BLM LIBRARY

88025641

RAVEN
MANAGEMENT PLAN

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DRAFT

RAVEN MANAGEMENT PLAN

FOR THE

CALIFORNIA DESERT CONSERVATION AREA

Prepared by:

U.S. Department of the Interior

Bureau of Land Management

California Desert District

1695 Spruce Street

Riverside, California

In Cooperation With:

DENVER, CO 80225.

California Department of Fish and Game

Region IV Office

12 3 4 E. Shaw Avenue

Fresno, California

California Department of Fish and Game
Region V Office
245 West Broadway
Long Beach, California

U.S. Department of the Interior

Fish and Wildlife Service

Laguna Niguel Field Office

Federal Building, 24000 Avila Road

Laguna Niguel, California

'

•

TABLE OF CONTENTS

Page

SUMMARY iv

INTRODUCTION 1

Purpose

Background 1

Program Goals 2

BACKGROUND 6

The Common Raven 4

Legal Status 4

General Life History 4

Status of the Raven in the California Deserts . 7

Management on Public Lands in California ... 13

The Desert Tortoise 15

Legal Status 15

General Life History 15

Status of the Tortoise in the California

Deserts 17

Management on Public Lands in California ... 18

Raven Predation on Tortoises 20

Impacts of Ravens on Other Resources 2 7

Efforts to Control Raven Populations 28

MANAGEMENT ACTIONS 3 4

Overview of Planned Tasks 34

Planned Actions 3 5

LITERATURE CITED 4 9

i

LIST OF TABLES

Page

Table 1. Increase in raven population in the

western United States deserts (FWS data - 1968

to 1988) 8

Table 2. Raven observations at active landfills

in the California Deserts during 1989 9

Table 3. Raven counts at sewage treatment sites

in the California deserts 10

Table 4. Roadside distribution of ravens in the

eastern Mojave Desert in the late 1960's compared

to the late 1980' s 11

Table 5. Results of road surveys conducted for

ravens in the California deserts during the

winter of 1988-1989 12

Table 6. Summary of east Mojave and northern

Colorado Desert surveys conducted by BLM staff .... 12

Table 7. Numbers of ravens observed per 100

survey hours on permanent desert tortoise study

plots between 1983 and 1988, and during 1989 .... 14

Table 8. Land ownership of desert tortoise

habitats containing 20 or more tortoises per

square mile in California in the 1970 's 18

Table 9. Numbers of small tortoise deaths

attributed to common ravens on 15 desert

tortoise study plots in California 23

Table 10. Changes in size-age class composition

of desert tortoises at five permanent study sites

in the western Mojave Desert between the 1970' s

and the present 2 5

Table 11. Toxicity of Starlicide to various

species of wildlife 30

Table 12. Results of raven control at the USMC

Twentynine Palms landfill during 1989 31

Table 13. Management actions, cooperating

agencies, and proposed time frames for initiation ... 36

li

LIST OF FIGURES

Page

Figure 1. Boundary of California Desert
Conservation Area, constituting raven
management area under review

Figure 2. Histograms of live tortoises captured

at the Desert Tortoise Natural Area interior

study site during 60-day spring surveys in 1979

and 1988 26

Figure 3. Locations and results of raven

monitoring efforts during the 1989 pilot raven

control program at the Desert Tortoise Research

Natural Area 3 2

in

SUMMARY

Common raven (Corvus corax) populations in the California deserts
have rapidly increased over the preceding 20 years. Raven
population increases appear to be a result of human-induced
alterations of desert regions that provide numerous perching and
nesting sites and stable food sources.

Raven predation on juvenile desert tortoises has increased
substantially within those areas exhibiting highest raven
population increases. Excessive predation by ravens has
significantly affected the desert tortoise in these areas,
lowering numbers of juvenile tortoises and altering tortoise
age-class composition.

This management plan was developed to increase the understanding
of the biology of the common raven in the California deserts,
test several methods of controlling ravens, and implement raven
control measures in a manner with the goal of restoring a
"healthy" predator-prey relationship between these two coexisting
species.

Specific management actions to accomplish this goal include
conducting research on raven movement patterns and feeding
behavior near landfills and at nest sites in desert tortoise
habitats, inventorying desert regions for raven nesting and
perching sites where tortoise remains are present, discouraging
raven nesting and perching where high predation rates on juvenile
tortoises occur, experimentally livetrapping and releasing ravens
at distant points to reduce tortoise predation, implementing a
program of selected raven control using lethal methods, and
monitoring results of management actions.

IV

INTRODUCTION

PURPOSE

BACKGROUND

The Bureau of Land Management (BLM or Bureau) , in cooperation
with other Federal, state, and local agencies, plans to implement
a regional management program for the common raven (Corvus corax;
hereafter referred to as "raven") . The area covered by this
management plan encompasses the 25-million acre California Desert
Conservation Area, including portions of Inyo, Kern, San
Bernardino, Los Angeles, Riverside, San Diego, and Imperial
Counties (Figure 1) .

This management plan was developed in response to excessive
predation rates by ravens on juvenile desert tortoises (Xerobates
aqassizii) , a state listed threatened and federally listed
endangered species. Desert tortoise populations in California
have declined precipitously in recent years. Contributing
factors include habitat loss and fragmentation, vandalism,
outbreaks of disease, and predation. The BLM, as the major
Federal agency administering public lands in desert tortoise
habitat, has a responsibility to protect this species. Specific
actions detailed in this plan are designed to limit excessive
raven predation on juvenile desert tortoises to a level that
allows for healthy recruitment into the adult population.

