
gular birds do or do not visit or disturb an egg after it
is laid.

The proper amount of moisture necessary for the preser-
vation of the embryo is probably attained by selection,
through a long process of adaptation to the surrounding
prevailing humidity; it would be valuable to know if eggs
of individuals of the same species, incubating in humid and
in arid regions, exhibit different egg shell structure in order
to compensate for humidity differences in such antipodal
regions. It is possible that in natural incubation the mois-
ture emanating from the, incubating parent's body, or from
the soil under a ground-nesting species, also may be a factor,
and help combat undue dessication during incubation. It
is said that the Egyptian plover (Pluvianus aegyptius),
whose eggs are partly incubated by the direct heat of the
sun, dampens its egg by contact with its previously water-
soaked feathers.

The first of these two requisites seems to be one of pure
physics only, and the second one largely of physics, with
perhaps an added element of physiology.

How important is the first in natural incubation I am

45

unable to state ; nevertheless, it seems important in artificial
conditions, since most poultry raisers insist that with hens'
eggs in artificial incubation it is very essential. The second
is obviously a sine qua non, yet being largely one of physics,
it does not seem in reality to affect the length of incubation,
but rather the life or death of the embryo. Hence, since
neither of the first two essentials for successful incubation
is one of a bird's physiology, it remains to take up the last,
and since the heat applied to eggs in natural incubation
emanates from the setting bird, it would appear that the
factor of the degree of temperature applied to the eggs is
one of pure physiology, a question of the production and
application of animal heat. Moreover, it can be said, with-
out fear of contradiction, that temperature is the most im-
portant of all the three factors (or conditions) just enumer-
ated. Now, inasmuch as the incubation heat comes from
the brooding parent (true or foster), excepting with the
megapods and (possibly for a part of daylight hours) with
ostriches and the Egyptian plover, it would appear that the
bodily temperature of .the incubation parent should be a
highly important factor in relation to the incubation period.
In brief, it seems to me that birds' temperatures should be
investigated, not only as such, but also as to any relation
they may bear to the incubation period, and also any other
facts cognate with birds' temperatures.

If a bird's temperature be highly necessary to successful
incubation, it would seem reasonable to predict that birds
have acquired habits, and conditions of body exhibited only
during incubation, which are calculated to facilitate the
application of, and conserve, the heat applied to the eggs
during this period.

A superficial consideration of birds' nests throws some
light on this question. While a goodly number of nests are
too flimsy, or in too close contact, apparently, with the earth
to aid in concentrating the heat applied to the eggs, never-
theless a majority of nests are so constructed as to retain
this heat most advantageously. It is obvious, however, that
nests may have been developed, and probably did arise, as
a protection to, and receptacle for, the eggs, yet the almost
innumerable instances where the nest materials are ideal as
insulating media show that out of this primary use of a nest
has grown the concomitant result of the conservation of the
all-important heat applied during incubation.

Reciprocally there are conditions normal to the bird, or
found only during the incubation time, which lend them-
selves to the application of a maximum amount of the par-
ent's heat to its egg, and to holding this heat at the most
advantageous level. Lucas (10) has maintained that the
majority of birds have no feathers on the abdominal area

46

in order to more successfully incubate their eggs, saying
"the bare spaces of the body of a bird are adaptive * * *
(the) belly (is) bare in most birds because of incubation,
and in ducks, penguins and auks becomes bare during incu-
bation." Pycraft draws attention to a further step in this
process, and says that the bare abdominal area becomes
seemingly inflamed, with its blood vessels more distinct than
normal, a condition not likely to be a true inflammation,
but a functional hyperaemia only, resulting in a more plenti-
ful and more frequently renewed blood supply to the parts,
in the end maintaining more easily the optimum incubation
temperature.

Before entering upon a larger consideration of bird
temperatures and their relation to the length of incubation,
it seems desirable to consider briefly a few facts concerning
the effects of heat applied to viable eggs.

There is a small amount of development in a fertile egg
before it leaves the body of the parent, but it is very slight,
and never reaches any advanced stage as is the case with
many near relatives of birds, the reptiles, with which ani-
mals incubation of the egg frequently goes on to completion
within the body of the female, and the young are born alive,
resulting in the fact that while many reptiles are ovovi-
parous, birds are never anything but oviparous. The slight
development in an egg which starts and goes on while it is
still within the female's body continues for a while at a
very slow rate after the egg is extruded, even under com-
paratively low temperatures, viz., 86° F. (38). Prolonged
or excessive chilling or over-heating promptly kills the
embryo. And both of these effects are persistently avoided
by incubating parents; thus ducks, etc., cover their eggs
with down or feathers when leaving the nest in cold weather,
and the ostrich shades its eggs with its body and wings from
the sun's excessive heat (160).

It is demonstrated that there is an optimum incubation
temperature, and it is almost demonstrated that each species
of domesticated bird has an optimum of its own, an opti-
mum which hatches the eggs in the shortest time possible,
and which probably varies little, if at all, with each species.

Nevertheless, it is extraordinary how, for example, a
hen's egg will hatch under what seem anything but optimum
conditions; thus, such eggs have been known to hatch in a
barn-yard manure pile (33).

H. Milne-Edwards gives the optimum (without desig-
nating for what species) as 104° F. Various other figures
have been given for this incubation optimum, a conflict
arising, most likely, because the optimum differs with differ-
ing species.

47

There is support to the further idea that the optimum
may vary according to the stage of the incubation. Job (95)
says that for ducks (sp?) the best temperature during most
of the incubation is 103° F., and slightly before or at hatch-
ing it -is 104° F. One might predict this. It is a fact that
with many birds, as the incubation period nears its close,
the bird's belly becomes more and more bare, permitting a
closer and closer contact of parent to eggs, a condition
facilitating heating of the eggs; and, too, that towards the
close of incubation the eggs themselves, especially in a large
set of eggs, produce and disseminate heat (160) ; these two
facts forming a combination calculated to subject the eggs
to a gradually rising temperature as the incubation nears
completion.

Furthermore, on the basis of what is known about avian
(embryonic and post-embryonic) temperatures, this gradual
rise of the incubation temperature would square with what
goes on within the egg as it is incubating. I am not doing
violence to the facts, nor yet making an overdraft on the
imagination, in believing that a new bird, in its embryonic
development, climbs up a series of evolutionary levels, from
low to high, and that under these circumstances one would
expect to find lower temperatures more fitting to the early
parts of the incubation period, and higher temperatures in
the later portions of the period, differences which may be
slight, yet none the less significant. The optimum incubat-
ing temperature (in artificial incubation) for the domestic
hen is given as 102° F. (early) and 103° F. (late) (33) ;
for ducks (sp?) for the first three weeks as 102° F., and the
last week 103° F. (34) ; for the ostrich 101° F (160), and
for the rhea (Rhea Americana) as 103° F. (13).

There is a large field for research in this question of
temperatures of artificial incubation; there is also an en-
gaging and unexploited field for investigation in the tem-
perature conditions of the nest in natural incubation; special
thermometers have been constructed to register this tem-
perature, but the data are too few to require special notice
here. This method of study might be undertaken with the
aid of the ordinary clinical thermometer, and it remains
to, and should, be vigorously prosecuted.

While it is known that there is a slight upward swing
of the optimum temperature towards the end of artificial
incubation, it has not yet been demonstrated in natural con-
ditions. Still so much points to the high probability of its
existence also in natural incubation that one can safely
accept it as tentatively demonstrated.

If there be an optimum incubation temperature, which
varies with the species and, also, according to the degree
of embryonic development in the egg, it would seem safe

to predict that differing bird species should exhibit, under
normal conditions of health, differing body temperatures,
and perhaps, in a given individual, this normal temperature
should be found to vary according to the stage of incuba-
tion, since the parent's body heat is that which develops its
embryo in practically all of the myriads hatched each year.

It then becomes self-evident that the optimum incuba-
tion temperature for any species is the temperature of the
incubating parent (true or foster).

The physiology of a bird's temperature is not nearly
so well known as that of man and other warm-blooded ani-
mals, but there is, nevertheless, enough information on the
subject to enable one to get a fairly comprehensive view of
its physiologic characteristics.

Birds are homoiothermic ; they have in health a body
temperature which is relatively characteristic and of a con-
stant curve, one peculiar to the family, the genus, or pos-
sibly even to the species. This temperature, with all species
so far studied, has a daily swing, being highest, with diurnal
birds, between noon and six in the evening, and lowest be-
tween midnight and six in the morning, these extremes
being reversed with nocturnal birds (167). The amplitude
of this daily temperature swing varies with different species,
and seems to be correlated more or less closely with the
bird's size, since Simpson (167) found it to be widest with
small birds and narrowest in large birds, there being 7.68° F.
between the extremes with a thrush, and 1.65° F. for a duck,
and with birds of intermediate size this swing was inter-
mediate in amplitude. This daily swing of body tempera-
ture corresponds fairly closely with the bird's activity, being
lowest when it is at rest, i. <?.* the hen's temperature is lower
while incubating than when active; it remains nearly con-
stant during most of the period of incubation, and rises only
when the hen becomes more active and "excited" at the
hatching of the eggs. It is said (164), however, that the
temperature of the nest is lowest in the first week of incu-
bation and highest at the end, a change possibly not due to
the hen's temperature alone, but also to the eggs themselves
producing heat as the embryos develop. This twenty-four
hour rise and fall of temperature also obtains with man,
and is also slightly correlated with his condition of activity
or rest; the curve can be reversed if his daily mode of life
be reversed, a fact throwing light on the reversed curve in
nocturnal birds. There is some evidence at hand showing
that the female carries a higher temperature than the male
(165-167), since with cormorants, guillemots, razorbills and
ducks the females have the higher temperature, the excess
varying between .07° F. and .5° F. This difference may
seem negligible, but it must be noted and borne in mind

49

when investigating this subject. It has been, determined
that a hen's temperature varies slightly with the season,
highest in the summer, and nearly, or exactly, identical in
May and October. I am inclined to believe that there is a
tendency to an increase of temperature during the mating
season, since I have found the cock ring-neck pheasant's
temperature much higher than the female's at this time of
the bird's physiological year, and Pickrill (141) says that
there is an increase of about 2° F. at such times with both
sexes of the ostrich. Ingestion of food and musciilar activ-
ity elevates (slightly) the temperature in man (156) and
seems to do so also in birds. The support to this latter
statement is indirect only. A setting hen's temperature
(166) is lower than that of an active "control" hen, and the
"control's" temperature is lower at night, as shown if taken
when the bird is gently lifted at night from the perch. An
under- fed or a starved hen (165) has a lower temperature
than the normal control bird ; this difference borders closely,
however, on the domain of pathology, into which it is in-
expedient here to enter.

Birds have bodily temperatures which are only slightly
affected (156) by climate. Individuals of the same species
in the Arctic region have the same temperature as those
in temperate zones, showing the wronderful control of body
temperature by the heat centre of the central nervous sys-
tem, and nothing shows more vividly the power of this con-
trol than the experimental demonstration that a sparrow,
with its feathers all clipped off, maintains its body heat,
under ordinary temperatures, quite easily at normal.

The age of a bird has no relation to its body tempera-
ture, except that altrical birds do not acquire a stable and
normal temperature curve until ready to leave the nest, or
at least until muscular co-ordination is nearly or quite per-
fected. Precocious birds seem to have a perfectly function-
ating temperature controlling centre at hatching, while con-
cerning the age and temperature, it may also be said that
ducks varying in age from four to twenty-four months
exhibited temperatures identical with those of older birds
(165). The rectal temperature of a hen, taken immediately
after it has laid an egg, is said to be 2° F. higher than nor-
mal (164). I am unable to say that all these peculiarities
of a hen's temperature apply also to all other birds, but until
shown otherwise, it is necessary to hold that they do. The
normal temperature of man is assumed to be the average
of the highest reading of the twenty-four hours, utilizing
as large a series of observations as possible, and all of normal
adults. Comparative physiology demands the same stand-
ard in other homoiothermic animals, having, of course, due
regard for individual peculiarities (i. e., nocturnal animals

50

and birds). Therefore, in this discussion, a bird's normal
temperature will be taken to mean, not the mean of the
records of twenty-four hours, but the highest in that period,
or the average of such highest records; hence, the most desir-
able bird temperature records are those taken between noon
and six in the evening, accompanied by notes as to the bird's
sex, age, method of securing the bird, and of other modify-
ing factors, some of which have just been outlined.

The way of securing a bird to take its temperature
varies, and it may have considerable influence on the result.
Sea birds were caught with a baited hook and line for such
purposes, nearly a century ago (165). I have found, using
trapped English sparrows, that the rectal temperature of
these birds is the same before and directly after being shot,
and Simpson (165), in studying other species, antedated me
in this conclusion more than thirteen years. This conclusion
is also true of flickers, since a male's temperature, directly
after being shot, was 106.6° F., while that of a female, taken
alive (and afterwards liberated), was 106.4° F.

This method of securing animal temperatures is approx-
imately accurate, and is substantiated by similar methods
with mammals (156) ; in all cases, care must be taken to note
if the specimen bleed profusely (becomes exsanguinated),
in which case there would be a SAvift descent of the tem-
perature, or if the brain be extensively damaged, in which
event it is possible that the temperature might be, for a
short time, abnormally high. Whatever the method of se-
curing a bird, a standard self-registering clinical thermom-
eter should be used, inserting it into the bowel a half -inch
or more, according to the bird's size, and held in place until
the mercury ceases to rise.

It seems self-evident that the ideal time to secure bird
temperature records would be while a bird is incubating, but
this, at present, would be difficult to do to any extent large
enough to be useful, and the next best time to take birds'
temperatures would be while they they were breeding or in
the brooding period. For the purposes of this study, any
and all recorded bird temperatures must be utilized, even
if all have been taken with little or no regard to their, bear-
ing on the length of incubation or to their relation to the
peculiarities of the daily temperature curve. The number
of published temperature records of birds is not large: aside
from some records scattered through the literature of human
and comparative physiology, and a few determined by my-
self, there are two principal sources of information on this
subject, the first being found in a list given by H. Milne-
Edwards (38), in 1863, and the second, a brief but highly
suggestive study published by Sutherland in 1899 (112).

51

Table No. 6 gives all the above mentioned records, plus a

few scattered in other publications, some given to me by
obliging friends, and a few determined by myself.

TABLE NO. 6
Bird Temperatures in Degrees Fahrenheit

Weight Authority

Ostrich, laying season 9 102.0 141

Ostrich, non-laying 100.0 141

Ostrich, breeding $ 104.0 141

Ostrich, average of five individuals 99.2 156

Ostrich, highest of five individuals 100.0 156

Emu 102.2 112

Emu avg. 103.1 165

Cassowary 102.56 112

Tinamou (Spotted) avg. 104.06 112

Tinamou (Rufous) 105.44 112

Apteryx (MantelPs) 99.32 112

Apteryx (Haast's) $ 100.22 112

Penguin (Eudyptula minor undina), Alive

avg. 6, 101.3 153

Penguin (Species?) 102.2 10

Penguin (Adelie) 102.5 10

Horned Grebe (Colymbus auritus) 105.26 165

Albatross (Sp?) (Taken in 1836 and 1837)

mean 103.82 165

Albatross (Sp?) 104.09 38

Diomedia (exulans or chlorhynchos ? )

mean of nine 105.27 165

Grand Albatross (taken in 1836 and 1837)

mean 102.92 165

Petit Albatross (taken in 1836 and 1837)

mean 106.16 165

Petrel (Sp?) 103.1 38

Grand Petrel Noir (taken in 1836 and 1837)

mean 103.46 165

Procellaria gracilis .• 101.66 165

Procellaria caperisis 103.5 165

Mutton Bird (Neonectris tenuirostris brevi-

caudis), alive avg. 7 juv.-100.2 153

Procellaria pelagica (Storm Petrel) 103.64 165

Petrel gris (taken in 1836 and 1837) ... .mean 103.28 165

Petrel damier (taken in 1836 and 1837) mean 105.26 165

Cormoran (Sp?) 106.16 38

Phalacrocorax carbo (Cormorant) mean 103.5 165

Phalacrocorax graculus (Shag) mean 106.4 165

Sula bassana (Gannet) mean 106.6 165

Black-Crown Night Heron $ May 2, directly

after being shot 102.3 182

Weight Authority

Heron (Sp?) 105.8 156

Heron (Sp?) 105.8 38

Merganser (Sp?) mean 106.75 165

Duck (Sp ? ) (24 individuals) avg. 107.8 156

Mallard Duck (tame, three removes from

wild) $ mean 106.7 165

Mallard Duck (tame, three removes from

wild) 9 : mean 106.88 165

Domestic Duck $ avg. maximum 107.24 167

Domestic Duck 9 avg. maximum 109.4 167

Domestic Duck $ avg. of fifty 107.5 165

Domestic Duck 9 avg. of sixty 108.0 165

Canard commune 109.58 38

Duck (Sp?) , 105.08 10

Anas rubripes (Black Duck) mean 106.34 165

Aix sponsa (Wood Duck) 107.6 165

Canard Millonin (Pochard?) 108.68 38

Marila affinis mean 106.2 165

Eider Duck 108.32 38

Oedemia nigra mean 106.34 165

Clangula clangula americana 104.72 165

Goose (Sp?) five individuals avg. 107.0 156

Goose, Domestic 106.7 147

Oie commune 106.7 38

Oie rieuse (Cackling Goose?) 109.4 38

Cygne a bee rouge 105.78 38

Prairie Falcon $ in December, immediately

after being shot 106.6 78

Faucon 104.9 38

Hawk (Kestrel?) avg. max. 108.32 167

Hawk (European Sparrow?) avg. max. 107.3 167

Tiercelet 106.52 38

Autour 109.58 38

Western Red-tail Hawk, in August, alive 106.2 78

Swainson's Hawk 9 alive, April 24 106.6 78

American Rough-leg Hawk $ in December,

after being shot 105.8 78

Osfraie 104.36 38

Gypaete 105.8

Globose Curassow 9 alive, in captivity. avg. 4-106.4 169

Globose Curassow $ alive, in captivity . avg. 3-106.4 169

Gellinotte (Sp?) 108.95 38

Lagopede (Sp?) 106.88 38

Natal Francolin (Francolin natalensis), in

captivity, 3 :30 p. m 107.9 97

Willow Grouse $ (taken in Arctic regions) . 109.04 156

Prairie Fowl 9 109.4 156

Prairie Fowl $ 109.76 156

53

Weight Authority

Turkey 109.0 156

Dindon 108.86 38

Guinea Fowl 110.0 156

Pintade 111.02 38

Pheasant (Sp?) 108.7 156

Ring-neck Pheasant $ avg. of two 107.5 109

Ring-neck Pheasant 9 106.0 108

Domestic Fowl 9 avg. max. 107.3 167

Domestic Fowl avg. of 111 individuals 106.9 156

Domestic Fowl $ 107.6 147

Domestic Fowl 9 avg. 106.55 38

Domestic Fowl 9 at large avg. of three 104.6 109

Domestic Fowl, laying, and at large 106.3 164

Domestic Fowl (Leghorn), laying hens. avg. 3 107.7 153
Domestic Fowl (Leghorn), laying hens,

