/Remoirs of tbe /IDuseum ot Comparative ZoiJlog?

AT HARVARD COLLEGE.
Vol. XL. No. C.

BREWSTER\S WARBLER

{HELMINTH OFHILA LE UCOBR ONCHIALIS )

A HYBRID BETWEEN
THE GOLDEN-WINGED WARBLER (HRLMIXTHOPHILA CHRYSOPTERA)

AND

THE BLUE-WINGED WARBLER {HELMINTHOPHILA PINUS).

BY
WALTER FAXON.

CAMBRIDGE, U. S. A.:

IPr(nte& for tbe /Uuseurn,

August, 1913.

/Demolrs of tbe /Duseum of Comparattpe ZoiJlofl?

AT HARVARD COLLEGE.
Vol. XL. No. (i.

BREWSTER^S WARBLER
(HEL3IINTH0PHILA LEUCOBRONCHIALIS)

A HYBRID BETWEEN
THE CTOLDEN-^YINGED WARBLER {HELMIXTHOPHILA CHRYSOPTERA)

AND

THE BLUE-WINGED WARBLER {HEUIIXrilOPHILA PIXUS).

BY
WALTER FAXON.

CAMBRIDGE, U. S. A.:

IPrtnteJ) for tbe /IMiseum.

August, 1913.

BRE WS^n<; R S WAR B LER ( HELMINTIIOI'IIILA L E U( 'OBlxONCHIA-
LIS) A HYBRID BETWEEN THJ-: GOLDEN-WINGED WARBLER
(HELMINTHOPHILA CHRY SOFT ERA) AND Tim BLUE-
WINGED WARBLER {HELMINTHOPHILA PINUS).

The real nature of Brewster's Warl)lcr lias long been a moot point with
ornithologists. Is it a true species, a mongrel, a color-phase of the Blue-winged
Warbler, or an atavistic form of the Golden-winged Warbler? Each of the four
propositions implied in this question has found advocates among the various
authors who have written on the topic, but until now no indubitable proof of
the true status of this bird has been obtained.

In Januarv, 1911,1 published a paper bearing on this subject, in the Memou-s
of the Museum of Comparative Zoology, 40, no. 2; I will here give a resume
of the facts recorded in it, a course which seems desirable because these facts
were misstated by the reviewer of my article, in The Auk for April, 1911.

In the summer of 1910, there bred within the confines of a swamp of about
fifteen acres in Lexington, Mass., a pair of Golden-winged Warblers, and two
male Golden-winged Warblers mated with two female Brewster's Warblers.
In the same swamp there was also a male Brewster's Warbler that unquestion-
ably was unmated. The progeny of the three pairs were closely observed from
the juvenile (in one case, from the natal) plumage up to tlie first winter plumage,
when the adult characters were acquired; the young of the pair of Golden-wings
were all Golden-wings; one of the Brewster's W'arblers that was mated witli a
Golden-wing brought forth a homogeneous brood of Brewster's Warblers, while
the other produced a mixed brood of Brewster's Warblers and at least one Golden-
winged Warbler. A striking thing about it was this : the j'oung birds of mixed
parentage were absolutely pure in plumage, — either Brewster's Warblers or
Golden-wings, without any tendency to combine, as "intermediates," the char-
acters of the two parents. They appeared to exemplify the transmission of
characters in accordance with JNIendel's Law, and from that time I had little
doubt that Brewster's Warbler itself would pr()\e to be a result of the union

312 BREWSTER'S WARBLER.

of flic Golden-winged Warbler and the Blue-winged Warbler, — a Mendelian
so-called dominant hybrid, a8 Mr. J. T. Nichols suggested in The Auk, Jan.,
1908, 25, p. 86.

Since my memoir was published, Golden-winged Warblers and Brewster's
Warblers have bred every year in the Lexington swamp. In 1911 Dr. W. M.
Tyler and I devoted a good deal of time to them, but without any very definite
results. There were during that season at least one male Brewster's Warbler,
and one female Brewster's Warbler, together with approximately three male
and three female Golden-wings; both Golden-wings and Brewster's Warblers
were reared by these buxls, but unfortunately we failed that summer to discover
how the old birds were paii-ed off.

In the sunmier of 1912 I lived so far away from Lexington that I was unable
to pursue the study of this interesting little colony of birds to any advantage.
However, in a visit to the place on the 27th of May, I found a male Brewster's
Warbler in song,^ and on the 4th of July Dr. Tyler saw a male Golden-wing
and a female Brewster's Warbler feeding a brood of young birds, one at least
of which was a young Brewster's Warbler, while in another part of the swamp
he met with a female Golden-wing with a brood containing Golden-wings and
Brewster's Warblers assuming their first winter plumage.