Raven control will take place only within those portions of the
CDCA that contain desert tortoise populations. The distribution
of the desert tortoise in the California deserts is not
continuous, and has been influenced by such factors as elevation,
topography, substrate, and level of prior disturbance.

This management plan addresses two aspects of raven control: (1)
reducing ravens from areas where excessive predation on desert
tortoises has been identified; and (2) experimentally attempting
other forms of control and applying these on a regional
long-term basis where proven successful. Actions to accomplish
this focus on: (1) selectively poisoning and shooting ravens to
reduce predation levels on a short-term basis; (2) concurrently
undertaking research on the ecology of the raven in the CDCA; (3)
concurrently testing nonlethal techniques of raven control; and
(4) implementing effective raven control techniques desert-wide
on a long-term basis.

Authorities under which this management plan has been developed
include the Federal Land Policy and Management Act of 1976, the
Sikes Act of 1974, as amended, the Migratory Bird Conservation
Act of 1929, as amended (and treaties pertaining thereto), the

California Endangered Species Act of 1984, and the Federal
Endangered Species Act of 1973, as amended.

PROGRAM GOALS

The objective of this management plan is to provide the framework
for initiating a multi-agency program with the goal of restoring
a balanced predator/prey relationship between the desert tortoise
and raven in the California California Desert Conservation Area.
Specific measures to accomplish this include actions to:

(1) reduce levels of juvenile desert tortoise
mortality caused by raven predation;

(2) permit increased recruitment of juvenile desert
tortoises into sub-adult and adult age-classes;

(3) improve understanding of the biology of the raven
through research and monitoring;

(4) acquire additional data on means of limiting raven
predation on juvenile tortoises;

(5) implement those measures that are found to be-
ef fective for raven management and control; and

(6) implement a monitoring program to determine
effectiveness of program actions at reducing raven
predation rates on desert tortoises.

Circumstantial evidence also suggests that predation by
increasing numbers of ravens in the California deserts may be
adversely affecting other wildlife species in addition to the
desert tortoise. Information on the effects of raven predation
on wildlife species in general is lacking. A second goal of this
management plan will emphasize the collection of such data.

Additionally, actions undertaken as part of the plan will provide
supplemental data that may be utilized to speed recovery and
delisting of the desert tortoise. Information from the raven
management program in California also will have application in
other portions of the geographic range where excessive predation
by ravens occurs.

An important assumption of the management plan is that the raven
is a resident of the desert and an integral component of the
desert ecosystem. As such, the raven will be managed in a manner
that allows for the perpetuation of desert populations.
Management relies on the selective control of ravens, and not on
the extirpation of the species throughout the California deserts.
An important goal of this management plan is therefore to
maintain ravens in desert areas at levels that are self-

\

CALIFORNIA DESERT CONSERVATION AREA

LNITEDSTATES
DfcP\RTMEMOT THE INTERIOR
81 REAL OF USD VHSAGEMENT

MEXICO

Figure 1. Boundary of California Desert Conservation Area, constituting
raven management area under review. Locations of comparatively high
density desert tortoise habitats are also shown (Adapted from Berry, 1984).

perpetuating but compatible with enhancement of desert tortoise
populations.

BACKGROUND

THE COMMON RAVEN

Legal Status

The raven is a protected species under the Migratory Bird Treaty
Act. Trapping or killing birds without a permit is prohibited.
The authority to issue permits to trap, manipulate, or kill the
species has been delegated to the U.S. Fish and Wildlife Service.

General Life History

Distribution

The raven is a large, all-black, member of the bird family
Corvidae. It has an extensive geographic distribution over the
northern hemisphere, ranging from Alaska, northern Canada,
southern Greenland, and most of Eurasia, south into Central
America and northern Africa (Goodwin 1976) . The geographic range
includes most of Alaska and the western continental United
States. The raven also occurs in Maine, Minnesota, Wisconsin,
Michigan, and Appalachia (Knight and Call 1980) . Historically,
the raven was extirpated from portions of the eastern and
northern United States, possibly as a result of trapping and
poisoning programs and the precipitous decline of bison herds
(Hooper 1977; Knight and Call 1980). More recently, ravens have
been recolonizing their historic range in portions of Alaska,
Michigan, and southern Canada, and expanding into previously
unoccupied areas in New England (Knight and Call 1980) .

Information on the occurrence of the raven in the California
deserts has been available, in part, by contract studies
undertaken by the BLM during preparation of the California Desert
Conservation Area Plan (Thelander 1977; Boyce 1977; Siperek 1977;
Alten 1977) , information compiled by Bureau wildlife staff on
raptor nest sites, and results of breeding and wintering bird
surveys conducted throughout the California deserts.

Social Behavior

Social behavior among ravens is complex. The species is highly
vocal and given to involved posturing and display (Goodwin 1976) .
Ravens are also highly gregarious and congregate in groups of 100
or more individual birds. Intelligence of these birds is evident
by their ability to "play" both individually (Dorn 1972; Elliott
1977) and in flying groups (Knight and Call 1980) . Ravens have
also been observed utilizing "tools" (Janes 1976; Montevecchi
1978; Heinrich 1988a).

Longevity

Ravens mature after 2-3 years of age. They reportedly mate for
life, although this aspect of behavior has been subject to debate
(Jollie 1976) . Raven longevity may exceed 50 or more years.