after exercise 108.4 153

Domestic Fowl, lifted from perch at night. . . 105.08 112

Domestic Fowl 9 setting avg. of two 103.6 109

Domestic Fowl, "setting, average of many". . 105.6 108

Domestic Fowl, brooding by day 107.06 112

Domestic Fowl, setting 106.7 164

Domestic Fowl $ avg. max. 107.5 167

Coq avg. 103.46 38

Bantam 9 avg. max. 107.8 167

Bantam, Leghorn, setting 104.0 109

Bantam $ avg. max. 107.39 167

Paon 107.15 38

Foulque 104.9 38

Sun Bittern (Europygia helias) in captiv-
ity, 3 :30 P. M 102.4 97

Pluvier 104.9 38

Hooded Dotterel— "One minute dead" 107.0 153

Killdeer 9 June, taken immediately after

being shot 106.6 78

Barge 107.98 38

Chiornis minor (Sheathbill), taken in 1836

and 1837 104.0 165

Stercoraire Pomer 104.54 38

Le Cordonnier (Megalestris skua?) mean 104.2 165

Rissa tridactyla mean 106.6 165

Mouette tridactyla 105.26 38

Mouette blanche 104.18 38

Larus argentatus 108.32 165

Goeland argente 108.14 38

Larus canus mean 107.1 165

Common Sea-gull, juv avg. max. 106.7 167

Larus fuscus mean 106.9 165

Goeland a manteau gris 105.26 38

54

Weight Authority

Mouette gris 106.52 38

Gull (Sp?) 100.4 139

Uria grylle mean 105.5 165

Uria troile mean 104.5 165

Alca torda mean 104.9 165

Guillemots 104.9 38

Pigeon, Domestic avg. 107.87 38

Pigeon, Domestic 107.6 147

Pigeon, Domestic 108.0 156

Pigeon. Domestic avg. 105.6 156

Pigeon, Domestic 107.24 156

Pigeon, Domestic $ 107.45 167

Pigeon, Domestic 9 brooding, noon 104.0 78

Pigeon, Domestic $ avg. max. 106.75 167

Speckled Pigeon (Columbia phasonota), in

captivity. 3 :30 P.M 110.+ 97

Mourning Dove, in captivity, with crippled »

wing, noon, Sept 106.2 78

Ruddy Quail (Geotrygon Montana) 110.+ 97

Road-runner $ taken 11 A. M. in October,

immediately after being shot 107.4 78

White-crested Turaco (Turacus corythaix),

in captivity, 3 :30 P. M 104.2 97

Ara oratrix, in captivity, noon, Sept 102.6 78

Parroquet 105.98 38

Para Motmot (Motmotus paraensis) in cap-
tivity, 3 :30 P. M 104.1 97

Barn Owl avg. max. 103.46 167

Long-eared Owl $ directly after being shot,

1 P. M.. May 2 104.2 182

Long-eared Owl, taken alive, June, 2 P. M. . . 103.3 78
Long-eared Owl $ directly after being

shot, 1 P. M., May 2 ! 103.4 182

Owl (Tawney ? ) avg. max. 105.08 167

Owl (Tawney?) avg. max. 106.0 167

Saw-whet Owl, in captivity, 3:30 P. M 104.3 97

Chat-huant 105.98 38

Owl (Horned?) avg. max. 105.98 167

Owl (Horned ? ) avg. max. 106.0 167

Choutte 106.52 38

Western Night-hawk, taken in daytime,

July, after being shot \ 104.2 78

Swift (European?) 111.2 156

Western Hairy Woodpecker $ 3 P. M.,

August, after being shot 105.5 78

Downey Woodpecker, in captivity, 3:30

P. M 108.3 97

55

Weight Authority

Williamson's Sapsucker 9 alive, 4 P. M.,

April 24 108.2 78

Red-headed Woodpecker 9 5 P. M., July,

after being shot 107.2 78

Ant-eating Woodpecker $ 3 P. M., Oc-
tober, after being shot 106.6 78

Lewis Woodpecker $ 4 P. M., August,

after being shot 107.0 78

Western Flicker $ 5 P. M., August, after

being shot 106.6 78

Western Flicker 9 taken alive Sept., 3

P. M 106.4 78

Passerine birds 107.6 to 111.2 112

Hammond's Flvcatcher $ noon, August,

after being shot 106.2 78

Grieve commune 109.4 38

Thrush (Sp?) avg. max. 108.86 167

Thrush (Sp?) 109*.0 156

Fieldfare 110.6 156

Redwing 109.9 156

Song Thrush avg. max. 108.2 167

Catbird, 2 P. M., in June, immediately

after being shot '. . . 106.4 78

Curved-billed Thrasher $ 3 P. M., in Oc-
tober, directly after shot 107.4 78

Water Ousle 9 3 P. M., in July, directly

after shot 106.4 78

Swallows (Sp?) 111.2 139

Bohemian Waxwing 9 alive, after strug-
gling, 5 P. M., March 5 107.2 78

Bohemian Waxwing 9 directly after being
shot, 5 :30 P. M 106.2 78

Bohemian Waxwing 9 directly after being

shot, 5:00 P. M 107.8 . 78

Bohemian Waxwing $ directly after being

shot, 5 :00 P. M 108.0 78

Warblers (Sp?) 109.4 10

Audubon's Warbler, 6 P. M., May 7 108.6 182

White-rumped Shrike 9 4 P. M., August,

directly after shot 108.4 78

Rocky Mountain Nuthatch 9 1 P. M., Au-
gust, directly after shot 107.0 78

Great Titmouse : 111.2 156

Greater Bird of Paradise (Paradisea

apoda) in captivity, 3 P. M 106.7 97

Magpie, juv., alive, 4 P. M., July . . avg. of five 106.5 78

Long-crested Jay, noon, July, directly after

shot 108.4 78

56

Weight Authority

Arizona Jay $ 4 P. M., October, directly

after being shot 109.0 78

Sooty Jay (Psilorhinus morio fuliginosa),

in captivity, 3 :30 P. M 110.+ 97

Jackdaw $ avg. max. 108.86 167

Jackdaw $ avg. max. 108.4 167

Choncas 107.78 38

Corbeau (Raven or Crow?) 109.22 38

Starlings avg. max. 109.25 167

Rocky Mountain Creeper $ 3 P. M., July,

directly after shot 108.2 78

Thick-bill Red- winged Blackbird $ 5P.M.,

directly after shot 109.0 78

Moirieau max. 112.1 38

Moineau min. 105.8 38

Western Meadow Lark $ directly after be-
ing shot, March 4, noon 107.2 175

Sparrows (Sp ? ) avg. 109.0 10

Bouvieuil 107.96 38

House Finch $ 5 P. M., alive 108.3 78

House Finch $ directly after being shot,

March 4, 10:30 A. M.' 106.0 175

Pine Siskin $ 2 P. M., July, directly after

shot 106.3 78

Sparrow (Sp?, probably P. domesticus) 107.8 156

English Sparrow, avg. 11, alive, 4 P. M., July 108.3 78

English Sparrow 9 avg. 6, alive, August 108.0 78

English Sparrow $ avg. 2, alive 108.1 78

English Sparrow $ June llth, directly

after being shot 109.4 78

English Sparrow $ alive, January 26th 109.0 78

English Sparrow $ alive, August * 107.8 78

Chestnut-collared Longspur $ October, di-
rectly after shot 109.6 78

Western Vesper Sparrow, 4 P. M., October,

directly after shot 109.0 78

Western Vesper Sparrow $ directly after

shot, April 24, 5 P. M 108.6 78

Bruant de niege 109.67 38

Shufeldt's Junco $ directly after being

shot, noon, January 107.50 175

Cassin's Sparrow $ 2 P. M., July, directly

after shot 108.0 78

Spurred Towhee $ 1 P. M., July, directly

after shot 108.0 78

Lark Bunting, in captivity, 3 P. M 110.+ 97

Bruant 109.04 38

Yellowhammer 109.8 156

57

There are several conflicts in the published records on
bird temperatures which are probably due to differences
brought about by season, sex, struggling, and, too, in the
early records, by imperfect instruments, for the modern ac-
curate self-registering clinical thermometer was unknown to
Milne-Edwards and his predecessors, its substitute being a
crude affair, and such as it was, only just beginning to be
used in physiological and clinical investigations.

Sutherland (112) concluded from his study of bird tem-
peratures that "the result seems to show that the higher the
bird in the zoological scale, the higher in general is the
temperature of the blood"; in other words, as birds have
risen in the zoological scale, their temperatures have become
elevated pari passu. In general, this conclusion is substan-
tiated by Table No. 6. If the writer is not in error, what
occurs in the whole class Aves is also found more or less
within the orders and families of the class — i. e., differences
of "highness," both in taxonomy and of temperatures, in
orders and in families ; if this be true, a steadily rising curve
of the temperatures found in the class Aves would not only
be unexpected but it would be suspicious.

What one would anticipate under these conditions is a
more or less steadily rising curve, subject to undulations
which are brought about by differences of temperature in
the families of the orders involved; this anticipation is real-
ized to a reasonable degree by the temperatures in Table No.
6, which show a distinct tendency to rise with the species.
and be subject to undulation when the linear classification
jumps from family to family. It seems to me that there is
more than chance in the parallelism between the rising body
temperature and the bird's elevation, despite the scanty
data, and despite the probable errors in both the data and
the classification.

Let us see if any other writers have been convinced of
this relation; Pembrey (156) says, "those animals which
are higher in the scale of evolution, such as birds and mam-
mals, have a high temperature, which is fairly constant and
independent of the temperature of the surrounding air."

Simpson (165) seemed surprised to find that different
families of the same order exhibited different temperatures,
saying, "even families of the same order appear to differ
considerably in body temperature," which, rather than a
surprising thing, is what ought to occur if Sutherland's law
be correct and apply to orders and families as it seems to
apply to the class. Simpson appears to have been familiar
with Sutherland's conclusion, and after denying that there
is a relation between the highness of a mammal and the ele-
vation of its temperature, he says that "the same appears
to be the case amongst Class Aves, above the Ratitae." I take

it that he based this adverse conclusion on his own work,
which included the "Turbinares, Staganopodes, Pygopodes,
and Longipennes." a group of families forming but a small
per cent, of the Class, and also taking in birds largely of
the lower levels. Furthermore, he based his conclusions
(seemingly) on the mean of the temperatures of the species
in the family, which seems untenable, if it be true (as he
points out) that there are differences of temperature
amongst the families of the orders, and the more untenable
if the species within the family also show different tempera-
tures. Simpson also based his conclusions on the average
temperatures of the twenty-four-hour period, which I feel
is not correct; unless I am much mistaken, man's normal
temperature is regarded as the average of the highest in
twenty-four hours, i. e., that taken between noon and six in
the afternoon, and such is the method I have utilized, when
possible, in my consideration of this phase of this study. I
am strongly of the opinion that graduations of temperature
occur primarily in the class, secondarily in the order, and
again in the family ; what occurs in the whole avian tree is
repeated in the secondary, tertiary, and even smaller
branches.

Seeking to confirm these ideas on grounds other than
those of these meagre records, one may justifiably ask, are
there any other facts (or reasons) which can point to, or
explain, the assumed co-existence of "high temperatures"
in "high birds," and "low temperatures" in "low birds?"

It is not possible here to go extensively into the question
of the physiology of animal heat, but it can be examined
briefly at one or two points where it applies to the ideas now
in hand. It has been shown (148) that, with all mammals
investigated up to date, each animal can go through only a
definite and fixed number of metabolic changes during its
period of post -embryonic development, a contention based
on the fact that all such mammals consume equal total num-
bers of calories per kilo of weight, from birth to maturity;
there are further indications that this ratio may also obtain
through the whole period of life. These facts mean that
the large mammal, which in general lives longer than the
smaller one, goes through its chain of metabolic changes
slowly, taking a long time to do so. while a small mammal
does the reverse, using up its metabolic-change quota (which
is identical in both mammals) swiftly. The large mammal
lives long and slowly, the small one briefly and swiftly, a
difference in "swiftness of life." What is known as to a
correlation between size and longevity in mammals tends to
confirm this theory of the "swiftness of life," for in the
scale of size represented by the elephant, camel, dog, and
mouse, variations in size and longevity are found to go hand

59

in hand. Another way to put this view of the swiftness of
life is that the rapidity of growth from birth to maturity
is proportionate to the metabolic intensity, which last is, as
said before, directly related to size ; the elephant takes years
to mature, the mouse can breed at the age of a few weeks.
This difference in the speed of life process in large and
smaller animals is not based on theory, but is demonstrable
by experimental methods (156), all of which show that the
metabolic intensity is proportionate to the animal surface
area, which is relatively larger in the smaller mammals
(148). Hand in hand wjth this skwness or swiftness of
metabolic processes are found corresponding differences in
the animal's physiology, particularly in the respiration,
heart rate, temperature, and possibly in the embryonic metab-
olism. There are some indications at hand suggesting that
this relation of swiftness of life and body size prevails in
the embryonic, as well as in the post-embryonic stages of
existence. There is suggestive evidence that the larger the
mammal the longer is its gestation period; with the mouse
it is three weeks and with the elephant at least eighteen
months, and animals intermediate in size show a fairly well-
defined intergrading in the gestation period length. Pem-
brey's remark that "the temperature of the smaller mam-
mals and birds is often higher than that of the biggest"
(156) gives a logical introduction to the question. Does this
relation of size and swiftness of life in mammals obtain also
with birds? Simpson (166) has definitely answered it in
the affirmative, so far as the hen is concerned, for he stated
that the large and most lethargic birds (i. e., of hens) had
a much lower temperature than the smaller and more active
ones. I am convinced that a thorough study of avian physi-
ology will show the same variation in the swiftness of life
in this class as is demonstrated in mammals. It seems quite
likely that increasingly intense metabolism and steady dimi-
nution in size of birds are related, and that the intense metab-
olism finds expression in many other ways in the function
of birds. For example, I have found a house wren's respira-
tion to be 160 per minute, and that of a house finch to be 100.
Rapid respiration and fast heart rate are always (in health)
co-existent; if one take the respiration: pulse rate index of
man, ?'. e., one to four, and apply it to these two species, it
would show the wren to have a pulse of 640, and the finch,
one of 400 beats per minute. Since there seems little known
about a bird's minute physiology, it might be hazardous to
assume that the human index can be correctly applied to
birds. However, there can be no question as to the amazing
rate of the heart beat in birds; if one hold. an English spar-
row or any small bird loosely but securely and gently in the
hand, and apply it closely to one's ear, the bird's heart, as it

beats, sounds like the rapid tick of a small watch*. All
these observations merely point to the exceedingly intense
metabolism and swiftness of physiologic processes in such
small birds, a combination of conditions that should produce
high bod}7 temperatures in small birds, and a brief study of
Table No. 6, shows clearly that the birds with highest tem-
peratures are, as a rule, the smallest of birds; moreover, it
is held by physiologists (151) that the power to produce heat
is proportionate to the activity or sluggishness of the animal
(not specifying class). These facts again lead to a belief in
the general correctness of Sutherland's law as to low and
high temperatures in "low and high" birds. There are some
relatively small birds which haA^e low temperatures, as, for
example, the apteryx. It seems to me that this is a good ex-
ample where it is not size but the primitive character of the
bird which determines the elevation of its temperature, but
this is not proven; it may be a true exception, or the low
record may be due to errors of observation. It is to be no-
ticed that one record of a pigeon, the speckled pigeon, shows
a very high temperature for a bird classed as relatively low ;
the whole of the facts in this, and similar cases, are not
known, and compel again one's suspending final judgment
pending further light.

In further support of the relation between elevation of
temperature and taxonomic standing, it may be recalled that
as birds have grown up, and away, from their proto-avian,
or proto-reptilian ancestors, they have become better and
better feathered, and feathers are said to have made birds
what they are, the warmest-blooded creatures in existence,
whence it follows that the farther they have traveled (with-
out later recession) from their primitive ancestry, the more
elevated have become their body temperatures. It seems to
me that weight of evidence supports Sutherland's hypoth-
esis, and for the purposes of this discussion it is held to be
true.

Now, if one holds that birds' temperatures are more
and more elevated as the species is higher and higher in its
phylogeny, one may ask, does this condition have any influ-
ence on the length of incubation? It must here be remem-
bered that it is possible that "swiftness of life" may embrace
the embryonic period of birds as it seems to with mammals,
and that the specific temperature and the incubation length
are co-ordinate, assuming this to be true for the moment

'Since the above was written I have learned that Buchanan (174)
has determined the heart rate in several bird species, using the electro-
cardiagram method for its detection. His results are almost incredible,
and show most strikingly the wonderful metabolic activity of birds,
especially the small species; Buchanan gives the heart rates as follows:
Gold-finch 900 to 925 per minute, green-finch 700 to 848, sparrow 745 to
850, pigeon 141 to 225, hen 304 to 345.