In the spring of 1913 I resumed my residence in Lexington and, in associa-
tion with Dr. Tyler, continued the study of the Warbler colony. The Golden-
winged Warbler arrived this year on the 6th of May, a rather early date for this
speqies." The population of the swamp, so far as the birds were concerned in
whom we were especially interested, consisted of a male Golden-wing, a female
Golden-wing, a male Brewster's Warbler, and a female Blue-winged Warbler.
The male Golden-wing was mated with the female Blue-wing, the male Brews-
ter's Warbler with the female Golden-wing. No nests were found, and the
manner in which the birds were paired ofT together was not determined till the
young birds had left the nests; in fact neither of the females was seen until then,
about the middle of June. All of the old birds were perfectly typical in plumage,
the male leucobrojichialis being one of the pure-p/lumaged specimens that show
not tlie faintest trace of yellow on the under parts. Here at last we had the
combination we had so ardently wished for, but hardly hoped to find, — the
Golden-wing mated with the Blue-wing, — and we now followed with keen inter-

' I take this opportunity to record anotln-r male Brewster's Warbler which I saw in full song near
Waldon Pond, Concord, Mass., on May 10, 1912.

' The average date of arrival of this bird in the neighborhood of Boston, based on my records for
24 years, is May 11. The earliest date within that period is May 3, 1905, the latest May 15, 1908.

RRFAVSTER'S WARBLER. 313

est the srowtli of the young of this pair in their slow progress from tlie nesthng
plumage to the first winter ])lumage. When first seen, on the 15th of June,
these young birds were probably not more than two days out of the nest, and
both Dr. Tyler and myself saw, or thought we saw, a deeper yellow tint on
their breasts and bellies than we had hitherto seen on young chrysopterae and
leucobronchiales of the same age; by the 20th of June, however, this yellow
tinge was much less pronounced, and by the 25th of the month, when the tails
of the little birds were about three quarters of their full length, the yellow was
scarcely apparent, the young still wearing the juvenile dress. Dr. Tyler had
fastened a platinum band on the leg of one of the little birds of this family on
the^ 15th of June; we were thus enabled to note the change of color in the same
individual as the days wore on, — a change doubtless due to the loss of the
fugacious, veiling, yellow tips of the juvenile feathers.

On the 27th of June we saw the first signs of the post-juvenile moult in
the loss of one set of wing-coverts with its yellowish bar. By the first of July
the contour feathers of the juvenile plumage were being extensively replaced
by those of the first winter plumage. As in former seasons (1910, 1911) amarked
difference was apparent in the amount of yellow on the under parts of the dif-
ferent indivitluals of the same brood. In at least one of the brood now under
consideration the yellow tint was very faint and it was clear even at this early
date that this young one was a leucobronchialis ; in other members of this same
brood the second winter plumage displayed a strong yellow tint on the throat
and along each side of the breast and belly, following the area of the ptcryla
vrntralis and leaving an ash-colored longitudinal band along the median line
of the abdomen, caused by the retention of some of the juvenile set of feathers.
As time went on, the yellow color gradually became fainter and restricted for
the most part to the breast, leaving the throat and abdomen ashy white; the
growth of a black trans-ocular stripe and yellow wing-coverts now perfected
the garb of Hclminlhophila leucobronchialis. On the 12th of July at least two of
this brood of young birds were as slightly tinged with yellow as the young
leucobronchialis figured on the plate of my memoir of 1911 (fig. 1). Not one
of this brood developed the least trace of the black throat and broad cheek-
patch of H. chrysoptera.

As I have previously said, the varying amount of yellow, at the same date,
in different young Brewster's Warblers of the same brood was noticeable in
all of tlic years in which we observed the growth of the winter plumage in these
birds. This may be the result of different degrees of precocity in the individ-

314 BREWSTER'S WARBLER.

uals composing the brood, or it may be connected with sex, since the female
of Brewster's Wai-bler, I think, always retains a tinge of yellow on the breast,
even in the adult. I fancy the rapid loss of the veiling yellow feather-tips is
occasioned by their disintegration and dropping off rather than by mere mechani-
cal abrasion and bleaching.

When the female pinus was first observed, although she was a typical pinus
in iiluniage, both Dr. Tyler and I surmised that she was a so-called impure iiinus
in blood, produced in accordance with the Law of Mendel from one of the many
crossings of chrysoptera with IcucobronchiaUs that, to our personal knowledge,
have occurred in this locality during the last four years; in other words, that she
belonged to the same family stock as the chrysopkrae and leucobronchiales in the
swamp. We even calculated, with lively anticipation, the chances of .seeing a
Lawrence's Warbler among her offspring, — a more than even chance provided
both she and her Golden-winged mate belonged to the mixed stock and raised
a brood of four or five young. This expectation (like many a hope staked on
young promise for its fulfilment) was not realized; all the young birds grew up
to be Brewster's Warblers; yet we had the full satisfaction of denK)nstrating
the true nature of Brewster's Warbler and removing the question forever from
the realm of conjecture. That all of the offspring of this pair of Inrds were
Brewster's Warblers would indicate that both of the parents were of pure blood.
By the Mendelian Law of transmission a pure chrysoptera mated with a pure
pinus should produce nothing but leucobronchiales; a pure chrysoptera and an
im]nu-c pinus will produce, on the average, chrysoptcrae and leucobronchiales
in equal numbers; an impure chrysoptera and a jiure pinus, in like manner,
pinus and leucobronchiales in the same proportion; while an imj^ure chrysoptera
united with an impure pinus would give rise to chrysopterae, pinus, leucobron-
chiales, and laivrencei in ecjual proportions.