Nesting Habits

Nests are constructed out of sticks (Dorn 1972; Stiehl 1978).
Nesting sites are highly variable, and have included abandoned
houses (Congdon 1948), cliffs (Jones 1935; Hooper 1977); oil
derricks (Bowles and Decker 1930) ; railroad trestles (Johnson
1899) ; windmills (Bowles and Decker 1930) ; transmission towers
and poles (Knight, University of Colorado, pers. comm. ; BLM
data) ; artificial platforms constructed specifically for raptors
(Howard and Hilliard 1980) ; highway billboards and overpasses
(White and Tanner-White 1988) ; and caves (Wolfe 1928) . In the
California Desert Conservation Area, raven nests have been
observed in tamarisk trees, Joshua trees, transmission towers,
and rock outcrops (Berry 1985; BLM data; Jim Raley, pers. comm.;
Peter Woodman, pers. comm.).

Nest construction usually begins in late winter-early spring
(Knight and Call 1980) . Stiehl (1978) has reported that ravens
will utilize the same nest site for several consecutive years.
Nest completion occurs about one to four weeks before eggs are
laid (Goodwin 1976) . Intraspecif ic defense of nesting
territories appears to be weak (Smith and Murphy 1973).

From three to six eggs are typically laid. These are incubated
by the female and hatch in approximately 18-20 days. Both sexes
care for young (Goodwin 1976) . Young fledge at about 39 to 45
days of age (Dorn 1972) . Parents continue to care for young
birds for another one to four weeks after fledging (Knight and
Call 1980) .

About two to four young birds successfully fledge per nest
(Stiehl 1978) . Sixty percent of raven nests were successful in
fledging one or more young in Oregon (Stiehl 1978) . Where
nesting failures occur, a replacement clutch may be laid (Stiehl
1978) . Home ranges of nesting adult ravens during a study in the
Great Basin Desert in Utah ranged from 1.86 to 2.98 square miles
(Smith and Murphy 1973) .

Feeding Habits

As food availability changes after the nesting season, birds may
congregate into increasingly larger and more mobile groups
(Stiehl 1978) . These groups may migrate to take advantage of
seasonal food sources during the winter months (Dorn 1972) .
During the breeding season, non-breeding birds may also flock

together at sites of food, such as garbage dumps (Dorn 1972) .
Communal roosting occurs with these groups of birds. Recorded
roosts have included transmission lines and towers (Young and
Engel 1988, Engel and Young 1989a), abandoned buildings (Temple
1974) , pine trees (Cadman 1947) , hemlock trees (Lucid and Conner
1974) , hay fields (Cushing, Jr. 1941) , bulrush stands (Stiehl
1978) , railroad bridges (Jim Raley, Quail Unlimited, pers.
comm.), and landfills (Knowles et al. 1989).

Ravens have been recorded scavenging on carcasses of a variety of
species, including deer (Odocoileus virginianus) , moose (Alces
alces) , domestic cows, domestic sheep, domestic goats, and
caribou (Ranqifer articus) (Nelson 1934; Heinrich 1988b). Austin
(1971) recorded instances of ravens feeding on a variety of
roadkills in the Mojave Desert. Species scavenged by ravens
included antelope ground squirrels (Citellus leucurus) ,
unidentified snakes, and black-tailed jackrabbits (Lepus
californicus) .

Ravens also opportunistically scavenge on human-produced food
sources, such as slaughterhouse offal (Goodwin 1976, Heinrich
1988b) and refuse (McManus 1935; Harlow et al. 1975).
Apparently, they will also scavenge human remains, since Goodwin
(1976) lists "battlefields" as raven foraging areas. Sources of
garbage accumulation that ravens can use as food supplies in the
California deserts include dumps and landfills, sewage ponds,
roadside pull-off s and rest areas, and campgrounds (Farrell, BLM,
pers. observ. 1988; Knowles et al. 1989). Ravens have also been
observed feeding on carcasses at roadside dump areas used by
livestock shippers in the eastern Mojave Desert (Farrell, BLM,
pers. obs. 1988) .

Raven diets vary seasonally and between habitats. Ravens are
opportunistic and highly adaptable in their feeding habits and
are able to quickly learn to exploit any food source that becomes
available. They are efficient predators and scavengers and, when
necessary, have a vegetarian diet (Bent 1946; Dorn 1972) that
includes cereal grains (Engel and Young 1989a). Temple (1974)
concluded that ravens obtain half their energy income through
scavenging and half through predation, although the majority of
time was spent hunting for prey. Species occurring most
frequently in the diet generally reflect those that are most
abundant and easily obtained in a particular area (Harlow et al.
1975; Marquiss and Booth 1986; Dorn 1972). In addition, raven
diets are influenced by seasonal food availability. Stiehl
(1978) observed ravens abruptly change their feeding behavior in
the month of July and concentrate on a dense population of
grasshoppers. He also reported heavy predation on waterfowl
eggs, a seasonally available food source.

Numerous animal species have been recorded from the diet (Nelson
1934; Bent 1946; Knowlton 1943; Murray 1943; Heinrich 1988a).

Dorn (1972) attempted to quantify diet by frequency of occurrence
in raven pellets analyzed near Jackson Hole, Wyoming. Mammals
comprised the largest portion of the diet (76%) , followed by
insects (33%) , plant material (10%), birds (7%), and fish (5%).
Harlow et al. (1975) estimated frequency of food composition by
pellet analysis near Mountain Lake, Virginia. He found that
mammal remains were most frequently encountered (74%) , followed
by plant materials (54%), and refuse (10%). Steihl (1978) found
that mammals also constituted a primary food source in the diet
of the raven (46%) , followed by bird eggs (26%) , bird parts
(14%), fish (5.6%), insects (5.5%), vegetation (2.5%), and
reptiles (1%) .