61

on hypothetical grounds alone. Thus, this relation of metab-
olic intensity, size, and length of incubation may account
for the loose relation known to exist between a bird's size
and its incubation length, a relation admitted in the discus-
sion on that theory. It seems to me that there is direct
experimental evidence of the effect of differing temperatures
on the incubation length ; it has been shown that an optimum
temperature is the most important of the three factors
necessary to successful incubation, and that this optimum
emanates from the incubating parent, and that with hens'
eggs, their usual minimum duration of incubation can be
shortened a few hours by carefully raising the temperature
of the incubator a little above the usual optimum, which
is identical under the hen and in incubator practice.

Until evidence is forthcoming to show that these two
conditions do not apply to all other birds' eggs, it seems
tenable to believe that as birds have slowly risen in evolu-
tionary height, their temperatures have also been corre-
spondingly elevated, and that these increasingly higher tem-
peratures have gradually shortened the periods of incu-
bation. What occurs with a hen's egg during one incubation
period through a slight elevation of the incubator tempera-
ture, has taken place in lesser degrees, in nature, for count-
less bird generations: each increment of temperature eleva-
tion, however slight, added by each succeeding ascending
generation, has correspondingly influenced the length of in-
cubation, always resulting in some shortening. This process
has been, probably, exceedingly slow, consuming ages in
accumulating a patent change, both in the body temperature
and in the incubation period yet, in the end, as we see it
today, it has swept on with striking results, the Passeres
having forged ahead, through their very high temperatures,
to a fourteen-day period, while the ostrich and other birds
with low temperatures are marooned at forty-two (or more)
days. From the foregoing it seems, to me, impossible to
escape the conviction that the true length of incubation is
-fixed or determined by the temperature of the incubating
parent, long with low temperatures, and brief with high
temperatures. Inasmuch as there is a goodly amount of
support to Sutherland's law of low temperatures with
"low" birds and higher temperatures with "high" birds, I
would tentatively enunciate the idea that the true length of
incubation is determined by the bird's position in the avian
scale of life, basing this hypothetical law on the assumed
relation of temperature elevation to incubation length, and
of temperature elevation to the species' position in avian
taxonomy.

If the various data collected together in this study are
now to be examined and interpreted from the viewpoints

62

just enunciated, it becomes necessary to arrange the species
affected, according to some reasonably acceptable classifica-
tion. It was with hesitation that I approached this question
of classification, and when the data finally had to be classi-
fied, I wrote to several of my professional ornithological
friends, requesting their opinion as to the best present avian
classification. All, as a unit, expressed themselves virtually
in the words of one who said, "There is no best classifica-
tion," saying further that the arrangement of avian tax-
onomy as given by Gadow, and slightly modified by Knowl-
ton (10), was as good as any, which is the one followed in
listing the incubation periods, and in the secondary tables
and lists given in this study.

In speaking of a species "lowness" or "highness," I do
not overlook the many difficulties attendant upon a deter-
mination of these levels. What I really would like to know,
viewing the question from the standpoint of this discussion,
is how far has a given species traveled from its proto-avian
ancestors, and not how much has it specialized. No linear
classification can show this, however perfect our knowledge
may be. nor can it exhibit the true positions of species in
one family in any given order as levelled with species in a
family in another order, nor yet can it give an adequate idea
of the relation of species of different families in the same
order. Many inconsistencies and contradictions appearing
under the present explanation of what controls the true
length of incubation are possibly due, not only to lacuna? in
our knowledge, and to errors in the records of incubation
lengths, but also to the shortcomings of a linear classifica-
tion. In other words, incorrect taxonomy, and the inability
to properly depict the relation of species in one order to
those in another, result in what appear to be severe disloca-
tions of incubation lengths from positions one would assign
to them under the present explanation.

Within natural groups (or families), the incubation
lengths should be more or less characteristic because the
members of such groups have diverged but little, inter se,
and being less plastic than many other characters, the incu-
bation length, under these circumstances, would diverge
still less, resulting in a condition substantiating Evans', (2)
first conclusion, to-wit, that natural groups (or families)
have incubation periods more or less characteristic to such
groups, a conclusion borne out by the incubation length data
given in Table No. 1; the gradations in some families be-
tween the accepted taxonomic positions assigned to the spe-
cies correspond surprisingly closely with the variations of
incubation length of the same species, especially if in such
families the largest species are ranked as lowest. If all
families be so arranged, the undulating curve of incubation

63

lengths becomes still more striking in its trend upward as
the species also trend upward.

Despite the difficulties inherent in a linear classification,
and the errors and conflicts in the incubation records, these
records show clearly that the incubation period becomes
shorter as the species mount the avian tree ; it is possible that
a rearrangement of the species according to some other clas-
sification might alter the curves radically and make other
interpretations necessary; it is equally possible that a re-
arrangement might strongly fortify the present conclusions.
Time and space forbid trying other classifications or com-
binations. It is obvious that the list of incubation periods
shows also that birds, in general, grow smaller as one follows
their life scale upwards, a fact which accords with the sug-
gestions as to "swiftness of life," size, and temperature.
One must not overlook the possibility that the character of
"swiftness of life" embraces the embryonic period (incuba-
tion) of life with birds, as it seems to do with the embryonic
period (gestation) of mammals.

There is much to sustain the belief that the intensity of
metabolism and the speed of embryonic development are
correlated (155), but the details of this evidence cannot,
however, be given here. That size and temperature are more
or less correlated both with mammals and birds is unmis-
takable ; that size, temperature, and taxonomic standing are
all parallel is not so evident, yet it looks as though the
lower birds are the larger ones, and that the tendency to
become smaller and to have shorter incubation periods, as the
avian tree expands upward, is fairly obvious. There is some
geologic evidence pointing to the probability that ancient
birds were generally larger, a fact well brought out by the
large size of birds recently found in the California asphalt
deposits; however, some living birds are small yet "low" in
degree, i. e., the kiwi, and some ancient birds were small yet
close to the reptile in some ways, the ichthyonis, for example.
This question of "lowness" or "highness" in birds, in the
present discussion, is a question of how far has a given
species journeyed away from its proto-avian stem, since it
seems probable that the farther a bird is from its primitive
ancestry, provided it does not later degenerate, the higher
will be its temperature. I doubt very much that the present
mainstays of taxonomy can alone measure this space between
pro-bird and super-bird. I believe that future students of
avian taxonomy will have to give more consideration, not
only to embryology, but also to bird physiology, in order to
correctly locate and plot the mile posts in a bird's taxonomic
journey.

On the other hand, experimental evidence demonstrates
that the usual length of incubation may be elongated by dif-

ferent bird temperature conditions, for it has been shown
(162) that the eggs of the Egyptian goose hatch under a
common hen in twenty-eight days, and under a muscovy
duck in thirty days. One more suggestive contribution to
the experimental evidence going to support the connection
between temperature, size, and incubation length can be
given here: Milne-Edwards (38) proved "that abnormal
elevation of incubation temperature during the first period
of incubation tends to diminish* the size of the chick, and
to produce dwarfs, though shortening the period"

If it be assumed that birds have in general grown
smaller as they evolved upward, the question can be inter-
polated here (in fact, it can be considered the "acid test" of
the truth of the assumption), have they benefited by their
smaller size? It would seem so, since the efflorescence of
the avian tree is made of the Passeres, practically all of
which are very small. Perhaps the diminishing size of birds
steadily accelerated their metabolic speed, which elevated
the body temperature (or vice versa), and initiated a physi-
ologic cycle, which is still revolving in ever diminishing
circles.

If a rise of temperature in an artificial incubator can
diminish the size of the chick, and shorten the incubation
period, is it impossible to have the same thing happen in na-
ture ? Because one cannot see or measure the slight decrease
in size, following an equally slight elevation of temperature
in a given species, is it not, however, possible that thousands
of such slight elevations of temperature, and of such minute
reductions in size, can be cumulative, and end in results
which are seen as existing conditions of today. It seems to
me not only possible, but highly probable.

For the sake of convenience, I will term the explanation,
(just elaborated), of factors fixing or controlling the true
length of incubation "the temperature and ascent theory."
A careful search through literature has disclosed but two
hints that avian "lowness" or "highness" might have some
effect on the incubation length. Both are more or less in-
direct, and the first is given in a few words by Fiirbringer
(102), who apparently considered this explanation, only to
reject it by saying, "Thus the number of incubation days,
which arranges itself more according to the size of the egg
and bird, than to the relationship boundaries, can be of no
great taxonomic* significance," with which conclusion I can-
not concur; the second is a suggestion made by Gadow
(150), who held that the developmental period (embryonic
plus nest life) stands in direct relation to the degree of "low-
ness" or "highness" of the bird, and that the "highest" birds

•Italics by W. H. B.

have the shortest incubation periods; so far as I am able to
grasp Gadow's words, he correlated this shortness of incuba-
tion with no definite physiologic process, and spoke of it
only incidentally in a discussion which related to the ques-
tion of a bird's being precocious or altricial. Arrigoni (12)
comes nearer to the final conclusion enunciated in this dis-
cussion, when he speaks of the length of the period of incu-
bation as being related to the vitality (sic) of the bird. I am
sure many others have grasped this idea, but I have been
wholly unable to locate any written statements to such effect.
These, then, are all the hints, direct or indirect, which sug-
gest a possible relation of a bird's taxonomic standing to the
length of its incubation period, that I have been able to dis-
cover in the ornithological literature at my command.

The evidence and data which I have been able to collect,
point to the idea that the true length of incubation yields to
change exceedingly slowly and with difficulty, much more so
than characters of more recent acquirement, as color, size,
etc. It is almost self-evident that the incubation periods of
birds have been gradually shortened and that they are still
slowly yielding, as in the past, to the influences outlined in
the foregoing, and will continue to do so until this process
of abbreviation becomes detrimental to the species. I am
firmly convinced that, once a given period becomes short-
ened, it remains so, becoming longer again only apparently,
and through such influences only, as slow or temporarily
suspend the embryonic development, and returns, either
in the next set of eggs, or with the first set of the next
generation, to the previous normal, after the warping influ-
ence ceases. Once a specie rises in "taxonomy," and has its
temperature coincidentally elevated, which shortens its in-
cubation period, this latter will remain constant under op-
timum conditions, even though the species later retrogrades
morphologically. I believe that in this case, the bird re-
mains physiologically stable, at the previous level, though
its co-existing morphologic level will have been lowered, and
that the bird will then have an incubation length which,
under the present theory, would appear shorter than its
morphology would predicate. On the contrary, a species
may "specialize" morphologically, appear as "high," yet re-
main at the pristine lower physiologic level, and have, as a
result, an incubation period longer than one would pre-
suppose for it, judging from its morphology.

I am inclined to believe, as a result of this investiga-
tion alone, that there are physiologic as well as morphologic
levels to be considered in taxonomy, and that these levels
may differ in the same individual. I am convinced that there
is a large and profitable field to be explored, in this question
of the possible differences in physiologic and morphologic

66

levels of taxonomy. The length of incubation with the
ostrich, and its temperature, would indicate for this species
a higher physiologic level, taxonomically, than does its
anatomy indicate for its morphologic level. It is higher
physiologically than morphologically.

Whatever may be the final decision as to the relation of
a bird's "lowness" or "highness" to its length of incubation,
I am convinced that there will be but one conclusion as to
the correlation of incubation temperature and the length of
incubation, and that it is highly probably that, ultimately,
the last word on this question will have to be said through
determination of temperatures taken between the eggs and
under the incubating parent (real or foster).

Collateral Evidence

Perhaps some additional light can be thrown on the
theory in hand, through a review of the incubation condi-
tions which prevail amongst birds' nearest relatives, the rep-
tiles, and by a scrutiny of the effects of temperature on eggs
other than those. of birds and reptiles. H. Milne-Edwards
(38) states that the period of incubation of silk-worm eggs
can be prolonged to fifty days if subjected to low tempera-
tures, that they hatch in thirty-four or thirty-six days if
kept at a temperature of from 77° to 86° F., and in sixteen
to eighteen days if they are maintained in a temperature of
86° to 95° F. Though it is true that these eggs belong to a
creature in a class widely separated, and very divergent
from the Class Aves, it is still quite suggestive that with
them is found a high grade of elasticity in the length of in-
cubation, which is governed by variations in the temperature
conditions; this is mentioned to'illustrate how potent an in-
fluence variations in temperature can have on embryonic
development. The close relationship between reptiles and
birds warrants a careful examination of the facts touching
on incubation of reptile eggs ; it is highly possible that the
incubation of such eggs is subject to the same variety of
control as those of birds, and that the same "temperature
and ascent" theory applies as well to reptiles', as to birds'
eggs. In line with this thought one may recall that the
evolution of "parental care" shows a series of steps from
low to high, broken it must be said, however, by striking
exceptions; the evolution of parental care is well seen in
reptiles and birds, taken as a whole, and has been said (155)
to be associated "with the need of higher temperatures for
development."

Reptile eggs, in all probability, have a normal minimum
duration of incubation, which may be peculiar to each genus,
i. e., a specific incubation length; such eggs withstand suc-
cessfully a much longer and some a far more severe chilling

67

spell than do those of birds, and they may bear, relatively
higher temperatures, i. e., the tuatara lizard's eggs 77° F.
(126) ; they probably remain viable for far longer periods,
their embryonic development persistently progresses at much
lower temperatures than does that of birds, and one would
predict, on these grounds, an exceedingly variable and elastic
apparent length of incubation, which is borne out by the
facts in the case. No reptile incubates its eggs, unless the
devoted care given by the female python to its eggs can be
so classed; this may, however, be a true but primitive kind
of incubation, a real beginning in the scale of incubation
habits, since it has been shown that a python's body tempera-
ture rises 10° F., above the surrounding air while covering
its eggs, and Heilmann (118) says the temperature of a
"quiet incubating snake" is about 50° to 53.6° F. If reptile
eggs respond to the same controlling influences as do those
of birds, it is possible that data on reptile incubation will
disclose such correlation; I have discovered a few facts,
which, while not nearly as numerous as might be desired, are
nevertheless quite suggestive. If I read aright, it appears
that the most primitive of all living reptiles is Sphenodon
punctatus, and on grounds outlined in this discussion, its
incubation period should be the longest of all living reptiles,
and it is, lasting from twelve to thirteen months, reckoning
from laying to hatching. It is necessary to recall, that about
five months of this period, extend over the New Zealand
winter, and that during this time the embryo ceases to de-
velop, and hibernates in ovo, as it were, until the environ-
mental conditions again become such that it can survive
when hatched.*

The next longest period of incubation amongst reptiles
is that of the European tortoise (Emys orbicularis) which is
probably a higher reptile than Sphenodon, though it is
placed in an order ranked below that of the Sphenodon ; in
southern Europe the eggs of this tortoise hatch in five to
six months, wrhile in northern Russia (120), they require
eleven months; a large part of this eleven months is the
season of Russian winter, and during its intense cold, the
developing tortoise embryo is in a condition of suspended
animation yet, deducting this time from the whole period
of incubation, the actual developmental period is still next
to the longest amongst reptiles. Then comes the alligator,
and in this instance one is on secure ground in having a

*It is possible that this reptile, which has persisted almost un-
changed since the Lower Permian, survived because it was able to
adjust, so to speak, its incubation to comparatively low temperatures,
which may have supervened in the course of geologic changes, and which
exterminated the then existing host of its huge contemporaries (dino-
saurs, etc.), whose probable long incubation periods required an equally
long period of relatively high temperatures.

record which has been checked frequently by eggs placed in
an artificial incubator, where they require sixty days to
hatch. The measuring of the true length of incubation
amongst other reptiles becomes more and more perplexing
as one takes up the succeeding ascending orders. It is ex-
ceedingly difficult to decide what is the true length of incu-
bation with many snakes and lizards because of the ease
with which the embryonic development of these animals
can be slowed down, or temporarily suspended, and because
of the high probability that the same reptile species may
exhibit all gradations between complete oviparity and ovovi-
parity, gradations which can cause a difference of days or
even weeks in the apparent length of incubation in the same
species. This possibility also is mentioned here because it
is probable that oviparity and ovoviparity are interchang-
able according to environment, in certain other species.

This causes a wide variation in the records of incubation
lengths of reptiles, and adds to the difficulty of reaching con-
clusions on the point now in mind, yet in spite of this and the
paucity of records, it seems to me that reptiles also exhibit a
tendency to shorter incubation periods as the species is
higher in its scale of life, which tendency seems to show in
the data given in Table No. 7.

TABLE NO. 7
Incubation Periods of Reptiles

Period Authority

Green Turtles Sy2 weeks 119

Loggerhead Turtles Sy2 weeks 119

European Tortoise 20 to M weeks 120

Tuatara Lizard 52 weeks 121

Crocodile 12 weeks 120

Alligator (in incubator) Sy2 weeks 122

Common Swift 6 to 8 weeks 123

Python reticulatus 6 to 8 weeks 125

Python molurus 10 weeks 120

Black Snake Sy2 weeks 123

Fox Snake 7 3/7 to 8 6/7 weeks 123

Corn Snake 6 to 8 weeks 123

Yellow Rat-snake 11 weeks 123

Ring Snake 56/7 weeks 123

Milk Snake 81/7 weeks 123

King Snake 6 to 8 weeks 123

Coral Snake 8 2/7 to 8 4/7 weeks 123

If reptile eggs vary in incubation lengths according to the
applied temperatures, it is proper to ask if there be any evi-
dence in the temperature of reptiles, showing trends similar
to those in birds ; reptiles are said to be "cold-blooded," but
it can be shown that this term is only relative. The average

69

temperature of reptiles with weak respiration is 1.8° to 5.4°
F. above the circumambient medium, and for others the fol-
lowing degrees have been given :

Temperature Authority

Turtles . .• 33.6° F. 38

Chameleon 1.8° F. (above air) 38

Lizards 1.3° to 14.6° F. (above air) 38

Vipers 4° to 11.3° F. (above air) 38

Boa (incubating) 10.8° F. (above air) 38

Snake (incubating) ..50.0° to 53.6° F. 118

Viper 68.0° F. (air being 58.0° F.) 156

Python 76.0° F. (air being 60.0° F.) 156

Turtle 84.0° F. (air being 79.0° F.) 156

The fact that the male boa, which does not incubate,
has a lower temperature than the female (156) is worth
noting.