It is interesting to recall to mind in this connection, that Dr. Tyler saw a
male Blue-winged Warbler near his house on the 6th of May of this year (Auk,
July, 1913, 30, p. 435); this bird sang the normal song of the Blue-winged
Warbler, — pretty sure evidence that he was a stranger from tlic South and not
a member of the native Lexington colony of mixed breed, all of whom without
exception sing the Golden-wing's song. It is highly j^robable that on the night
of the 5th-6tli of May, when there was a heavy migration of W^arblers in this
region, a small flight of Blue-winged Warblers invaded eastern Massachusetts
and that the female Blue-wing that mated with the Golden-wing in the Lexington
swamp came in with that flight.

RRKWSTEH'S WARIiLER. 315

The history of iho otlior i)air in tho swamp, a male Brewster's Warbler and
a female Ciolden-win^', may l)p told in a lew words. As in one of tlie two cases
of a male Golden-wing joined with a iVnialc Brewster's Warbler, considered in
detail in my memoir published in 1911, a majority of their issue were Brewster's
Warblers, but one of them a male (!oIden-wing. Dr. Tyler banded two of the
little birds belonging to this brood on the 19th of June, when they were but a day
or two out of the nest and as like each other as two peas from one pod; one of
these grew up to be a typical Brewster's Warbler while the other, its own brother,
became a typical male Golden-wing. If any of the birds that were banded
(three in all) return and breed in their native place next summer, we may be
able to establish a family pedigree for these interesting hybrids, extending
through three generations, complete as regards both the male and the female
lines.

In nn' paper published in 1911, after stating the different hypotheses pro-
posed in order to explain the relations existing among the Golden-winged, Blue-
winged, Brewster's, and Lawrence's Warblers I added, half in jest, that the only
hypothesis left for a new-comer in the field was this: that the Golden-winged
and the Blue-winged Warblers themselves were merely two forms of one species.
Curiously enough, not long after this I found that this very opinion had been
expressed, and in a most unexpected quarter: in a letter dated Edinburgh,
Sept. 15, 1835, Audubon wrote to Bachman that he suspected the Golden-
W'inged Warbler and the Blue-winged Warbler were one species ! ' That Audubon
at that early date, ignorant (as he was assumed to be) of the existence of
Brewster's and Lawrence's Warblers, and but superficially acquainted with the
Golden-wing, should suspect that two birds so diverse as the Blue-wing and
the Golden-wing were one species seemed incomprehensible, and in the light of
what we now know about these birds, his surmise seemed to presuppose an
almost superhuman faculty of prevision.

As a possible explanation of Audubon's letter I have only this to offer:
in the winter of 1876-77 Dr. Spencer Trotter - discovered in the collection of the
Academy of Natural Sciences of Philadelphia a specimen of Brewster's Warbler
without a label, the third specimen known up to that time; on the bottom of

'This letter is amony; the many unpuhhshed MS. letters of Audubon in the Wade eoUeetion,
generously presented to this Museum by Mr. John E. Thayer.

2See Proc. Acad. Nat. Sei. Phila. for 1877, Jan. 1, 1878, p. 292; Bull. Nuttall Ornithol. Club,
Jan., 1878, 3, p. 44, Jan., 1879, 4, p. 59.

316 BREWSTER'S WARBLER.

the stand was written in the autograph of John Cassin, "J. C, 20 October,
1862", and also a badly blurred legend "Not [note?] from Bell." An appeal to
J. G. Bell elicited the response that he remembered shooting a peculiar Warbler
in Rockland Co., N. Y., about the year 1832, — a Warbler something like a
Golden-wing, but lacking, although in high plumage, the black throat of that
species; a great many years afterward, he sold this specimen in Philadelphia
but knew nothing of its ultimate fate. Dr. Trotter justly inferred that the Phila-
delphia Academy specimen was in all probability the very bird shot by Bell.

Now as Audubon was intimately associated with Bell, is it not possible that
he had examined this example of Brewster's Warbler? In that case, seeing that
this bird's characters were in part those of the Blue-wing, in part those of the
Golden-wing, he may have inferred the interbreeding of these two birds, and so
(rather unwarrantably, it is true) their identity. If this be not the ex-
planation of the passage in Audubon's letter to Bachman I have no other to
suggest.

When Audubon came to publish his account of the Golden-winged Warbler
in 1839 (Ornithological Biography, 1839, 5, p. 154) he said not a word about
its connection with the Blue-winged Warbler.

IliiillllllllHI

)44 066 301

I hi
953

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