Fluctuations in prey populations can influence food intake of
ravens, as evidenced by regular monitoring undertaken in the
Snake River Birds of Prey Natural Area, Idaho (Kochert et al.
1975, 1976, 1979, 1980, 1981).

Ravens may be extremely cautious when approaching potential food
sources. Heinrich (1988c) has speculated that this behavior may
be a result of prior persecution by humans or a behavioral
mechanism to reduce vulnerability when approaching food.

Raven Status in the California Deserts

Historical Perspective

Ravens were uncommon in the deserts of California in the first
half of the 20th century. When Eugene Cardiff, Curator of
Natural History at the San Bernardino County Museum, first began
searching for ravens in the western Mojave Desert in the early
1940' s, he looked for about two years before he could locate a
raven to collect for the museum (Eugene Cardiff, pers. comm.).
Ravens were very scarce in the deserts of eastern California
between the 1920's and 1940's also. Johnson et al. (1948), in a
survey of the eastern Mojave Desert encompassing the Providence,
New York, and Clark Mountains and adjacent areas, reported few
observations of ravens. They saw so few ravens over a several
year period that they noted that the raven was only a summer
resident.

Raven populations grew substantially between the 1940's and
1970' s and 1980' s throughout the California deserts and parts of
the Southwest. Data from both formal surveys and experienced
observers indicate that increases in raven populations have been
significant and that population growth continues. One of the
more important sources of trend data is the U.S. Fish and
Wildlife Service (Service) , which developed a comprehensive
breeding bird survey for the United States and Canada (Robbins et
al. 1986). Using experts in bird songs and calls, the Service
established a network of routes for breeding bird surveys and
began to gather data in the 1960's. In the deserts, surveys were

usually conducted in May along established routes 25 miles long.
The observer-listener made 50 stops at 0.5-mile intervals to
record observations. Routes were established in 1968 and have
been run consistently for 22 years.

In the Mojave Desert of California, southern Nevada, and extreme
southwestern Utah, raven numbers have increased approximately
14.9% annually in the 20-year period between 1968 and 1988
(Unpubl. data, Service). Since 1968, the raven population has
increased 15-fold or 1528%. In the same time period, raven
numbers increased 9.1% annually for a total population increase
of 474% in the Colorado and Sonoran desert regions of California
and Arizona, respectively. Data for the Great Basin Desert in
California and west-central Nevada indicate raven populations
increased 5.05% annually and populations almost doubled (168%
total increase) in the last 20 years. In the southern California
basin extending from the Los Angeles-San Bernardino area south to
San Diego, raven populations also have experienced significant
increases of 7.5% annually with a 3-fold population increase
(328% total increase) during the last 20 years. Increases are
statistically significant at P<0.01 for the Mojave and
Colorado/Sonoran deserts and the southern California basin. Fish
and Wildlife Service data are summarized in Table 1.

Table 1. Increase in Raven population in the
western United States deserts (FWS data - 1968
to 1988) .*

Region

No. of Annual Trend
routes (% annual change)

Change (%)
in 20 years

Mojave Desert

14

+14.973

+ 1528

< 0.01

Colorado-

Sonoran Desert 10

Great Basin Desert 16

+ 9. 134
+ 5.047

+ 474
+ 168

< 0.01
< 0. 1

So. California
Basin

15

+ 7.539

+ 328

< 0.01

*Unpublished data courtesy of U.S. Fish and
Wildlife Service, North American Breeding
Bird Survey Program, Laurel, Maryland.

Other data from experienced observers, published papers, and
reports support the findings of the Service. In the western
Mojave Desert, there are many examples of high raven numbers.
Eugene Cardiff reports substantial increases in the species in

the general region and has observed flocks of 100 to 400 birds in
the Harper Dry Lake and marsh areas near Barstow (Cardiff, San
Bernardino County Museum and San Bernardino Audubon Society
files) . Mike Phillips and Mark Hagen, biologists at Edwards Air
Force Base, report flocks of 100 to 200 ravens at the base
landfill in 1988. Groups of over 100 ravens have been regularly
observed at the U.S. Marine Corps Air Ground Combat Center at
Twentynine Palms (Roger Twitchell, USMC, pers. comm. 1989).
Large numbers of ravens, ranging to over 100 birds, have also
been observed at landfills for the desert communities of Boron,
Apple Valley, Victorville, and Baker (Table 2) .

Table 2. Raven observations at active landfills in
in the California desert during 1989*

Site

Total Number

Maximum Count

Frequency of

Ravens Observed

Birds

Refuse Burial

Ridgecrest

444

82

Daily

Randsburg

31

15

Transfer Stat.

Boron

427

150

Daily

Hinkley

15

7

Daily

Yermo

115

74

Weekly

Newberry Springs 203

67

Weekly

Barstow

113

27

Daily

Apple Valley

950

200

Daily

Victorville

816

250

Daily

Camp Rock

278

66

Weekly

Landers

208

45

Daily

29 Palms

220

76

Daily

USMC 29 Palms

Base 752-827

150

Daily

Baker

444

130

Weekly

Nipton

71

24

Periodic

Kelso

17

9

Periodic

Searchlight

71

28

Periodic

Needles

99

26

Daily

Goffs

54

30

Periodic

Essex

71

28

Never

Amboy

1

1

Periodic

Vidal Junction

4

Periodic

Desert Center

225

35

Weekly

Blythe

3

2

Daily

Palo Verde

0

0

Never

Indio

123

41

Daily

•*Source: Knowles et al. 1989; BLM data 1989
**Raven numbers for the USMC 29 Palms Base landfill
were derived over 14 days of monitoring between
April 18, 1989 and August 16, 1989.