There is shown in the figures just given (see Table
No. 7) enough tendency for the temperatures to rise as the
species gets "higher" in its life scale to give color to the pos-
sibility that with reptiles, as with birds, the temperatures
and the species become elevated concomitantly. From the
"swiftness of life" point of view, it is interesting, and per-
haps important, to note that the quiet, sluggish reptiles have
lower temperatures than do the active ones.

A relationship between reptiles and birds, should, and
does, leave in both, traces of a common ancestry; can one
find indications of this in the incubation periods of birds?
If the length of incubation in birds be, as I am convinced it
it, a deep-rooted, inelastic and inherited characteristic, it
should exhibit some proto-reptile or some proto-reptile-bird
peculiarity or peculiarities; the data show, both with birds
and reptiles a decided tendency to arrange themselves in
groups of periods having a septenate multiple, i. e., 14, 21,
28, 42 and 56 days. This septenate tendency has been ex-
plained on the hypothesis that it is an inheritance from
ancestors which were aquatic, and subject to, and probably
modified by, maximal tides every twenty-eight days. If this
be true, it would bespeak an amazing tenacity in the persist-
ence in birds, of this very ancient character. It may not,
however, be a correct explanation, because the same sep-
tenate or monthly spacings are to be found attached to the
functions of mammals, i. e., gestation and rut. It does not
seem possible that such an old influence should have left an
impress which is visible in a stem so far removed from the
main trunk as is that of the mammal branch.

Taxonomy, the "Ascent Theory" and the Data
I know full well that real and apparent inconsistencies
and conflicts will be found in the correlation of these three,

70

when the present data are viewed from several different
points. Some come promptly to mind ; according to modern
taxonomists, the megapodes are near relatives of the do-
mestic hen, and they should have, by this token, a length of
incubation somewhat similar to that of the hen, yet it is in
reality much longer, and far longer than one would predict,
from .my point of view, with birds of the comparatively high
standing of the megapodes. Hence their incubation length
does not seem to fit in with this "ascent theory ;" how it fits
in with the temperature side of the present explanation I do
not know, as I have no temperature records of these queer
birds. What are the possibilities in the premise? These
birds may upset the whole argument, or they may be one of
a few exceptions, or there are two other possibilities. It has
been maintained in this discussion that the true length of
incubation is fixed by the parents temperature as applied to
the eggs,- thus this method of fixing cannot apply to the
megapodes because their eggs are hatched, not by heat sup-
plied by the parent bird, but by that arising in the decaying
mound, its temperature being given as 95.0 to 96.0 F. (131),
which are the lowest successful incubation temperatures, for
birds' eggs, known to me. It might be assumed, in the first
place, that the mound incubation habit is very ancient, dat-
ing from the time when the megapodial ancestors were much
less removed from the pro-bird than at present, and that
the ancestral species had at that time, a comparatively low
temperature, approximating that of the present incubation
mound. If the mound building habit arose at that time,
and continued to the present, it would explain the inappro-
priately long incubation because, on this last assumption, it
has not varied during all the past ages, notwithstanding that
in the interim there have been great change and variation
in the morphology of the group. The eggs have, during all
this time, been hatched by a temperature which has varied
little, if at all, resulting in an unchanging incubation length.
On the contrary, it is possible that this mound incubation
habit is of recent origin, and that the present recorded meg-
apod length of incubation is an apparent one only, being the
true length plus X days of slowing down of embryonic de-
velopment caused by the low temperature of the mound; it
is difficult to picture just how, under this suggestion, the
eggs successfully became adapted to such abnormal pro-
longation of incubation. This might be tested experiment-
ally by subjecting megapode eggs to varying temperatures
in artificial incubators; it also would give an added light if
one knew what is the true length of incubation of the genus
Maleo, since eggs of this genus are said to be incubated, not
in mounds, but by the sun's heat or by hot springs. The
case of the megapodes falls without the confines of my

71

present theory, as regards taxonomic standing, but within
the control of the temperature explanation. The incubation
length of the flicker, not a very high, or a small bird, is
given as eleven to twelve days, which seems unduly brief in
the light of this study. This short period (if^ it is correctly
estimated] may be due to temperature conditions, since this
species nests in tree holes, where there is little heat loss,
and lays a large complement of eggs, which, as they develop,
would tend to higher and higher temperatures as the em-
bryos grow. With other hole-nesting species laying few
eggs, this latter effect is not present, an absence which may
help to account for longer incubations with these species.

What has happened in the case of parasitic cowbirds
and cuckoos since their eggs are incubated by foster parents ?
An answer to this problem will only be had when the tem-
peratures of the parasitic birds and of their dupes are
known, as well as the temperatures of such cuckoos and
cowbirds given to autogenous incubating. It is not too late
to learn the last, as there are still some of the species of
these genera which incubate their own eggs. In the devel-
opment of this parasitic habit, it undoubtedly was at first a
"hit or miss" arrangement; if the parasitic eggs were given
too high heating by the foster parents, their embryos died,
and the reverse could also be true, though an incubating tem-
perature slightly below the optimum might result in a suc-
cessful, but prolonged, incubation. Such a "hit or miss"
method, after repeated trials through a long stretch of time,
together with a possible tendency of the female cowbird to
return, when mature, to a nest similar to that in which it was
raised, would eventuate in a variety of selection, which
would secure a large percentage of successful incubations.
The cuckoo is said (110) to lay eggs which are, proportion-
ately to itself, the smallest of all birds' eggs ; has this arisen
because of the possibly higher temperatures of the foster
parents?

What happens when a quail's egg is hatched by a ban-
tam hen, or what would happen if a magpie's eggs were
placed under a bantam?

It seems difficult to reconcile a mourning dove's incu-
bation period, which is said to be fourteen days, with that
of the robin's of equal length, because taxonomists are
agreed that the latter is a much higher bird; perhaps both
of these species have traveled, physiologically, equally far
from the pro-avis ancestor, but along quite divergent roads.
The ostrich is regarded, by most classifiers, as lower than the
emu, yet its incubation period is not as long as that of the
latter. This example may not be a real conflict with the
present "ascent theory," but the lack of concord may be due
to an error in taxonomy, because at least one ornithologist

72

(120) believes that the emu is a lower and a more primitive
bird than the ostrich, and his classification is supported by
the position taken in this paper, i. e., that the longer is the
incubation period, the lower is the bird. The secretary bird
is believed to be a primitive survival, not greatly changed,
from its pro-hawk-heron ancestor ; if this be correct, a long
incubation period would be predicted, viewed from the
"ascent theory," a period as long, or longer, than all of its
near relatives, and such is the case (barring one or two
questionable exceptions). These and similar questions must
be decided by experimental work; they are but a few of the
many fascinating ones uncovered by a study of the length of
incubation.

It is possible that many conflicts, and inconsistencies in
this new theory, will be cleared up when the present incuba-
tion records are checked and corrected by new observations,
undertaken to eliminate, or allow for, the conditions which
apparently increase or decrease the true length of incubation,
and when a method of recording avian classification will
have been devised which does not project the vision of the
mind's eye along a single line, and which will graphically
portray the relation of genera to genera, irrespective of fam-
ily, or order. Perhaps such a three-dimension demonstration
scheme is impossible of attainment.

The chances of any given explanation being correct are
larger if the results it foreshadows can be shown to be
beneficial ; if the bird's taxonomic position, temperature, size,
and length of incubation be correlated, it is pertinent to ask,
has the combination been beneficial to the race? It would
seem so, judging from actual experience in nature, since so
large a number of existing species are of the highest levels,
have diminished in size, have acquired short incubation
periods, have developed high temperatures and have con-
comitantly flourished and multiplied. Do short incubation
periods prove helpful to the species ? Is there any advantage
to the robin that it gets its young out of the nest and able,
more or less, to shift for themselves in four weeks from the
laying of the eggs, while a goose, a turkey, or a red-tail
hawk, in the same time is able to complete only the em-
bryonic development of its young, and has still ahead of it,
the long post-embryonic period of development, and all
the care incidental to it, plus such post-nidicular time, during
which it must give more or less care to its young? Is there
any benefit to a species to be able to put two broods of young
into the field, while another species can, in the same time, put
but one? It seems to me that these questions can be an-
swered only in the affirmative. If it be true that the number
of eggs laid in a set is in direct ratio to the dangers en-
countered by the species (110), it also would seem reasonable

73

to believe that a species would be better able to successfully
maintain its status in the face of its daily dangers, through
the increased prolificity made possible by the short incuba-
tion period permitting more, and perhaps equally large
broods to mature during the breeding season; this is but
another way of making the eggs proportionate to the dan-
gers meeting the species in its daily life, and may be looked
upon as a result of the higher birds having high temper-
atures.

More broods each season inflict greater responsibility
and labor on the parents; this affects but two, while the
greater multiplication of individuals certainly more than
counterbalances this. As said above, Gadow holds that the
shortened incubation period, and its correlated long nest
period (?) reflects benefits on both parents and young ; I
believe it does help the young, but as to the parents, I am
inclined to feel that for them, the beneficial effects are ne-
gated by the disadvantage of more labor, wear and tear, etc.
In all other creatures, the parent is so universally disre-
garded, so to speak^ for the good of the young, that I am
skeptical as to there being an exception with birds. The
question also arises of two species, which young are better
able to fend for themselves at the end of a given time, view-
ing the question from the standpoint of the possible bene-
ficial or injurious effects of a short or a long incubation
period, those hatched after a short or those after a long in-
cubation? Two months after hatching are young robins, or
young quail, better able to shift for themselves, and to meet
and overcome the perils of their daily life ? It seems to me
that the advantage lies with the higher bird, because of its
shorter incubation period, which supports the belief that the
diminishing lengths of incubation delivers benefit to the
species. It is self-evident that such a continual shortening
of the incubation period as I have assumed to have been in
progress in the past, and which is probably still going on,
may have for a given species adventages and disadvantages
which cannot now be clearly apprehended since some may
be disappearing and others only just coming into effect.

If my contention be correct, that the true length of in-
cubation is a fixed and persistently inherited characteristic,
and determined by the bird's temperature, and its position
in its scale of life, it then becomes possible that this char-
acteristic will be useful as an aid in allocating the correct
taxonomic position of a species, taking a place (perhaps a
very minor one) with anatomy, embryology, splanchnology,
ptilosis, etc. ; it seems to me that there are larger possibilities
for usefulness in these directions for this character, than
there are for such an unmeasurable and indeterminate char-

74

acter as the nervous system, which last has been used to place
the Corvidae at the apex of the avian tree. If this "tempera-
ture and ascent theory" of the control of the length of in-
cubation, with birds, be correct, it will, at least, bring about a
semblance of order where before more or less confusion pre-
vailed ; if it be fallacious, and untenable, then I can at least
hope that this discussion may point out the lines of inves-
tigation along which, in the future, ornithologists will have
to work in order to solve what in the past has been an
unanswered riddle.

Correct or incorrect this discussion may, I hope, draw
attention to the importance of gathering accurate data on
the true length of incubation with birds, and its relation
to their physiology and taxonomy.

It appears to me that some of the previous explanations
as to what fixes, determines, or controls the length of avian
incubation have held more or less of the truth, because they
concern conditions which are effects of a single underlying
cause, but I believe that there is no experimental evidence
whatsoever, that differences in the size of bird, age of fe-
male, longevity of the parents, condition of the young at
hatching, the size of the egg, or its shell, or the yolk size, or
various telluric conditions ever permanently alter the true
length of incubation; there is abundant experimental evi-
dence (both accidental and intentional) that variations in
temperature can prolong the time of incubation, and can
shorten it to a slight extent, and presumptive evidence that
this effect of variation in temperature in shortening the
length of incubation, does in the end bring about the large
variety of incubation lengths found with birds today.

Data Needed for Further Study of This Problem

The prosecution of this study has uncovered a lament-
able dearth of information in many phases of the questions
at irsue, and in now calling attention to a few of these de-
plorably incomplete chapters of ornithology. I would remind
ornithologists that in them they will find openings which
will lead to splendid opportunities for original research in
almost unexplored and unexploited fields, fields which may
soon be swept out of existence before the devastating inarch
of civilization. Data needed :

Exact length of incubation period of birds and reptiles,

Exact length of incubation of birds in polar and tropical
regions,

The period of viability of birds' eggs,

The weights of birds, preferably of the breeding female,

The weights of birds' eggs,

The effects of superheating on birds' and reptiles' eggs,

75

The optimum incubation temperatures of birds' and rep-
tiles' eggs,

Bird temperatures,

Temperatures under the incubating bird,

Reptile temperatures,

Minutiae of bird physiology.

Conclusions

While final judgment and decision must be suspended
on many, if not all, of the points at issue in this discussion,
suspended at least until a much larger, and perhaps an en-
tirely rewritten accumulation of data on all of the above
questions has been made, I feel justified, at this time, in
drawing the following tentative conclusions :

Minor: That

Our present incubation records include the true and the
apparent lengths, the latter in the majority, probably,

The true length of incubation can be shortened (arti-
ficially) with extreme difficulty, but prolonged with ease,

The true length of incubation is loosely related to the
size of the species, and still more loosely to the size of the
egg,

There is little or no relation between the true length of
incubation and precocity,

There is no relation between the true length of incuba-
tion and the longevity of the species, nor between it and the
body-weight : egg-weight index,

There is no relation between the true length of incuba-
tion and the age of the female, or the size of the egg yolk,

Condition of the parents, faithfulness of the parents
while incubating, viability of the egg, thickness of the egg
shell, and influences listed under the head of "Telluric Con-
ditions," have no lasting effect on the true length of incu-
bation; when any of these conditions seem to modify the
incubation length, it is merely an action of slowing the
embryonic development, and causes no permanent change in
the length of the incubation period.

Major: That

There is a true or specific length of incubation,

The true or specific length of incubation is a deep-seated,
inelastic, and persistently unchanging (in human time
measures) character,

Bird temperatures are closely related to the taxonomic
"lowness" or "highness" of the species;

A bird's temperature determines, or fixes, the true length
of its incubation period, and that only an abiding change in

76

the bird's temperature can permanently alter the true length
of its incubation period,

Whence I would suggest the following

Hypothetical Law,

That the true or specific length of incubation is fixed by
the "lowness" or "highness" of the species, particularly by
the physiologic "lowness" or "highness," both of which are
fixed by the distance the species has traveled (without re-
treating) from its proto-avian ancestor.

TABLE NO. 1

Incubation Periods*
Family — Species , Period Authority

Struthionidae

Struthio Camelus— Ostrich 36 to 40 days 1

Struthio Camelus — Ostrich 45 to 48 days 9

Struzzo 50 to 60 days 12

Ostrich 50 to 60 days 11

Ostrich 42 to 49 days 10

Ostrich 42 days 7

Ostrich 42 days 8

Ostrich (In incubator) 42 days 6

Rheidae

Rhea Americana — Common Rhea 35 days 1

Rhea Americana — Nandu 39 days 9

Rhea Americana — Great-billed Rhea 6 weeks 94

Rhea darwinii — Darwin's Rhea 30 to 31 days 13

Dromaeidae

Dromaeus novae-hollandiae — Emu 56 days 1

Dromaeus novae-hollandiae — Emu . . . about 8 weeks 10

Dromaeus novae-hollandiae — Emu 58 days 9

Dromaeus novae-hollandiae — Emu . . . about 8 weeks 14
Dromaeus novae-hollandiae — Emu (under

hen) 7 weeks 17

Dromaeus novae-hollandiae — Emu (Under

normal parents 9 & $ ) 57 days 17

Dromaeus novae-hollandiae — Emu (Under

normal parents $ only _ ... 63 days 17

Emu 50 days 15

Emu 58 days 94

Dromaeidae "the period being 80 days" 16

Dromaeidae 70 to 80 days 25

Casuariidae

Casuarius bennetti — New Britain Casso-
wary— (Mooruk) 42 days 1

*The author is confident that some of the incubation-length records
included in this list are grossly incorrect, yet they are incorporated in
the data, with a feeling that they will, in the future, be more accu-
rately determined, and in that form recorded, and thus be corrected.

77

Family — Species Period Authority

Casuarius australis — Cassowary

"probably 7 weeks or over" 14

Casuarius galeatus ( ?) — Cassowary 9 weeks 1

Cassowarie 52 days 117

Crypturidae

Rhynchotus ruf escens — Tinamou 21 days 1

Apterygidae

Apteryx australis — Kiwi 6 weeks 1

Spheniscidae

Aptenodytes Fosteri — Emperor Penguin

*. "some 7 or 8 weeks" 20

Emperor Penguin 7 weeks 138

Aptenodytes Pennanti — King Penguin. . .