Lesser numbers of birds are regularly observed at sewage
treatment sites at Ridgecrest, Baker, and Boron (Table 3]

At

10

sewage ponds on the China Lake Naval Weapons Center, flocks of
more than 100 ravens are common (Knowles et al. 1989).
Knowles et al. (1989) have observed ravens feeding on filtered
rags at sewage ponds and on the sewage in the ponds.
Significantly, only five of 17 ponds under study showed
consistent use by ravens.

Table 3. Raven counts at sewage treatment
sites in the California deserts*

Site Total Number Maximum Count

Ravens Observed Birds

2

20

43

2

36

2

2

2

3

0

3

47

3

0

0

2

0

* Source: Knowles et al. 1989.

In the eastern Mojave Desert, James Van Remsen conducted bird
surveys in 1975-1976 and suggested that ravens were much more
common during his field work than in earlier decades (Berry 1985;
BLM files for the East Mojave Unit Resource Analysis: Wildlife,
Needles and Riverside, California). By 1975-1976, the raven was
a permanent resident (most seen/day = 15) , whereas it was
considered a summer resident only in the 1920' s and 1940' s
(Johnson et al. 1948).

Road Survey Results

Austin (1971) conducted a study of the roadside distribution of
the common raven in the eastern Mojave Desert of California and
southern Nevada during 5,500 miles of travel between 1967 and
1969. More recently, regional inventories for ravens have been
conducted under Bureau contract during 1989 (Knowles et al.
1989), and by BLM wildlife staff biologists Jim Farrell, Bill
Montgomery, and Phillip Hughes during 1988-1989. Data

California City

5

Inyokern

76

Ridgecrest

249

Federal Prison

4

East Boron

153

West Boron

4

Barstow

8

Yermo Marine Corps Base

2

Yermo Union Pacific RR

3

Silver Lakes

0

Victorville

6

Baker

181

Needles

3

Park Moabi

0

Blythe

0

Cactus City

3

Indio

0

11

summarizing results of Austin's (1971) and Farrell's (1989)
findings are provided in Table 4.

Table 4. Roadside distribution of ravens in the
eastern Mojave Desert in the late 1960's compared
to the late 1980 's*

Type of Road

Years

Number

of

Number of

No.

Ravens/

Season

Ravens

Miles

100 Miles

Main Roads

Nov-Feb

1967-69

38

1125

3.38

Mar-Apr

1967-69

13

862

1.51

May-Aug

1967-69

11

1463

0.75

Sep-Oct

1967-69

17

870

1.95

Totals

79

4320

1.83

Nov-Feb

1988-89

674

5603

12.0

Secondary Roads

Totals 1967-69

1132

0.79

Nov-Feb
May-Aug

1988-89
1988

85
154

1517
2215

5.6
6.95

♦Sources: Austin (1971) ; Farrell (1989)

This road survey information has been supplemented with a
contract survey of four major regions of the California deserts
(Knowles et al. 1989) and BLM staff in the Needles area. Knowles
et al. (1989) conducted vehicle transects in the west Mojave,
Ivanpah Valley, Fenner-Chemehuevi Valleys, and southern Colorado
Desert of California during the winter of 1988-89. They recorded
sightings of 4,328 ravens during 26,239 miles of driving on paved
and unpaved roads. Ravens were most abundant in the western
Mojave Desert and Ivanpah Valley. Significantly fewer ravens
were observed in the Fenner-Chemeheuvi Valleys and southern
Colorado Desert. Observations along paved roads accounted for
92% of all raven sightings during this survey (Table 5) . These
results also are consistent with a concurrent survey conducted
during 1989 for the Southern California Edison Company in the
California deserts, showing a higher incidence of raven
observations close to paved roads (Dr. Richard Knight, Colorado
State University, pers. comm.).

12

% Rov

ite
Dirt

Ravens/100

miles

Paved

Paved

Dirt

78

22

33

31

56

44

22

3

59

41

4

2

Table 5. Results of road surveys conducted for
ravens in the California deserts during the winter of
1988-1989*

Region % Raven Oservations

Paved Dirt

West Mojave 79 21

Ivanpah Valley 89 11

Fenner-Chemehuevi 77 23

Southern Colorado 93 7 59 41 4 1

TOTAL 92 8 6644 20 9

*Source: Knowles et al. 1989

Between November 1988 and May 1989, Bill Montgomery and Phillip
Hughes of the BLM Needles Resource Area Office staff conducted
surveys of paved and unpaved roads in the eastern Mojave Desert
and northern Colorado Desert. Numbers of ravens observed and
mileages driven were recorded during routes on freeways, County
paved and unpaved, and dirt roads. A total of 840 observations
of ravens were made during 12,036 miles of driving. Observations
of ravens during this survey were significantly higher on dirt
roads than for all classes of paved roads (Table 6) .

Table 6. Summary of East Mojave and northern Colorado
Desert surveys conducted by BLM staff*

Route Type

Ravens

Miles

Ravens/

Observed

Driven

100 Miles

Freeway"

194

3758

5.2

County Paved

180

1868

9.6

County Unpaved

44

1244

3.5

State Highway

197

3659

5.3

Dirt

188

912

20.6

Powerlines

37

597

6.1

* Source: Montgomery and Hughes data

13

Observations on Tortoise Study Plots

Data are being gathered on numbers of ravens observed flying
over, perching, or otherwise present on desert tortoise permanent
study plots. So far, data have been gathered on 10 of the 15
permanent study plots (Table 7) . With the exception of the Goffs
study plot where survey efforts were more intensive between 198 3
and 1986, data were gathered during 60-day spring surveys of
tortoise study plots varying in size from 1.0 to 1.6 sguare
miles. The primary purpose of the 60-day spring survey is to
gather data on desert tortoises. Sightings of ravens and other
predatory birds are recorded as they occur and are opportunistic
observations. The raven data from the tortoise plots provides a
means of comparing the numbers of ravens seen per unit time
during the spring months of March through June when tortoises are
active. The data (presented in numbers of ravens observed per
100 hours of tortoise survey) can be compared from different
desert regions and can offer an index of abundance of ravens in
the open desert.

Numbers of ravens observed per 100 survey hours on tortoise study
plots ranged from 0.41 to 45.32. A total of 14 ravens were
observed per 100 hours of monitoring effort during the "pilot"
raven control effort conducted at the Desert Tortoise Natural
Area during 1989. Sites with significant losses of juveniles
include areas where frequencies of raven observations range from
about 3 to 45 ravens per 100 survey hours.

Management on Public Lands in California

Management of the common raven on Public Lands administered by
the Bureau of Land Management has been largely passive. The
raven has been afforded protection as a result of
implementation of management actions for the protection of
wildlife species in general. Such actions have included:
restrictions on vehicle use; designation of wilderness study
areas; protection of specific localities as Areas of Critical
Environmental Concern; implementation of general mitigation
measures to protect wildlife during a variety of activities; and
increased ranger presence to enforce applicable laws.

14

Table 7. Numbers of ravens observed per 100 survey
hours on permanent desert tortoise study plots
between 1983 and 1988, and during pilot raven control
program monitoring during 1989*

Region

Total No. Raven Seen/Mo.
Year March April May June

Number
ravens/
100 hours

Western Mojave Desert

Fremont Valley

Desert Tortoise
Natural Area

Kramer Hills
Stoddard Valley
Eastern Mojave Desert

Shadow Valley
Goffs

1987

Colorado Desert

Ward Valley
Chemehuevi Valley
Chuckwalla Valley
Chuckwalla Bench

16

33

n.d

14.76

1988

n.d.

99

90

n.d.

45.32

1989

n.d.

3

22

0

20

1988

35

96

66

n.d.

41.04

1988

n.d.

26

91

n.d.

28.92

1988

n.d.

45

15

3

20.81

1983

n.d.

3

19

4

3. 32

1984

n.d.

7

0

2

1.5

1985

n.d.

0

3

0

0.76

1986

n.d.

11

9

2

3.9

1987

n.d.

14

22

0

8.73

1988

9

12

n.d.

n.d.

3 . 32

1987

n.d.

0

2

0

0.41

1988

9

21

5

0

6.01

n.d. = no data collected for that month
♦Sources: Berry, BLM data

15

THE DESERT TORTOISE

Legal Status

The desert tortoise was State listed as "threatened" in
California on June 22, 1989 [California Code of Regulations,
Section 670.5(b)(4) of title 14], and federally listed as
"endangered" under the emergency listing provisions of the
Endangered species Act on August 4, 1989 (54 Federal Register
32326-32331) . Reasons for both Federal and State listing include
loss or deterioration of habitat, disease, predation, and
collection for pets. Contributing factors include urbanization,
vandalism and poaching, release of sick pet animals into wild
populations, excessive predation on juvenile tortoises by ravens,
motorized vehicle use, and route proliferation. The desert
tortoise receives legal protection afforded under both the
California Endangered Species Act and the Federal Endangered
Species Act of 1973, as amended. The U.S. Fish and Wildlife
Service is currently seeking comment on a Proposed Rule to extend
the Federal endangered status of the desert tortoise past the
240-day timeframe allowed under the emergency provisions of the
Endangered Species Act (54 Federal Register 42270-42278) .

General Life History

Distribution

The desert tortoise (Xerobates agassizii) is distributed over
portions of the Mojave, Colorado, and Sonoran deserts of the
southwestern United States and adjacent Mexico. It occupies a
variety of habitats, ranging from Mojave Desert plains and
valleys, Sonoran Desert bajadas and lower mountain slopes, and
thorn scrub forest in northern Mexico. Associated plant species
include creosote bush (Larrea tridentata) , burrobush (Ambrosia
dumosa) , saltbush (Atriplex spp.), Joshua trees (Yucca
brevifolia) , ocotillo (Foquieria splendens) , saguaro cactus
(Carnegiea gigantea) , and palo verde (Circidium f loridum)
(Woodbury and Hardy 1948; Schwartzmann and Ohmart 1977; Bury et
al. 1978; Berry 1975, 1984)

Burrows

The tortoise is highly adapted for digging, with shovel-shaped
front legs used to scoop out earth during burrow construction
(Woodbury and Hardy 1948). Burrows are constructed to escape
predators and to avoid harsh temperature extremes and
dessication. Burrows constructed include a shallow and short
"pallet" that is used on a regular basis during seasonal activity
periods by foraging animals and a deeper and more extensive
burrow that is used for overwintering or estivation (Woodbury and
Hardy 1948; Berry 1975). Burrows may be constructed under the

16

canopy of a bush or shrub (Berry 1972) , in the open (Burge 1976) ,
along wash embankments (Woodbury and Hardy 1948) ; or under
boulders (Coombs 1977) .