. . ."said to be 7 weeks" 93

Pygoscelis Adaliae — Adele Penguin 37 days 20

Pygoscelis Adaliae — Adele Penguin

(more accurate) 34 days 20

Pygoscelis Adaliae — Adele Penguin 31 days 20

Adelie Penguin 33 to 36 days 133

Johnny Penguin 33 to 37 days 134

Eudyptes Chrysocome — Tufted Penguin

about 6 weeks 14

Catarrhactes chrysocome — Crested Penguin

about 6 weeks 1

Sphenicus demersus — Black-footed Pen-
guin 56 days 94

Gavidae

Gavia immer — Loon "about a month" 19

Gavia immer — Loon 29 days 3

Colymbus arcticus — Black-throated Diver. 28 days 1
Podicipedidae

Western Grebe 21 days 18

Podicipes nigricollis — Eared Grebe . about 24 days 1

Podicipes Minor— Little Grebe 20 or 21 days 1

Podicipes Cristatus — Great-crested Grebe

21 to 24 days 1

Podicipes novae-hollandiae— Black-
throated Grebe 23 days 17

Podilymbus podiceps — Pied-bill Grebe . . .

21 (?) days 3

Diomedeidae

Diomedea exulans — Wandering Albatross

"about 3 months" 14

Diomedea melanophrys — Black-eyebrowed

Albatross 60 days 1

Diomedea melanophrys — Black-eyebrowed

Albatross '. ."about 60 days" 14

Turbinares (?) 35 days 25

78

Family — Species Period Authority

Procellariidae

Fulmarus glacialis — Fulmar "about a month" 1

Fulmarus glacialis — Fulmar 50 to 60 days 12

Puffinus tenuirostris — Short-tailed Petrel

(Mutton Bird) 56 days 14

Procellaria pelagica — Storm Petrel . . 24 to 25 days 1

Procellaria pelagica — Uccello del tempeste 36 days 12

Storm Petrel (in incubator) 35 days 2

Oceanodroma leucorhoa — Leache's Petrel

30 (?) days 3

Phaethontidae

Phaethon americanus — Yellow-billed Trop-
ic Bird 28 days 3

Yellow-billed Tropic Bird (at least) 28 days 26

Yellow-billed Tropic Bird (in incubator) .28 days 27

Pelecanidae

Pelecanus erythrorhynchos — White Peli-
can ." 29 to 30 days 3

White Pelican 29 days 30

Pelecanus onocrotalus — Pelikan 36 to 38 days 9

Pelecanus occidentalis — Brown Pelican. . .28 days 3

Pelican "said to be 49 days" 28

Pelican (Sp?) 30 days 1

Phalacrocoracidae

Phalacrocorax carbo — Cormorant ... .28 to 29 days 1

Phalacrocorax carbo — Kormoran 28 days 9

Phalacrocorax pelagicus pelagicus — Pela-
gic Cormorant (Est. by W.H.B.) 26 days 21

Phalacrocorax urile — Red-faced Cormo-
rant 21 days 3

Phalacrocorax Sp.? — Cormorant 28 days 28

Sulidae

Sula piscator — Red-legged Gannet 45 days 14

Sula bassana — Gannet 39 days 3

Sula bassana — Gannet 39 days 1

Gannet (38 to 44 days) usual is 42 days 28

Gannets 39 days 25

Sula serrator — Australian Sula

"supposed to last only 33 days" 29

Gannet "about 33 days" 14

Ardeidae

Botaurus lentiginosus — Bittern 28 days 3

American Bittern (at least^ 4 weeks 39

Ixobrychus exilis — Least Bittern

(estimated) 17 days 40

Botaurus stellaris— Bittern 23 days 1

Ardetta Minuta— Little Bittern . . . . 16 or 17 davs 1

Family — Species Period Authority

Ardea occidentalis — Great White Heron

"about 30 days" 1

Ardea herodias herodias — Great Blue

Heron 28 days 3

Great Blue Heron "about 28 days" 41

Ardea Cinerea — Fishreiher 25-28 days 9

Ardea Cinerea — Heron 25 to 26 days 1

Herodias garzetta — Seidenreiher 23 days 9

Herodias timoriensis — Egret about 4 weeks 14

Herodias timoriensis — White Heron 4 weeks 42

Ardea ludoviciana — Louisiana Heron. .. .21 days 1
Butorides virescens virescens — Green

Heron 17 days 3

Green Heron about 16 days 43

Nycticorax nycticorax naevius — Black-
crowned Night Heron 24 (?) days 3

Nycticorax nycticorax — Nachtreiher 21 days 9

Circoniidae

Circonia Alba— White Stork 23 days 1

Circonia circonia — Weisser Storch ... 32 to 38 days 9

Ibididae

Ibis Aethiopica — Sacred Ibis about 21 days 1

Ibis molucca — Australische Ibis 24 days 162

Guara alba— White Ibis 21 days 3

Guara alba— White Ibis 20-23 days 107

Carphibis spinicollis — Stachelibis 22 days 162

Plegadis falcinellus — Sichler (Bay Ibis)

21-22 days 9

Plegadis autumnalis — Glossy Ibis 21 days 3

Phoenicopteridae

Phoenicopterus ruber — Flamingo 28 days 193

Phoenicopterus Sp. ? — Flamingo 30 to 32 days 1

Palamedeidae

Chauna chavaria — Tschajia (Derbian

Screamer) 42 days 9

Anatidae

Mergus americanus — American Merganser

(at least) 28 days 31

Mergus americanus — Merganser 28 days 3

Mergus merganser — Goosander 28 days 1

Mergus serrator — Red-breasted Merganser

26 to 28 days 1

Red-breasted Merganser (under hen)

"about 4 weeks" 32

Mergus serrator — Red-breasted Merganser

26 to 29 days 3

Mergus cucullatus — Hooded Merganser. . .31 days 1

80

Family— Species Period Authority

Fulvous Tree Duck 26-28 days 95

Tree Duck 26-28 days 95

Dendrocygna Fulva — Gelbe Baumente .... 32 days 162

Chenonette jubata — Hahnen Gans 28-29 days 162

Casarca casarca — Rote Kasarka 29 days 162

Anas platyrhynchos — Mallard 26 to 28 days 3

Mallard (varies from 23 to 29) 27 to 28 days 95

Anas boscas — Wild Duck 26 days 1

Ducks 28 days 25

Anas domesticus — Domestic Duck. .27 and 28 days 1

Duck 28 days 8

Duck— Common 28 days 33

Ducks 26 days 34

Duck— Common 24-29 days 9

Anas Rubripes— Black Duck 26-28 days 3

Black Duck 26-27 days 95

Anas angusterostris — Marbled Duck. . 25 to 27 days 1

Barbary Duck 30 days 33

Gadwell 28 days 95

Polionetta poecilorhyncha - - Flecken-

schnablente about 29 days 162

Chaulelasmus streperus — Schnatterente . . .

26 days 162

Anas superciliosa — Australische Wildente

27-28 days 162

Mareca Penelope — Widgeon 24 to 25 days 1

Mareca penelope— Pfeifente 22-23 days 162

European Widgeon 24 days 95

Querquedula crecca — Common Teal 22 days 1

Green-winged Teal 21 to 23 days 95

Blue-winged Teal 21 to 23 days 95

Cinnamon Teal 21 to 23 days 95

Querquedula cyanoptera — Blaufliigelente . .

26 days 162

Querquedula circia — Gargone}^ 21 or 22 days 1

Querquedula querquedula — Knakente

about 24 days 162

Spatula clypeata — Shoveller 28 days 1

Shoveller 22 to 24 days 95

Dafila acuta— Pintail 22 or 23 days 1

Aix Sponsa — Wood Duck 25 days 1

Wood Duck 28-30 days 95

Lampronessa sponsa — Brautente 31 days 162

Aix galericulata — Mandarin Duck 30 days 95

Mandarin Duck 28-30 days 1

Redhead 28 days 95

Nyroca f erruginea — White-eyed Duck ....

22 to 23 days 1

81

Family — Species Period Authority

Nyroca nyroca — Moorente 28 days 162

Fuligula cristata— Tufted Duck 22 to 23 days 1

Fuligula f uligula — Rheierente 25 to 26 days 162

Metopiana peposaca — Peposacaente . . 27 to 28 days 162

Netta rufina — Kolbenente 28 days 162

Somateria mollissima — Eider Duck 28 days 1

Somateria mollissima boreal is — Northern

Eider 36 (?) days 3

Somateria mollissima boreal is — Northern

Eider 25-26 days 137

Somateria dresseri dresseri — American

Eider 25-26 days 137

Somateria spectabilis — King Eider 25-26 days 137

Cereopsis novae-hollandiae — Cereopsis

Goose .35 days 1

Cereopsis novae-hollandiae — Cereopsis

Goose about 30 days 97

Cereopsis novae-hollandiae — Hiihnergans...30 days 162

Anser cinereus — Grey-lag Goose 28 days 1

Anser domesticus — Domestic Goose 30 days 1

Goose 30 days 8

Geese 30 days 34

Goose 30 days 33

Geese 28-29 days 9

Anser brachyrhynchus — Pink- footed Goose 28 days 1

Anser arvensis— Acker Gans 28-29 days 162

Chen hyperboreus — Schneegans 28-29 days 162

Branta canadensis canadensis — Canada

Goose 28 to 30 days 3

Bernicla canadensis — Canada Goose . . 28 to 29 days 1

Canada Goose 17 days 35

Canada Goose 28-30 days 97

Bernicla poliocephala — Ashey-headed

Goose 30 days 1

Bernicla sandvichensis — Sandwich Island

Goose 31 days 1

Chloephaga magellanica — M agellangans . .

28-29 days 162

Philacte canagica — Emperor Goose 24 days 163

Chenalopex aegyptiaca — Egyptian Goose

27 to 28 days 1

Alopochen aegypticus — Nilgans, under

hen (domestic) 28 days 162

Alopochen aegypticus — Nilgans, under

Muscovy Duck 30 days 162

Tadorna cornuta — Shelldrake (common) .30 days 1

Tadorna casarca — Ruddy Shelldrake 30 days 1

Pairina moschata — Turkenente 35 days 162

82

Family — Species Period Authority

Olor cygnus — Whooping Swan 35 to 40 days 3

Cygnus musicus — Whooper 35 days 1

Cygnus olor — Mute Swan 35 days 1

Cygnus olor— Swan 35 to 42 days 36

Cygnus Melanocoryphus — Schwartzhals

schwann 35 days 162

Schwane (Cygnus) 35 days 9

Swan 35 to 40 days 25

Swan "between 5 and 6 weeks" 25

Swan 42 days 8

Cygnus nigricollis — Black-necked Swan.. 35 days 1

Cygnus atratus — Black Swan 35 days 1

Chenopis atrata — Schwartzer Schwann ... 35 days 162

Cathartidae

Sarcorhamphus gryphus — Kondor 55 days 9

Condor (in captivity) 54 days 25

Sarcorhamphus gryphus Condor 54 days 2

Cathartes Californicus — California Vul-
ture 29 to 31 days 1

Cathartes aura septentrionalis — Turkey

Vulture ("close to") 30 days 192

Turkey Buzzard (Est. by W.H.B.)..37 (?) days 55

Cathartes atrata— Black Vulture 28 to 30 days 35

Cathartes atrata— Black Vulture 32 days 1

Black Vulture 30 days 44

Cathartes urubu — Rabengeier 40 days 9

Cathartes uruba — Black Vulture 30 days 3

Cathartes uruba — Black Vulture about a month 100

Gypogeranidse

Serpentarius reptilivorus — Secretary Vul-
ture about 6 weeks 1

Falconidae

Prairie Falcon 21 to 28 days 44

Falco peregrinus — Peregrine Falcon. 18 to 19 days 1

Falco peregrinus anatum — Duck Hawk.. 28 days 3

Duck Hawk 28? days 44

Falco subbuteo — Hobby 21 days 1

Falco columbarius cdlumbarius — Pigeon

Hawk 21 ? days 3

Pigeon Hawk 21 ? days 44

Tinnunculus alaudarius — Kestrel 27-28 days 1

Falco sparverius sparverius — Sparrow

Hawk 29 to 30 days 3

Sparrow Hawk 21 ? days 44

Sparrow Hawk 29-30 days 56

Sparrow Hawk about 21 days 35

Sparrow Hawk 29-30 days 64

Audubon's Caracara 28 ? days 44

S3

Family — Species Period Authority

Buteonidae

Pernis apivorus — Honey Buzzard 21 days 1

Circus hudsonius — Marsh Hawk 26 to 28 days 3

Marsh Hawk "is nearly 4 weeks" 57

Marsh Hawk _ 31 days 43

Marsh Hawk 21 days 44

Circus aeruginosus — Marsh Harrier

"21 to 24 days group" 1

Circus cyaneus "21 to 24 days group" 1

Accipiter velox — Sharp-shinned Hawk. . .21 days 3

Sharp-shinned Hawk 21 days 44

Accipiter cooperi — Cooper's Hawk 24 days 3

Cooper's Hawk 24 days 44

Accipiter nisus — Sparrow Hawk 21 days 1

American Goshawk 28 ? days 44

Harris Hawk 28 ? days 44

Buteo borealis borealis— Red-tailed Hawk.28 days 3

Red-tailed Hawk 28 days 44

Red-tailed Hawk 32 days 194

Buteo borealis calurus — Western Red-
tailed Hawk 28 days 3

Western Red-tailed Hawk 28? days 44

Buteo lineatus — Red-shouldered Hawk. . .

27 to 28 days 3

Red-shouldered Hawk 28 ? days 44

Red-shouldered Hawk "less than 28 days" 57

Zone-tailed Hawk 28? days 44

Buteo swainsoni — Swainson's Hawk. . ..25-28 days 3

Swainson's Hawk . . . . 28 ? days 44

Swainson's Hawk about 25 days 58

Buteo platypterus — Broad-winged Hawk

23-25 days 3

Broad-winged Hawk 21*24 days 44

Buteo vulgaris — Buzzard 21 days 1

Buteo vulgaris — Common Buzzard 31 days 2

Urubitinga anthracina — Mexican Gos-
hawk (Black Hawk) 24-28 days 3

Mexican Goshawk 21-28 days 44

Archibuteo lagopus sancti-johannis —

Rough-legged Hawk 28 days 3

American Rough-legged Hawk 28? days 44

Archibuteo ferrugineus — Ferruginous

Rough-legged Hawk 28 days 3

Ferruginous Rough-legged Hawk 28? days 44

Ferruginous Rough-legged Hawk. ..about 25 days 59

Milvus ictinus — Kite "21 to 24 days group" 1

Pandion haliaetus carolinensis — Osprey .27-28 days 3

Osprey 21-28 days 44

84

Family — Species Period Authority

Fish Hawk 28 days 65

Osprey 24-28 ? days 66

Aquila chrysaetus — Golden Eagle 30 days 1

Aquila chrysaetos — Golden Eagle 35 days 3

Golden Eagle 28 days 44

Golden Eagle 25-35 days (prob. avg.) 30 days 60

Golden Eagle .' "at least 32 days" 61

Golden Eagle • 35 days 63

Golden Eagle 30 days 63

Aquila naevia — Spotted Eagle 21 days 1

Aquila imperialis — Imperial Eagle 35 days 1

Haliseetus albicilla — Grey Sea Eagle 30 days 3

Haliseetus albicillis — Gray Sea Eagle

"lasts about a month" 60

Haliseetus leucocephalus — Bald Eagle .... 30 days 3

Bald Eagle 28 days 44

Bald Eagle 36 days 57

Bald Eagle 30-36 days 60

Bald Eagle (in captivity) 31 days 99

Nesaetus fasciatus — Bornelli's Eagle 40 days 1

Vultur monachus — Monchsgeier 51 days 9

Vulture monachus — Kuttengeier 51 days 162

Gyps fulvus— Griff on Vulture 42 days 176

Gypaetus barbatus — Avvoltorio degli ag-

nelli (Lammergeier) 20 days 12

Cercaetus gallicus — Short-toed Eagle

about 28 days 1

Megapodidae

Lipoa ocellata— Malles Fowl .. "from 38 to 41 days" 191
Lipoa ocellata — Mound Bird

"takes a little over 5 weeks" 131

Lipoa ocellata rosinae — Malles Fowl . 58 to 77 days 142
Megapodius duperreyi — Scrub Fowl

from 5 to 6 weeks 14

Megapodius duperreyi — Scrub Fowl . about 6 weeks 131
Catheturus lathami— Bush Turkey . . . about 6 weeks 14
Catheturus lathami — Bush Turkey . . . about 6 weeks 131

Cracidae

Crax globicera — Globose Currassow

(Probably 28) 28-31 days 169

Crax carunculata— Yarrell's Hokko. . . .28-29 days 162

Phasianidae

Colinus virginianus — Virginian Colin

"toward 4 weeks" 1

Colinus virginianus — Bobwhite . . : 24 days 3

Bobwhite Quail 24 days 44

Bobwhite 24 days 45

85

Family — Species Period Authority

Bobwhite 24 days 35

Bobwhite (Quail) 23 to 24 days 95

Plumed Quail 21 days 44

Scaled Quail 21 days 44

Callipepla californica — California Part-
ridge 21 days 1

Lophortyx californica californica — Cali-
fornia Quail 24? days 3

California Quail 21-28 days 44

Lophortyx californica vallicola — Valley

Quail 24? days 3

Valley Quail 28 days 44

Lophortyx californica gambeli — Gambel's

Quail 24 days 3

Gambel's Quail 21-24 days 44

Caccabis rufa — Partridge (Red-Legged) .

23 & 24 days 1

Caccabis petrosa — Barbary Partridge 19 or 20 days 1

Partridge (Sp?) 24 days 33 .

Perdix perdix — Hungarian Partridge ....