Activity Periods

Periods of seasonal activity coincide when food supplies and
water are available. Principal food plants include a variety of
annuals (Berry 1975, Burge and Bradley 1976; Coombs 1977) , and
perennials (Hansen et al. 1976). Tortoises eat perennial
grasses, a few half shrubs (such as Sphaeralcea ambiqua) , and
occasionally the flowers of perennial shrubs. Tortoises will
emerge during or immediately after spring or summer rainfall to
drink (Medica et al. 1982).

Reproduction

Age to maturity is approximately 15-2 0 years (Woodbury and Hardy
1948) . Animals may reach 100 years of age (Woodbury and Hardy
1948) . Courtship is complex, and consists of initial trailing of
a female by a male. Males will often preceed copulation by
circling, butting, ramming, and biting the female. Males may
mate with several different females over the course of a season.
Most mating occurs close to the female's burrow (Berry 1986).

Typically 1-2 clutches of from 2 to 14 eggs (Miller 1955; Berry
1974; Turner et al . 1987) are deposited during the spring or
early summer in or near the burrow. During a study of the
species at the Ivanpah Valley, California, tortoises laid their
first clutch of eggs during mid-May. A second clutch was
subseguently laid by 30% of the adult females during mid-June
(Turner et al. 1984). Mean clutch size, determined during a
population study near Goffs in the eastern Mojave Desert, was 4.5
(Turner et al. 1987). The eggs hatch in approximately 105 to 135
days (Coombs 1974). Hatchlings and juvenile tortoises have very
thin shells that gradually harden as animals mature (Patterson
1978) . Nest defense by female desert tortoises from predators
such as gila monstors (Heloderma suspectum) has been observed
(Vaughan and Humphrey 1984) .

Social Behavior

Desert tortoises exhibit dominance hierarchies. Individual
fighting or posturing between animals may occur between both
males and females. Females are typically subordinate to males.
Interactions between animals may include head bobbing and
ramming. The "aggressor" in such instances is usually the larger
and dominant animal. Outcomes of such encounters may result in
the subordinate individual rapidly leaving the area or
withdrawing into its shell (Berry 1986) .

17

Desert tortoise rely on a combination of sight, smell, sound, and
tactile signals. They will frequently sniff the soil and each
other, and use their sense of smell to follow trails of other
tortoises. Visual signals are used to communicate and include
head bobbing, gaping, withdrawing limbs and head into the shell,
and elevating the neck. Vocalization can occur, but use of sound
for communication has received little attention to date (Berry
1986) .

Home Range and Movements

Home range size for desert tortoises studied in the Ivanpah
Valley, California, ranged from approximately 5 acres to 220
acres, with an average of about 50 acres for both sexes (Turner
et al. 1981; Medica et al. 1982). Based on a comparison of
results of different studies, males typically have a
significantly larger home range than females (Berry 1986) .
Although home range appears to be relatively small, individual
animals are capable of rapid linear movement. Males may move
over 1000 yards per day within their home range. Tortoises have
also been recorded moving distances of over four miles over an
extended time period (Berry 1986) .

Tortoise Status in the California Deserts

Abundance

Desert tortoise populations and habitats are subjected to a
number of man-induced actions, including mineral exploration and
development, urbanization, road construction, motorized vehicle
recreation, grazing by domestic livestock, and energy
development. The cumulative effects of such activities are
direct loss of individual tortoises, habitat fragmentation and
degradation, and habitat loss (Berry and Nicholson 1984).

In California, data on distribution and densities of tortoises
have been derived from a database of over 2000 strip-transects
and 30 study plots. Desert tortoises are distributed within
approximately 40,156 square miles, 84% of which has low densities
estimated at 0 to 20 tortoises per square mile (Berry and
Nicholson 1984) . Approximately 73% of habitat in California is
on public lands administered by the BLM (Table 8) .

Data gathered over periods ranging from 9 to 17 years on
permanent study plots indicate that tortoise populations have
declined precipitously in several areas since 1979-80. Areas
experiencing severe declines include the "core" populations in
the western Mojave Desert and southern Colorado Desert in
California (Berry et al. 1987, 1988). Outbreaks of two different
diseases, excessive raven predation, vandalism, and vehicle kills
have contributed to population declines exceeding 50% in the last
four to seven years.

18

Table 8. Land ownership of desert tortoise habitats
containing 20 or more tortoises per square mile in
California in the 1970' s.*

Est. Tortoise
Densities
(no./sq. mile)

Land Ownership
BLM

(sq. miles)
Private

**

Totals
(sq. mi. )

20 to 50

2,687

729

3,416

50 to 100

1,334

478

1,812

100 to 250

559

314

873

250

47

225

272

Total

s

4,627

1

r746

6,373

*Source: Berry and Nicholson (1984)
**Gross estimates that do not deduct habitat lost as a
result of developments such as roads, small towns,
off-road vehicle staging areas, etc. Actual habitat
is less than shown.

In 1989 the BLM and California Department of Fish and Game
contracted with two internationally recognized experts in
reptilian diseases for assistance in identifying the pathogens
involved in the disease outbreak in the western Mojave Desert.
In addition, surveys were conducted on the epidemiology of the
disease at the Desert Tortoise Natural Area, particularly to
gather data on distribution and frequency of disease symptoms in
tortoises (Faunawest 1989). In May and October of 1989, meetings
were held with research veterinarians and other scientists to
determine protocols for handling tortoises, for design of future
research projects, and to share information about disease issues.