21 (to 26?) days 46

Perdix perdix — Rephahn 24 days 162

Perdix Cinerea — Partridge 25, days 1

Coturnix communis — Quail 21 days 1

Synoecus australis — Brown Quail, under

hen 14-21 days 14

Coturnix novae-zealandiae — New Zealand

Quail 21 days 42

Coturnix novae-zealandiae — New Zealand

Quail 21 days 1

Tetrao tetrix— Black Grouse 26 days 1

Tetrao urogallus — Capercaillie 26 days 1

Dusky Grouse 18-24 days 44

Dendragapus obscurus obscurus — Dusky

Grouse 24 days 3

Sooty Grouse 18-24 days 44

Dendragapus obscurus fuliginosus — Sooty

Grouse 24 days 3

Canchites canadensis canace — Spruce

Grouse 17 days 180

Bonasa umbellus — Ruffed Grouse 21 days 1

Bonasa umbellus umbellus — Ruffed Grouse 21 days 3

Canada Ruffed Grouse 17 days 35

Ruffed Grouse 24-28 days 44

Ruffed Grouse 24 days 95

Bonasa umbellus togata — Canadian Ruffed

Grouse 21 days 3

Canada Grouse 17 ? days 44

Family — Species Period Authority

Canada Grouse 24-28? days 44

Bonasa betulina — Hazel Grouse 21 days 1

Lagopus lagopus lagopus — Willow Ptar-
migan 18 ? days 3

Lagopus lagopus — Schnee Huhn 26 days 9

Lagopus lagopus — Schnee Huhn 26 days 162

Willow Ptarmigan 17 days 44

Lagopus rupestris — Rock Ptarmigan 24 days 1

Lagopus scoticus — Hed Grouse 24 days 1

Lagopus albus — Willow Ptarmigan 24 days 1

Tympanuchus americanus americanus —

'Prairie Chicken 21 days 3

Prairie Hen 21-28 days 44

Heath Hen 24 days 47

Cupidonia cupido — Prairie Hen 18 or 19 days 1

Columbian Sharp-tail Grouse 21 days 44

Pediaecetes phasianellus columbianus —

Columbian Sharp-tail Grouse 21 days 3

Prairie Sharp-tail Grouse 21 days 44

Pediaecetes phasianellus campestris —

Prairie Sharp-tail Grouse 21 days 3

Centrocercus urophasianus — Sage Grouse . . 22 days 3

Tetrao urophasianus — Sage Grouse 21-22 days 1

Sage Hen 21 days 44

Sage Hen "about 3 weeks" 48

Meleagris gallopavo silvestris — Wild Tur-
key 28 days 3

Meleagris gallopavo — Turkey 26-28 days 1

Wild Turkey 28 days 44

Mexican Wild Turkey 28 days 44

Meleagris gallo-pavo — Domestic Turkey.. .28 days 1
Meleagris americana — Domestic Turkey . .

"four weeks of incubation" 49

Turkey 28 days 8

Turkey 28 days 33

Numida meleagris — Guinea Fowl 25 days 1

Numida meleagris — Guinea Fowl 28 ? days 51

Guinea 25 days 33

Guinea Hen 28 days 8

Ceriornis satyra — Horned Tragopan

(Horned Pheasant) 28 days 1

Ceriornis temmincki — Temminck's Trago-
pan 28 days 1

Lophophornis impeyanus — Himalayan

Monal 28 days 1

Crossoptilon mantchuricum — Mantchurian

Crossoptilon 28 or 30 days 1

87

Family — Species Period Authority

Euplocamus albi-cristatus — White-crested

Kaleege 26 days 1

Euplocamus nycthemerus — Silver Pheas-
ant ! 26 days 1

Euplocamus nycthemerus — Silver Pheas-
ant (under hen) 26 days 108

Euplocamus melanotus — Black-backed

Kaleege 24 days 1

Euplocamus Horsfieldi — Purple Kaleege. .24 days 1

Gennaeus, species of 24-25 days 97

Gennaeus nvcthemerus 26 days 162

Gennaeus lineatus 26 days 162

Gennaeus melanotus 26 days 162

Gennaeus albocristatus 26 days 162

Phasianus colchicus — Common Pheasant .. 23 days 1

Phasianus colchicus — English Pheasant 23-24 days 3

Phasianus mongolicus 24-25 days 162

Phasianus versicolor 26 days 162

Phasianus torquatus 26 days 162

Phasianus principals — Prince of Wales

Pheasant 24 days 108

Phasianus reevesi — Reeves's Pheasant 24 days 108

Syrmaticus reevesii 24-25 days 162

Syrmaticus reevesii — Reeves's 24 days 108

Pheasant (sp. ?) 25 days 33

Pheasant (sp?) 24 days 8

Pheasant (sp.?) 23-24 days 25

Pheasant (sp.?) 24 days 50

Pheasant Ring-neck .24 days 108

Syrmaticus wallichi — Cheer Pheasant 28 days 1

Calophasis mikado — Mikado 28 days 68

Calophasis ellioti 24-25 days 162

Thaumalea picta — Golden Pheasant 22 days 1

Golden Pheasant 21 days 108

Chrysolophus pictus — Golden Pheasant. 21-22 days 97

Chrysolophus pictus — Gold Fasan 23-24 days 162

Chrysolophus amherstiae — Lady Amherst

Pheasant " 22-23 days 97

Chrysolophus amherstiae — Amherst Fasan

23-24 days 162

Gallus various — Forked-tail Jungle Fowl.. 21 days 1
Gallus domesticus — Hen (Dorking var.) . .

20-21 days 1

Hen 21 days 33

Hen 21 days 8

Gallus domesticus — Bantam 21 days 1

Polyplectron chinquis — Peacock Pheasant 21 days 9

Polyplectron chinquis — Peacock Pheasant 21 days 1

Argus giganteus — Argus Pheasant 24 days 1

Family — Species Period Authority

Argusianus argus — Argus 26 days 9

Pavo cristatus — Peafowl 28 days 1

Peafowl 28 days 33

Peafowl 28 days 8

Rallidae

Rallus Crepitans — Clapper Rail 14 days 1

Porzana Carolina — Sora Rail 14? days 3

Porzana manietta — Spotted Crake 21 days 1

Porzana parvia — Little Crake 21-24 days 1

Crex pratensis — Corn Crake 21-24 days 1

Gallimila chloropus — Moor Hen 21 days 1

Porphyrio caeruleus — Purple Gallinule.23-25 days 130

Ocydromus anstralis — Weka Ralle 28 days 9

Eulabeornis Ypacha — Ypakaha Ralle 21 days 9

Fulica atra — Coot (European) 22 days 1

Fulica americana — Coot 14 days 3,

Gruidae

Grus eommunis — Crane 28 days 1

Griis japonensis — Mandschurischae Kran-

ich 30-31 days 9

Grus viridirostris — Mantchurian Crane ... 30 days 1
Grus canadensis — Kanadische Kranich . . . 33 days 162
Anthropides paradisea — Paradieskranich

34-35 days 9

Anthropides virgo — Jungfernkranich 28 days 9

Cariamidae

Cariama Cristata — Seriema 28 days 9

Otididae

Otis Tarda— Great Bustard "about 4 weeks" 1

Rhinochetidae

Rhinochetus jubatus — Kagu (in incuba-
tor) 36 days 10

Rhinochetus jubatus — Cagon (in captiv-
ity) ' 36 days 143

Eurypygidae

Eurypyga helias — Sun Bittern 27 days 1

Charadriidae

Recurivrostra avocetta — Avocet 17-28 days

Scolopax rusticula — Woodcock 20 days

Philohela minor— Woodcock 20-21 days

Gallinago major—Great Snipe 16 to 18 days

Gallinago caelestis — Common Snipe 20 days

Tringa alpina — Dunlin 22 days

Totanus hypoleucus — Common Sandpiper

22 days

Totanus calidris — Redshank 14 to 16 days

Totanus calidris — Redshank 23 days 2

Totanus calidris — Pettigola 23 days 12

Family — Species Period Authority

Totanus ochropus — Green Sandpiper.21 to 24 days 1

Machetes pugnax — Ruff 16 days 1

Bartramia longicauda — Bartram's Sand-
piper 17 ? days 3

Actitis mascularius — Spotted Sandpiper

15-16 days 3

Numenius arquata — Curlew 30 days 1

Haematopus vulgaris — Lapwing 25 to 26 days 1

Eudromias morinellus — Dotterel 18 or 20 days 1

Zonifer tricolor— Brust-Schild Kubitz ... .28 days 162

Charadrius pluvialis — Golden Plover 27 days 1

Aegialitis hiaticula — Ringed Plover

22 and 23 days 1

Aegialitis vociferus— Killdeer (about) .. .26 days 149
Oxyechus vociferus — Killdeer (probably

26y2-27i/2) ..28 days 172

Haematopus palliatus — Oystercatcher . . . 14 ? days 3
Recurvirostra ostralegus — Oystercatcher . .

23 to 24 days 1

Oedicnemidae

Oedicnemus scolopax — Stone Curlew 16 or 17 days 1
Laridae

Stercorarius catarrhactes — Great Skua

about 4 weeks 1

Megalestris maccormicki — Skua Gull 4 weeks 20

Rissa tridactyla — Kittiwake 26 days 1

Larus glaucus — Glaucus Gull 28 days 1

Larus Marinus — Mantel Mo we 26 days 9

Larus argentatus — Herring Gull 26 days 1

Larus argentatus — Herring Gull 26 or 27 days 3

Herring Gull one set 24 days 22

Herring Gull two sets 25 days 22

Herring Gull five sets 26 days 22

Herring Gull four sets 27 days 22

Herring Gull three sets 28 days 22

Larus ridibundus — Black-headed Gull . . .

22 and 23 days 1

Larus f ranklini — Franklin's Gull ... 18 or 20 days 3

Franklin's Gull 18 or 20 days 23

Sterna caspia — Caspian Tern about 20 days 1

Sterna Fluviatilis — Common Tern

almost exactly 21 days 1

Sterna Fluviatilis— Common Tern. . .22&23 days 1

Sterna hirundo — Common Tern 21 days 3

Sterna hirundo — Rondine di Mare 21-23 days 12

Sterna arctica — Arctic Tern 15 or 16 days 1

Sterna dougalli — Roseate Tern 21 days 3

Sterna minuta — Little Tern 14 to 16 days 1

90

Family — Species Period Authority

Sterna fuscata— Sooty Tern 26-29 days 3

Sooty Tern 26 days 24

Sterna fuliginosa — Sooty Tern 15 or 16 days 14

Hydrochelidon niger surinamensis — Black

Tern 17 to 19 ? days 187

Hydrochelidon niger — Black Tern ... 15 or 16 days 1

Noddy Tern 32-35 days 24

Anous stolidus — Noddy 35-36 days 3

Anous stolidus— Noddy Tern 35-36 days 106

Alcidae

Fratercula arctica — Puffin 36 days 1

Fratercula corniculata — Horned Puffin

(Est, by W.H.B.) 25-32 days 21

Cepphus grylle — Black Guillemot 21 days 3

Uria grylle — Black Guillemot 24 days 1

Lorn via troile — Guillemot 30 & 33 days 1

Alca torda — Razor-bill 30 days 1

Pteroclidae

Pteroclurus alchata — Pin-tailed Sand

Grouse "about 25 days" 1

Syrrhaptes paradoxus — Pallas's Sand

Grouse 28 days 1

Columbidae

Columba fasciata fasciata — Band-tailed

Pigeon 18-20 days 3

Band-tailed Pigeon 18-20 days 44

Red-billed Pigeon 18-20 days 44

Columba livia — Rock Dove 16 to 18 days 1

Columba livia — Domestic Dove 14 days 3

Columba livida— Tauben 17-18 days 9

Pigeon 18 days 8

Pigeon (almost exactly) 17 days 92

Columba domestica— Rock Dove .... "about 2 weeks" 52

Columba aenas — Stock Dove 17 to 18 days 1

Columba palumbus — Ring Dove 17 days 1

Ectopistes migratorius — Passenger Pigeon

16 days 1

Ectopistes migratorius — Passenger Pigeon

14 days 3

Passenger Pigeon 18-20 days 44

Passenger Pigeon ("almost to a day") 14 days 53

Zenaidura macroura carolinensis — Mourn-
ing Dove 12-14 days 3

Mourning Dove 13 days 92

Mourning Dove 14 days 44

Mourning Dove 14 days 97

Melopelia asiatica — White-winged Dove..

18 days 3

91

Family — Species Period Authority

White-winged Dove 18 days 44

Turtur risorius — Collared Turtle Dove. . .15 days 1

Turtur risorius — Ring Dove about 15 days 92

Turtur communis — Turtle Dove 16 to 17 days 1

Chaemepelia passerina terrestris — Ground

Dove 12 days 3

Ground Dove 14 days 44

Mexican Ground Dove 14 days 44

Ocyphaps lophote's — Crested Pigeon 14 days 1

Ocyphaps lophotes — Crested Dove, .about 14 days 97

Caloenas nicobarica — Nicobar Pigeon 28 days 1

Goura coronata — Crowned Pigeon 28 days 1

Crowned Pigeons (Gouridae) 28 days 116

Cuculidae

Geococcyx calif ornicus — Roadrunner 18 days 3

Coccyzus americanus americanus — Yellow
billed Cuckoo 14 days 3

Coccyzus americanus americanus — Yellow-
billed Cuckoo about 14 days 35

Coccyzus erythroplhalmus — Black-billed

Cuckoo 14 days 3

Cuculus canorus — Kuckuck 11 days 9

Cuculus canorus — Cuckoo 13 to 14 days 1

Cuculus pallidus — Pallid
Cuckoo about 12 or 14 days 14

Chalcococcyx lucidus — Broad-billed Bronze
Cockoo .' 19? days 14

Trichoglossidae

Trichoglossus novae-hollandiae — Swain-
son's Lorikeet 21 days 1

Psittacidae

Cacatua cristata — Great White-crested

Cockatoo 21 days 1

Cacatua goffini — Goffin's Cockatoo 21 days 1

Cacatua rosicapilla — Roseate

Cockatoo "about 21 days" 1

Colopsitta novae-hollandiae —

Cockateel "about 20 days" 1

Licmetis nasica — Long-billed

Cockatoo "about a month" 14

Licmetis tenuirostris — Slender-billed

Cockatoo "about 21 days" 1

Cyanorhamphus novae-zealandiae — New

Zealand Parrakeet "about 18 days" 1

Ara ararauna — Blue and Yellow

Macaw 20-25 days 1

Family — Species Period Authority

Melopsittacus imdulatus — Warbling Grass

Parrakeet "about 20 days" 14

Melopsittacus undulatus — Waved Parra-
keet .16 to 20 days 1

Neophema (?) venusta — Blue-winged

Grass Parrakeet 19-22 days 14

Euphema bourkei — Bourke's Parra-
keet "about 17 days" 1

Euphema Pulchella — Torquoisine Parra-
keet "about 18 days" 1

Euphema splendida — Splendid Grass

Parrakeet "about 18 days" . 1

Parrots — period of incubation is

approximately 21 days 10

Psephotus haematonotus — Blood-rumped

Parrakeet about 17 days 1

Lathamus discolor — Swift Parrakeet 21 days 1

Carolina Parrakeet (in captivity) 21 days 98

Parrots— Small 16-20 days 162

Parrots— Large 25-30 days 162

Coraciidae

Coracias garrulus — Roller 18 to 20 days 1

Alcedinidae

Ceryle alcyon — Belted Kingfisher 16 days 1

Ceryle alcyon— Belted Kingfisher 23-24 days 3

Alcedo insipida — Kingfisher 14 days 1

Halcyon vagans — New Zealand King-
fisher 17? days 42

Halcyon vagans — New Zealand King-
fisher 19 days 1

Halycon sanctus — Sacred King-
fisher about 17 days 14

Upupidae

Upupa epops — Hoopoe 16 days 1

Strigidae

Aluco pratincola — Barn Owl 21-24 days 3

Barn Owl 21 ? days 44

Barn Owl 3 to 3i/2 weeks' 57

Asio otus — Long-eared Owl 27 days 1

Asio wilsonianus — Long-eared Owl 21 days 3

American Long-eared Owl 21 ? days 44

Long-eared Owl "about 3 weeks" 30

Long-eared Owl "about 21 days" 35

Long-eared Owl "about 3 weeks" 57

Asio flammeus — Short-eared Owl 21 days 3

Short-eared Owl . ..21? days 44

Family — Species Period Authority

Scleoglaux albif acies — Laughing Owl .... 25 days 42

Barred Owl 21-28 days 44

Syrnium aluco — Tawney Owl 21 days 1

Cryptoglaux arcadia arcadia — Saw-whet

Owl 21? days 3

Saw-whet Owl 21 ? days 44

Otus asio asio — Screech Owl 21-26 days 3

Screech Owl 21 days 44

Screech Owl 25 days 67

Screech Owl 22 days 57

Macf arland's Screech Owl 21-28 ? days 44

Bubo virginianus virginianus — Great-horned

Owl 28-30 days 3

Great-horned Owl "probably about 4 weeks" 57

Great-horned Owl 21-28 days 44

Bubo virginianius pallescens — Western

Horned Owl 28 days 3

Western Great-horned Owl 28 ? days 44

Bubo virginianius pacificus — Pacific Horned

Owl 28 days 3

Bubo turcomanus — Turkman Uhu 27-33 days 9

Bubo Ignavus— Eagle Owl 21 to 24 days 1

Nyctea scandiaca — Snowy Owl 32 days 1

Speotyto cunicularia hypoga?a — Burrowing

Owl 21-28 days 3

Burrowing Owl 21 ? days 44

Athene noctua — Little Owl 14 to 16 days 1

Micropallas whitneyi — Elf Owl 14 days 3

Elf Owl .' 14? days 44

Caprimulgidae

Antrostomus vocif erus — Whip-poor-will. . . 17 days 3

Chordeiles virginianus — Nighthawk. 16 to 18 days 3

Chordeiles virginianus — Nighthawk. "a fortnight" 1

Caprimulgus europaeus — Nightjar 15 days 1

Trochilidae

Trochilus colubris — Ruby-throated Hum-
mingbird 10 days 1

Trochilus colubris — Ruby-throated Hum-
mingbird 14 days 188

Trochilus colubris — Ruby-throated Hum-
mingbird 14 days 65

Trochilus colubris — Ruby -throated Hum-
mingbird 15 days 179

Archilochus. colubris— Ruby-throated

Hummingbird 14 days 3

Archilochus alexandri — Black-chinned

Hummingbird 13 days 3

94

Family— Species Period Authority

Calypte costae — Costa's Hummingbird .... 14 days 3

Calypte anna — Anna's Hummingbird 14 days 3

Selasphorus rufus — Rufous Hummingbird.12 days 3

Florisuga atra — Black Hummingbird 12 days 1

Hummingbirds 12-14-18 days 71

Micropodidae

Cypselus apus — Swift 16 or 17 days 1

Cypselus melba — Alpine Swift

"a little over 2 weeks" 1

Chsetura pelagica — Chimney Swift 18 days 3

Chaetura pelagica — Chimney Swift 19 days 184

Chimney Swift * 22 days 194

Trogonidae

Hapaloderma narina — Narina Trogon 20 days 1

Picidae

Gecinus viridis — Green Woodpecker. . 16 to 18 days 1

Dendrocopus major — Great Spotted
Woodpecker 14 to 16 days 1

Dendrocopus medius — Middle Spotted
Woodpecker 15 days 1

Dendrocorpus minor — Lesser Spotted

Woodpecker 14 days 1

Dryobates villosus villosus — Hairy

Woodpecker 14 days 3

Dryobates villosus hyloscopus — Cabanis'
Woodpecker 15 days 3

Dryobates pubescens medianus — Downy

Woodpecker 12 days 3

Dryobates scalaris bairdi — Texas Wood-
pecker 13 days 3

Xenopicus albolarvatus — White-headed
Woodpecker 14 days 3

Picoides americanus americanus — Three-
toed Woodpecker 14 days 3

Sphyrapicus varius nuchalis — Red-naped

Sapsucker 14 days 3

Sphyrapicus ruber ruber — Red-breasted

Sapsucker 12-14 days 3

Phlaeotomus pileatus pileatus — Pileated
Woodpecker 18 days 3

Picus martius — Great Black Wood-
pecker 16 to 18 days 1

Melanerpes erythrocephalus — Red-headed

Woodpecker 14 days 3

Asyndesmus lewisi — Lewis's Woodpecker. . 14 days 3

Centurus carolinus — Red-bellied Wood-
pecker 14 days 3

Family — Species Period Authority
Centurus aurifrons — Golden-fronted Wood-
pecker 14 days '3

Centurus uropygilais — Gila Woodpecker .. 14 days 3

Colaptes auratus luteus — Flicker 11-14 days 3

Flicker 11-12 days 70

Flicker "about 16 days" 69

lynx torquilla — Wryneck 14 days 1

Tyrannidae

Muscivora forficata — Scissor-tailed Fly-
catcher 12-13 days 3

Tyrannus tyrannus — Kingbird 12-13 days 3

Kingbird 14 days 43

Tyrannus verticalis — Arkansas King-

'bird 12-13 days 3

Tyrannus verticalis — Arkansas Kingbird . . 14 days 78
Tyrannus vociferans — Cassin's King-
bird 12-14 days 3

Myiarchus crinitus — Crested Fly-
catcher 13-15 days 3

Myiarchus cinerascens cinerascens — Ash-
throated Flycatcher .' 15 days 3

Sayornis phoebe— Phoebe 12-14 days 3

Phoebe 15 days 72

Phoebe 15-16 days 189

Sayornis sayus — Say's Phoebe 12 days 3

Say's Phoebe 12? days 43

Nuttalornis borealis — Olive-sided Fly-
catcher 14 days 3

Myiochanes virens — Wood Pewee 12-13 days 3

Empidonax difficilis difficilis — Western

Flycatcher 12 days 3

Empidonax trailli trailli — Traill's Fly-
catcher 12 days 3

Empidonax trailli alnorum — Alder Fly-
catcher 12 days 3

Empidonax minimus — Least Flycatcher. . .12 days 3

Least Flycatcher 14 days 43

Empidonax wrighti — Wright's Fly-
catcher 12 days 3

Pyrocephalus rubinus mexicanus — Ver-
milion Flycatcher 12 days 3

Menuridae

Menura superba — Lyre Bird ("sometimes

extends over a month") 31 days 14

Lyre Bird "about a month" 25

Menura victorae — Victoria Lyre Bird

"about 5 or 6 weeks" 17

Menura victoriae — Victoria Lyre Bird ... 51 ? days 14

96

Family— Species Period Authority

Alaudidae

Otocoris alpestris leucolaema — Horned

Lark 11-14 days 146

Alauda arvensis — Sky Lark 14 days 1

Alauda arvensis — Lodola 13-14 days 12

Alauda arvensis — Skylark 13-14 days 2

Alauda arborea — Wood Lark about 15 days 1

Alanda cristata — Crested Lark

"about a fortnight" 1

Motacillidae

Motacilla alba— White Wagtail 14 days 1

Motacilla alba — Ballerina 14 days 12

Motacilla lugubris — Pied Wagtail 13 days 1

Motacilla lugubris — Pied Wagtail

"middle of the 14th day" 2

Anthus pratensis — Meadow Pipit. . . 13 and 14 days 1

Anthus trivialis — Tree Pipit 13 days 1

Pycnonotidae

Pycnonotus leucotis — Weissorbulbul 11 days 162

Muscicapidae

Muscicapa grisola — Spotted Flycatcher. . .13 days 1

Muscicapa atricapilla — Pied Flycatcher. . .14 days 1
Rhipidura tricolor — Black and White

Fantail 15 days 14

Rhipidura albiscapa — White-shafted

Fantail 16 days 14

Rhipidura albiscapa — White-shafted .

Fantail 12 days 17

Melurus superbus — Superb Warbler. about 14 days 17

Melurus cyaneus — Blue Wren 14 days 14

Gervgone albigularis — White-throated

Bush Warbler "about 12 days" 17

Melanodryas petroeca (?) — Hooded

Robin 16-17 days 14

Turdidae

Hylocichla mustelina — Wood Thrush 14 days 3

Wood Thrush 12 days 35

Hylocichla ustulata ustulata — Russet-backed

Thrush 14 days 14

Hylocichla ustulata swainsoni — Olive-backed

Thrush 10-13 days 3

Hylocichla guttata pallasi — Hermit

Thrush 12 days 3

Hermit Thrush 12 days 89

Turd us musicus — Song Thrush 15 days 1

Turdus viscivorus — Missle Thrush 15 days 1

Turdus merula — Blackbird .. . .14 and 15 davs 1

Family — Species Period Authority

Turdus merula — Blackbird 15 days 2

Merula merula — Merlo 15 days 12

Planesticus migratorius migratorius —

Robin 11-14 days 3

Robin (Eastern) exactly 14 days 90

Robin 13-14 days 72

Robin 14 days 91

Robin (Western) 14 days 43

Robin (Western) exactly 14 days 78

Accentor modularis — Hedge Sparrow 14 days 1

Ruticilla phoenicurus — Redstart 13 days 1

Ruticilla phoenicurus — Redstart 14th day 2

Ruticilla phoenicurus — Codirosso 14 days 12

Ruticilla titys— Black Redstart 13 days 1

Erithacus rubecula — Redbreast 14 days 1

Erithacus rubecula — Pettirosso 13-15 days 12

Erithacus rubecula — Redbreast 13-14 days 2

Daulias luscinia — Nightingale "a fortnight" 1

Sialia sialis sialis — Bluebird 12 days 3

Sialia sialis sialis — Bluebird 14-15 days 96

Mimidae

Mimus polyglottos polyglottos — Mocking-
bird 10 days 3

Mimus polyglottos — Mocking-bird 14 days 1

Dumetella carolinensis — Catbird 12-13 days 195

Catbird .more than 11 days 43

Toxostoma rufum — Brown Thrasher. . .13-14 days 196

Brown Thrasher 12 ? days 80

Toxostoma curvirostre curvirostre — Curved-
billed Thrasher 13 days 3

Cinclidae

Cinclus aquaticus — Dipper 15 days 1

Troglodytidae

Thryothorus ludovicianus ludovicianus —

Carolina Wren '. 12 days 3

Thryothorus ludovicianus miamensis —

Florida Wren 14 days 3

Thryomanes bewicki — Bewick's Wren. .10-15 days 3

Troglodytes ae'don aedon — House Wren 11-13 days 3

Troglodytes parvulus — Wren 13 days 1

Anorthura troglodytes — Scricciolo 10 days 12

Telmatodytes palustris palustris — Long-
billed Marsh Wren 10-13 days 3

Chamaeidae

Chamcea fasciata fasciata— Wren Tit. .15-18 days 128

98

Family — Species Period Authority

Sylviidae

Acrocephalus palustris — Marsh Warbler. . . 13 days 1
Acrocephalus turdoides — Great Reed

Warbler 14-15 days 1

Acrocephalus phragmitis — Sage Warbler. . 15 days 2

Sylvia curruca — Lesser Whitethroat — 12-14 days 1

Sylvia cinerea — Whitethroat 11-13 days 1

Sylvia atricapilla — Blackcap 14 days 1

Sylvia hortensis — Garden Warbler 15 days 1

Sylvia sylvia — Sterpazzola 10 days 12

Phylloscopus trochilus — Willow Wren 13 days 1

Phylloscopus ruf us — Chiff chaff 13 days 1

Sericornis frontalis — White-browed

Scrub Wren 21-23? days 14

Prinia maculosa — Capocier 14 days 1

Geobasileus reguloides — Buff-rumped

Thornbill "about 12 days" 17

Regulidae

Regulus cristatus — Goldcrest 12 days 1

Hirundinidae

Progne subis subis — Purple Martin 12-15 days 3

Martin 12-15 days 81

Pertocheildon lunifrons lunifrons — Cliff

Swallow 12-14 days 3

Hirundo erythrogastra — Barn Swallow. . .11 days 3

Hirundo erythrogastra — Barn Swallow. . .13 days 1

Hirundo rustica — Swallow 15 days 1

Hirundo rustica — Swallow

.."second half of the 15th" 2

Hirundo rustica — Rondine 15 days 12

Hirundo neoxena — Welcome Swallow 14 days 17

Iridoprocne bicolor — Tree Swallow ....... 14 days 3

Chelidon urbica — Martin 13 days 1

Cotile riparia — Sand Martin 12 or 13 days 1

Ampelidae (or Bombycillidae)

Bombycilla cedrorum — Cedar Wax-
wing 10-12 days 3

Cedar Waxwing (probably) 14 days 82

Bombycilla cedrorum — Cedar Waxwing. .16 days 170

Ptilogonatidae

Phainopepla nitens — Phainopepla 16 days 3

Artamidae

Artamus superciliosus — Wood

Swallow about 14 days 116

Laniidae

Lanius ludovicianus ludovicianus —

Loggerhead Shrike 12-13 days 3

Family — Species Period Authority

Lanius ludovicianus migrans — Migrant

Shrike 13-16 days 3

Lanius excubitor — Great Grey

Shrike 15 to 16 days 1

Lanius minor — Lesser Grey

Shrike " 15 to 16 days 1

Lanius collurio — Red-backed Shrike 14 days 1

Lanius pomeranus — Woodchat 15 days 1

Falcumculus frontatus— Shrike Tit 18-20 days 14

Vireonidae

Vireosylva olivacea — Red-eyed Vireo. . .12-14 days 3

Vireo srilvus — Warbling Vireo 12 days 1

Lanivireo solitarius solitarius — Blue-headed

Vireo 10-11 days 3

White-eyed Vireo 7? ? days 35

White-eyed Vireo 16 days 43

Sittidae

Sitta caesia— Nuthatch 13-14 days 1

Sitta canadensis — Red-breasted Nuthatch. . 12 days 3

Paridae

Panthestes atricapillus atricapillus —

Chickadee 11-14 days 3

Parus major— Great Tit "end of 14th day" 2

Parus caeruleus — Blue Titmouse 14 days 1

Parus palustris — Marsh Titmouse. "about 13 days" 1

Parus ater — Coal Titmouse 14 days 1

Acredula rosea — Long-tailed Titmouse.. 11-13 days 1

Panurus biarmicus — Bearded Titmouse — 14 days 1

Oriolidae

Oriolus galbula — Golden Oriole 15 days 1

Corvidae

Pica nistica — Magpie 18 days 1

Pica pica hudsonia — Magpie 16-18 days 3

Magpie "between 15 and 20 day's" 43

Cyanopolius cyanus — Chinese Blue

Magpie "about 18 days" 1

Garrulus glandarius — Jay 16 days 1

Cyanocitta cristata cristata — Blue Jay. .15-17 days 3

Cyanocitta cristata cristata — Blue Jay .... 17 days 190

Cyanocitta stelleri stelleri — Steller's Jay. .16 days 3
Cyanocitta stelleri frontalis — Blue-fronted

'Jay 16 days 3

Aphelocoma woodhousei — Woodhouse's

Jay 16 days 3

Aphelocoma calif ornica — California Jay . . 16 days 3
Aphelocoma sieberi arizonae — Arizona

Jay 16 days 3

100

Family — Species Period Authority

Perisoreus canadensis canadensis — Canada

Jay 16-18 days 3

Canada Jay ! 18? days 73

Corvus corax — Raven 18 to 19 days 1

Raven 18 to 19 days 25

Corviis corax principalis — Northern

Raven 20-21 days 3

Corvus cryptoleucus — White-necked

Raven 21 days 3

Corvus brachyrhynchos — Crow 18 days 3

Corvus ossif ragus — Fish Crow 16-18 days 3

Corvus frugilegus — Rook 17 and 18 days 1

Corvus cornix— LHooded Crow 18 to 20 days 1

Corvus corone — Carrion Crow 18 to 20 days 1

Nicifraga caryocatactes — Nutcracker. . .17-18 days 1
Nicifraga columbiana — Clark's Nut-
cracker 16-17 days 3

Nicifraga columbiana — Clarke's Nut-
cracker 22 days 129

Cvanocephalus cyanocephalus — Pinon

Jay *. 16 days 3

Sturnidae

Sturnus vulgaris — Starling 14 days 1

Sturnus vulgaris — Starling 11-14 days 3

Temenuchus pogadarum 14-16 days 116

Poliopsar malabaricus 14-16 days 116

Poliopsar andarrensis 14-16 days 116

Meliphagidae

Ptilotis notata — Yellow-spotted Honey

Eater .* 14 days 14

Ptilotis notata— Yellow-spot Honey Eater . 14 days 17
Zosteropidae

Zosterops coerulescens — White Eye. .about 10 days 14

Zosterops coerulescens — Silver Eye .... 9 to 10 days 1

Zosterops palpebrosa — White Eyes 10-11 days 116

Certhiidae

Certhia familiaris— Brown Creeper. . .12-13? days 88

Certhia familiaris — Tree Creeper 15 da}^s 1

Mniotiltidae

Protonotaria citrea — Prothonotary

Warbler .14 days 3

Helmintheros vermivorus — Worm-eating

Warbler 13 days

Vermivora pinus — Blue winged Warbler. .10 days 3

Blue-winged Warbler 10 days 83

Blue-winger Warbler 10-14 days 84

Vermivora chrysoptera — Golden-winged

Warbler . . . " 10 days 3

101

Family — Species Period Authority

Vermivora rubicapillus rubicapillus —

Nashville Warbler 11-12 days 3

Dendroica aestiva aestiva — Yellow

Warbler 10-11 days 3

Yellow Warbler 10 days 43

Yellow Warbler 11 days 85

Dendroica coronata — Myrtle Warbler. .12-13 days 3

Dendroica magnolia — Magnolia Warbler. .12 days 3

Magnolia Warbler 12-13? days 86

Dendroica pensylvanica — Chestnut-sided

Warbler . . .'. 10-11 days 3

Dendroica pensylvanica — Chestnut-sided

Warbler 13? days 183

Dendroica virens — Black-throated Green

Warbler 12 days 3

Black-throated Green Warbler 12-14 days 87

Dendroica palmarum hypochrysea — Yellow

Palm Warbler ! 12 days 3

Dendroica discolor — Prairie Warbler 14? days 3

Seiurus aurocapillus— Ovenbird 12 days 3

Seiurus noveboracensis — Water Thrush 14 days 1

Geothlypis trichas trichas — Maryland

Yellowthroat 12 days 3

Icteria virens virens — Yellow-breasted

Chat 15 days 3

Icteria virens virens — Yellow -breasted

Chat 12 days 1

Setophaga ruticilla — Redstart 12 days 3

Tanagridae

Scarlet Tanager 13 days 35

Pyranga rubra — Summer Tanager 12 days 1

Ploceidae

Lagonosticta minima — Blood

Finch "about a fortnight" 1

Lagonosticta minima — Blood Finch 12 days 116

Hypantica sanguinerostris — Blutschnabel-

weber 11 days 162

Poephila gouldie— Grass Finch 12-13 days 116

Icteridae

Dolichonyx oryzivorus— Bobolink 10 days 3

Molothrus ater — Cowbird "nearly a fortnight" 1

Cowbird 10-11 days 74

Molothrus ater ater — Cowbird 10 days 3

Argentine Cowbird 11% days 74

Xanthocephalus xanthocephalus — Yellow-
headed Blackbird 10? days 3

Yellow-headed Blackbird 10? days 75

102

Family — Species Period Authority

Agelaius phoeniceus phoeniceus — Red-winged

Blackbird 10-14 days 3

Strunella magna magna — Meadowlark. . 15-17 days 3

Strunella neglecta — Western Meadowlark. .15 days 3
Icterus cucullatus nelsoni — Arizona Hooded

Oriole 12-14 days 3

Icterus spurius — Orchard Oriole 12 days 3

Orchard Oriole 12-14? days 72

Icterus galbula — Baltimore Oriole 14 days 1

Icterus galbula — Baltimore Oriole 14 days 3

Icterus bullocki — Bullock's Oriole 14 days 3

Euphagus carolinus — Rusty Blackbird .... 14 days 3
Euphagus cyanocephalus — Brewer's Black-
bird 14 days 3

Brewer's Blackbird 12 days 43

Quiscalus quiscula quiscula — Purple

Grackle 14 days 3

Quiscalus quiscula rcneus — Bronzed

Grackle 13-16 days 3

Megaquiscalus major major — Boat-tailed

Grackle 15 days 3

Megaquiscalus major macrourus — Great-
tailed Grackle 15 days 3

Fringillidae

Hesperiphona vespertina vespertina —

Evening Grosbeak '. 13-14 days 3

Coccothraustes vespertina — Evening

Grosbeak . . , 14? days 76

Ligurinus chloris — Greenfinch 14 days 1

Fringilla coelebs — Chaffinch 12 days 1

Pyrrhula europaea — Bullfinch 13 days 1

Ery th rospiza githaginea — Trumpeter

Bullfinch "within a fortnight" 1

Carpodacus purpureus — Purple Finch 13 days 3

Carpodacus mexicanus frontalis — House

Finch exactly 14 days 77

Carpodacus mexicanus frontalis — House "

Finch 13? days 3

Loxia pityopsittacus — Parrot Crossbill . 14-15 days 1

Loxia cupvirostra — Crossbill 14 days 1

Linota cannabina — Linnet 14 days 1

Astragalinus tristis tristis — Goldfinch. .12-14 days 3

Carduelis elegans — Goldfinch 13-14 days 1

Toeniopygia castanotus — Chestnut-eared

Finch 14 days 14

Emblema picta — Painted Finch

"took exactly 14 days" 17

Chrysomitris spinus — Siskin 13-14 days 1

103

Family — Species Period Authority

Sycalis flaveola — Saffron Finch about 14 days 97

Serinus Canarius — Canary 13-14 days 1

Serinus Canarius — Canary exactly 14 days 78

Canary 13-14 days 54

Canary 14 days 8

Serinus canarius — Canary 14 days 171

Serinus hortulanus — Serin 13 to 14 days 1

Passer domesticus — House Sparrow.13 and 14 days 1
Passer domesticus — European House

Sparrow 12-14 days 3

Passer domesticus — English Sparrow.. .12-13 days 104

Passer luteus 13-14 days 116

Plectrophenax nivalis nivalis — Snow

Bunting 21 ? days 3

Poaecetes gramineus gramineus — Vesper

Sparrow 11-13 days 3

Passerculus sandwichensis savanna —

Savannah Sparrow 12 days 3

Chondestes grammacus strigatus — Western

Lark Sparrow 12 days 3

Zonotrichia albicollis — White-throated

Sparrow 12-14 days 3

Spizella passerina passerina — Chipping

Sparrow 10-12 days 3

Chippy 10 days 72

Spizella pusilla pusilla — Field Sparrow ... 13 days 3

Field Sparrow "12 days or more" 43

Junco hyemalis — Slate-colored Junco... 11-12 days 3
Melopsiza melodia melodia — Song

Sparrow 10-14 days 3

Melopsiza georgiana — Swamp Sparrow. . .13 days 3
Passerella iliaca schistacea — Slate-colored

Fox Sparrow 12-14 days 3

Papilo erythrophthalmus erythroph-

thalmus— Towhee 12-13 days 197

Cardinalis cardinalis cardinalis —

Cardinal 12 days 3

Cardinalis cardinalis cardinalis —

Cardinal 12-14? days 79

Paroaria cucullata 14 days 116

Paroaria larvata 14 days 116

Subernatrix cristata 14 days 116

Volatinia jacarini 10 ? days 116

Zamelodia ludoviciana — Rose-breasted

Grosbeak 14 days 3

Rose-breasted Grosbeak 9 ? ? days 43

Coryphospiza cruentus — Purpurkronfink. .11 days 162

Passerina cyanea — Indigo Bunting 12 days 3

104

Family— Species Period Authority

Indigo Bird 10? days 35

Passerina amaena — Lazuli Bunting 12 days 3

Passerina eiris — Nonpareil 14 ? days 80

Emberiza citrinella — Yellow Hammer 14 days 1

TABLE NO. 2— BIBLIOGRAPHY

1. Evans— Ibis, Vol. Ill, p. 52.

2. Evans— Ibis, Vol. IV, p. 55.

3. Burns— Wilson Bull. No. 90, p. 275.

4. Gadow— Encyclop. Brit, Edit. XI, Vol. Ill, p. 976.

5. A. O. U. Check List, Third Edit. 1910.

6. "Arizona" (Phoenix, Ariz.), Vol. IV, No. 3, p. 5.

7. U. S. Dept, Agric. Yearbook, 1905, p. 400.

8. Am. Poult. Jour. Yearbook, 1914, Breeder's Table.

9. Die Vogel, Reichenow, Band I.

10. Knowlton, Birds of the World, 1909.

11. Coues— Key to North Am. Birds, 1884.

12. Arrigoni-Uccelli Europei, 1902.

13. Proc. Zool. Soc., London, Apr. 21, 1885, p. 325.

14. Campbell, A. J. — Nests and Eggs of Australian Birds.

15. International Encyclop. Vol. XIV, p. 554.

16. Shufeldt— U. S. Nat, Mus. Report, 1892, p. 462.

17. North — Nests and Eggs of Birds of Australia and
Tasmania.

18. Youth's Companion, May 28, 1914, p. 287.

19. Bird-Lore, Jan.-Feb. 1915. p. 69.

20. Levic — Antarctic Penguins, 1914.

21. Willet— Auk, Vol. XXXII, p. 297.

22. Butcher and Bailey— Auk, 1903, p. 431.

23. Roberts— Auk, July, 1900, p. 279.

24. Watson— Auk, Apr. 1909, p. 213.

25. Newton— Dictionary of Birds, 1893.

26. Gross— Auk, Jan. 1912, p. 58.

27. Plath— Bird-Lore, Vol. XV, No. 6, p. 347.

28. Gurney— The Gannet.

29. Birds of New Zealand, Vol. II, p. 181.

30. Barrows— Birds of Michigan, 1912.

31. Harper— Bird-Lore, Sept.-Oct,, 1914, p. 341.

32. Strong— Auk, Oct. 1912, p. 482.

33. U. S. Dept. Agric. Farmers Bull. No. 236, p. 8.

34. U. S. Dept, Agric. Farmers Bull. No. 64.

35. Dugmore — Bird Homes.

36. Encyclop. Britt. Edit. XI, Vol. 26, p. 180.

37. Forbush— Game Birds, p. 220.

38. H. Milne — Edwards — Lecon, sur la physiologic et an-
atomie .... animeau.

39. Butch— Bird-Lore, Mch.-Apr., 1915, p. 108.

105

40. Chamberlain-B. Rhett — Personal communication.

41. Cameron— Auk, July, 1906, p. 254.

42. Hutton and Drummond — Animals of New Zealand,
p. 199.

43. Saunders, A. A. — Personal communication.

44. Bendire— U. S. National Mus. Special Bull. No. 1.

45. Hodge, C. F.— Nature Study Pamphlet, Bobwhite, p.
13.

46. U. S. Dept, Agricult. Yearbook, 1909, p. 252.

47. Field— Bird-Lore, Vol. XV, No. 6. p. 356.

48. U. S. Dept, Agricult, Bull. No. 24, Biological Survey,
p. 24.

49. U. S. Dept. Agricult, Farmers Bull, No. 200, p. 23.

50. Tegetmier — Pheasants, p. 14.

51. U. S. Dept. Agricult. Farmers Bull. No. 234, p. 12.

52. Townsand— Auk. Vol. XXXII, p. 310.

53. Deane— Auk, July, 1896.

54. Powell, Mrs. Cuthbert — Personal communication.

55. Townsend— Bird-Lore, Vol. XVI, No. 4, p. 279.

56. Sherman— Auk, Vol. XXX, p. 408.

57. U. S. Dept. Agricult. Bull. No. 3.

58. Cameron— Auk, Vol. XXX, p. 170.

59. Cameron— Auk, April, 1914, p. 160.

60. Fisher— U. S. Dept. Agricult, Bull. No. 27, Biological
Survey.

61. Cameron— Auk, 1908, p. 253.

62. Cameron— Auk, Vol. XXXI, p. 160.

63. Findley— American Birds, p. 245.

64. Kobbe— Auk, 1900, p. 15.

65. Chapman — Birds of Eastern North America, 1912, p.
68.

66. Abbott — Home Life of the Osprey.

67. Sherman— Auk, April, 1911, p. 158.

68. Beebe— Bull. N. Y. Zool. Soc., Vol. XVII, p. 1906.

69. Sherman— Auk, Vol. XXX, p. 412.

70. Sherman- Wilson Bull. No. 72, p. 143.

71. Ridgway— U. S. Nat. Mus. Report, 1890, p. 284.

72. Nichols, J. T. — Personal communication.

73. Warren— Auk, Jan., 1899, p. 17.

74. Bendire— U. S. Nat. Mus. Report, 1893.

75. Roberts— Auk, 1909, p. 377.

76. Fleming— Auk, April, 1903, p. 214.

77. Bergtold— Auk, Vol. XXX, p. 55.

78. Bergtold — Personal observation.

79. Harvey— Auk, Jan., 1903, p. 56.

80. Weston, F. M. — Personal communication.

81. Jacobs — Story of a Martin Colony.

82. Saunders— Auk, July, 1911, p. 325.

83. Bowdish— Auk, Jan., 1906, p. 16.

106

84. Wright— Auk, Oct., 1909, p. 345.

85. Auk, Oct., 1913, p. 604 (quoting Biggleston, Wilson
Bull., Vol. XXV, p. 49).

86. Stanwood— Auk, Oct., 1910, p. 386.

87. Stanwood— Auk, July, 1910, p. 291.

88. Bluebird— Vol. VII, No. 4, p. 101.

89. Stanwood — Nature and Culture, May, 1913 (vide Auk,
Oct., 1913, p. 613).

90. Judson— Bird-Lore, Vol. XVII, No. 3, p. 213.

91. Tyler— Auk, Vol. XXX, p. 395.

92. Cole and Kirkpatrick— Rhode Island State College,
Exp. Station Bull. 162.

93. N. Y. Evening Post, Nov. 6, 1915, "Penguins in Am.
Mus. Nat. Hist."

94. Crandall, Lee S.— BuU. N. Y. Zool. Soc., Vol. XVHI,
No. 5, p. 1262.

95. Job — Propagation of Wild Birds, 1915.

96. Boulton— Bird-Lore, March-April, 1916, p. 123.

97. Crandall, Lee S. — Personal communication.

98. Nowotny— Auk, 1898, p. 30.

99. Crandall— Bull. N. Y. Zool. Soc., Oct., 1909, p. 583.

100. Beebe— Bull. N. Y. Zool. Soc., Jan., 1909, p. 466.

101. Claus, Zoologie, Band ii, p. 34.

102. Fiirb ringer- Untersuch. zur Morph Vogel.

103. Standard Encyclop., Vol. IX, p. 395.

104. Wood— Am. Encyclop. Ophthalmology, Vol. IV, p.
2585.

105. Knight — Birds of Maine (quoted by>Burns (3) ).

106. Thompson— Bird-Lore, Vol. V, p. 80.

107. Beebe— Zoologica, Vol. I, p. 242.

108. Kendrick, W. F. — Personal communication.

109. Kendrick, W. F., and Bergtold, W. H.— Personal ob-
servation.

110. Ingersoll— Harper's Monthly, Vol. XCVE, p. 40.

111. Rubincam, H. C. — Personal' communication.

112. Sutherland— Proc. Zool. Soc., London, 1899, p. 787.

113. Collins, E. W., and Bergtold, W. H.— Personal ob-
servation.

114. Beebe— Bull. N. Y. Zool. Soc., July, 1904, p. 166.

115. Pycraft— Encyclop. Brit., Edit. XI, Vol. 9, p. 14.

116. Page — Aviaries and Aviary Life.

117. Renshaw— Aviculture Mag., Vol. 7, No. 3, p. 82.

118. Heilmann-Dansk Ornith. Forening. Tidesk., March,
1915, p. 147.

119. Hornaday — American Natural History.

120. Pycraft — Infancy of Animals.

121. Denby— -Nature, "Vol. LIX, p. 340.

122. California Alligator Farm, Los Angeles — Personal
communication.

107

123. Detmers— Keptile Book.

124. Detmers— Zoologica, Vol. I, p. 237.

125. Detmers— Bull. N. Y. Zool. Soc., July, 1904, p. 158.

126. Hawes and Swinnerton — Trans. Zool. Soc., London,
Feb., 1901.

127. Jewett— Condor, March, 1916, p. 75.

128. Newberry— Condor, March, 1916, p. 65.

129. Skinner— Condor, March, 1916, p. 64.

130. Whitaker— Ibis, 1899, p. 502.

131. Le Souef— Ibis, 1899, p. 9.

132. Gurney— Ibis, 1899, p. 19.

133. Oustalet— La Nature, 1900, p. 378.

134. Murphy, R. C. — Personal communication.

135. Beebe— Bull. N. Y. Zool. Soc.. Jan., 1906, p. 258.

136. La Nature, July 6, 1912.

137. Beetz and Townsand— Auk, July, 1916, p. 287.

138. Pycraft^-History of Birds, 1914.

139. Mills— Animal Physiology, p. 402.

140. Curtis — Maine Agric. Exp. Station, Bull. No. 228,
June, 1914.

141. Pickerell, Arizona Ostrich Farm — Personal communi-
cation.

142. Bellchambers — So. Australian Ornithologist, April,
1916 (vide Auk, July, 1916).

143. Menegaux — Revue Francais d'Ornith., Oct., 1915 (vide
Auk, Jan., 1916).

144. Internat. Encyclop., Vol. VIII, p. 660.

145. Ridgway — Man. No. Am. Birds, 4th Edition.

146. Macdonald— Auk, Oct., 1916, p. 435.

147. Smith, R. W. — Physiology of Domestic Animals.

148. Howell— Text Book of Physiology, 1915.

149. Bates— Wilson Bull., No. 96, p. 151.

150. Gadow— Bonn's Tier. Reich, Vogel, VI. 4, p. 698.

151. International Encyclop., Vol. I, p. 518.

152. Phillips, Clyde and Bergtold, W. H.— Personal ob-
servation.

153. Morgan — So. Australian Ornithologist, July, 1916.

154. Beebe— Bull. N. Y. Zool. Soc., Oct., 1908, p. 459.

155. Encyclop. Britt,, Edit. IX, Vol. XX, p. 411.

156. Pombery — Schafer's Physiology, Vol. I, p. 785.

157. Fuertes, Louis — Personal communication.

158. Mousley— Auk, July, 1916, p. 281.

159. Beebe— Bull. N. Y. Zool. Soc., Jan., 1908, p. 397.

160. Pickerell— U. S. Dept, Agric. Yearbook, 1905, p. 401.

161. Hatch — Birds of Minnesota, ,p. 73.

162. Heinroth— Zool. Beobachter, Vol. XLIX, No. 1, p. 14.

163. Blaauw— Avicult. Mag., Vol. VII, No. X, 1916.

164. Simpson— Trans. Royal Soc. Edinburgh, Vol. XL VII,
part 1, No. 5, p. 607.

108

165. Simpson— Proc. Royal Soc. Edinburgh, Vol. XXXII,
part 1, No. 5, p. 19.

166. Simpson— Proc. Royal Soc. Edinburgh,, Vol. XXXII,

gart 1, No. 11, p. 10.
impson and Galbraith — Jour. Physiology, Vol. 33,
p. 225.

168. Crisp, E.— Proc. Brit. Soc. Adv. Sci., 1864 and 1865,
p. 92.

169. Wilbur, A. P., and Watts, E. A. — Personal communi-
cation.

170. Post, K. C.— Wilson Bull., Dec., 1916. p. 178.

171. Wetmore, Alex.— U. S. Dept. Agric. Farmers Bull.,
No. 770, p. 16.

172. Sherman, A. R.— Wilson Bull., Dec.. 1916, p. 196.

173. Spohn and Riddle— Am. Jour. Physiology, Vol. XLI,
No. 3, p. 408.

174. Buchanan — Jour. Physiology (Cambridge), Vol.
XXXVIII, 1909, p. 62-66.

175. Colo. Mus. Nat. Hist., F. C. Lincoln — Personal com-
munication.

176. Evans— Birds, 1898.

177. Oberholser— U. S. Nat. Mus. Bull. 86.

178. Meyer— Auk, 1916, p. 82.

179. Hitchcock— Bird-Lore, March-April, 1917, p. 79.

180. Eaton— Birds of New York.

181. Lanning— Wild Life in China, 1911.

182. Colo. Mus. Nat. Hist, R. J. Niedrach— Personal com-
munication.

183. Wilbur— Youth's Companion, Feb. 22, 1917, p. 111.

184. Day— Bird-Lore. June, 1899, p. 78.

185. Lucas— Auk, Vol. IV, p. 2.

186. Campbell— Bird-Lore, Jan.-Feb., 1903, p. 5.

187. Seton and Chapman— Bird-Lore, Jan.-Feb., 1904, p. 1.

188. Lemmon— Bird-Lore, May- June, 1901, p. 108.

189. • Whitten— Bird-Lore, May- June, 1902, p. 95.

190. Lemmon— Bird-Lore, May-June, 1904, p. 89.

191. Campbell— Bird-Lore, Jan.-Feb., 1903, p. 5.

192. Jackson— Bird-Lore, Nov.-Dec., 1903, p. 184.

193. Moore— Bird-Lore, Sept-Oct., 1902, p. 162.

194. Hegner— Bird-Lore, Sept.-Oct., 1906, p. 154.

195. Shufeldt— Auk, July, 1893, p. 304.

196. Heil— Bird-Lore, May-June, 1908, p. 101.

197. Heil— Bird-Lore, July-Aug., 1909, p. 158.

ERRATA—

Pages 52 to 57, inclusive— Read "Temperature" for "Weight"

at heads of second columns.

Page 100, line 9— Read"Falcunculus" for "Falcumculus."

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