Management on Public Lands in California

In 1979 the BLM designated the desert tortoise as a "sensitive"
species. According to the California Desert Conservation Area
(CDCA) Plan of 1980 (BLM 1980) , sensitive species are provided
special consideration and attention during project planning and
implementation. The CDCA Plan also contained designated Wildlife
Habitat Management Areas (WHMAs) and Areas of Critical
Environmental Concern (ACECs) for the desert tortoise in
California. These are the western Mojave Desert (WHMA) , western
Rand Mountains (ACEC) , Desert Tortoise Research Natural Area
(ACEC) , Shadow Valley (WHMA) , Ivanpah Valley (WHMA) ,
Fenner/Chemehuevi Valleys (WHMA) , and the Chuckwalla Bench (ACEC)
(BLM 1980) . Management plans for the Desert Tortoise Research

19

Natural Area and the Chuckwalla Bench were completed in 1988 and
1986, respectively (BLM 1986; BLM 1988). An additional
management plan for the desert tortoise population in the Fremont
Valley-Rand Mountain area of eastern Kern County is nearing
completion (BLM 1989a) . Development of four regional habitat
management plans encompassing tortoise populations in California
was subsequently recommended (Sievers et al. 1988), and adopted
by the BLM.

The BLM, as a "lead" Federal agency responsible for management of
Public Lands containing a significant portion of desert tortoise
habitats, has implemented several actions specifically for the
desert tortoise. Aside from designation of special management
areas, the Bureau has undertaken systematic transects over large
portions of the geographic range of the desert tortoise to
determine distribution and relative abundance; established
permanent trend study to monitor population trends over time;
instituted a program to acguire inholdings of private lands in
two Areas of Critical Environmental Concern; undertaken specific
studies to determine effects of land management actions on the
desert tortoise; required some project-specific mitigation
measures for the species; and assumed a lead agency role towards
development of comprehensive management strategies for the desert
tortoise.

In 1986, a Bureau-wide Task Force was established to develop a
framework for management of the desert tortoise and its habitat
throughout the geographic range of this species in the United
States and to enhance cooperation and coordination between
government agencies (Kennedy et al. 1987). The Task Force
produced a report, Management of Desert Tortoise Habitat, and set
the goal of ensuring that viable tortoise populations exist on
Public Lands. In 1988, another team was assembled to begin
implementation of the Task Force's recommendations (Spang et al.
1988) . The team completed a report, Desert Tortoise Habitat
Management on the Public Lands: A Rangewide Plan, in November of
1988. The plan contains 14 management objectives covering a
range of subjects, including management of off-highway vehicle
use, livestock grazing, and energy/mineral development.

At the same time that the BLM Washington Office was working on
national rangewide recommendations for the desert tortoise, the
Bureau's California Desert District Office established a
California Desert Tortoise Workgroup composed of Bureau tortoise
experts as well as representatives from the California Department
of Fish and Game, Desert Tortoise Council, and Desert Tortoise
Preserve Committee. The California Workgroup was charged with
developing recommendations for definitive actions that might be
taken to improve, protect, and enhance tortoise populations and
habitat (Sievers et al . 1988). The Workgroup's report contains
47 recommendations for changes in management, including a
specific statement about reducing the level of raven predation on

20

tortoises desert-wide in California. The BLM California Desert
District Office is currently working on implementing some of the
47 recommendations, as well as addressing the 14 management
objectives in the rangewide report.

In 1989, the Bureau has also established a Management Oversight
Group (MOG) , to oversee consistency of management direction and
information collection within the four western states with desert
tortoise habitat (Utah, Nevada, Arizona, and California) . The
MOG is comprised of Bureau officials and representatives of State
wildlife agencies and the U.S. Fish and Wildlife Service to
oversee consistency of management direction and information
collection on the species. Each state also established a
Technical Advisory Committee to provide recommendations for
research and management of the desert tortoise. In California,
the California Desert Advisory Council, consisting of
representatives from conservation groups, use interests,
recreation interests, local governments, and the public, has
established a Coordination Committee specifically to review
actions relating to the desert tortoise.

RAVEN PRE DAT I ON ON TORTOISES

Evidence of Predation

Ravens use tortoises for food. They have been observed killing
tortoises several times. Ted Rado (BLM), Tom Campbell (Naval
Weapons Center, China Lake) , and Jim Farrell (BLM) have observed
ravens in the process of killing tortoises. Laura Stockton
(pers. comm. ) and Bev Steveson (pers. comm. ) , tour guides for the
Desert Tortoise Preserve Committee, rescued a juvenile tortoise
that had been attacked by a raven at the Desert Tortoise Natural
Area.

Woodman and Juarez (1988) described a raven nest and perch site
with exceptionally high levels of predation on juvenile tortoises
at the BLM's Kramer permanent study plot for tortoises. They
collected remains of 190 juveniles killed between 1984 and 1987
and concluded that ravens accounted for probably 185 (97%) of the
deaths. In the spring of 1988, they collected additional fresh
remains of juvenile desert tortoises from the nest and perch
area, bringing the total number of juveniles killed between
spring 1984 and spring 1988 to 250 (Woodman, pers. comm).
Woodman and Juarez (1988) noted that ravens began killing larger
juvenile tortoises, and after these were removed from the
population, focused on progressively smaller juveniles.

John Wear, a biological consultant, collected shells of 24
freshly killed juvenile desert tortoises under a raven nest in
the Chemehuevi Valley in 1985 (Berry 1985). In the fall of 1988,
Jim Farrell (BLM, pers. comm.) found 46 shells here. Additional
tortoise shells were also located at this location in 1989 (Jim

21

Raley, pers. comm.) . In the spring of 1988, Bureau biologist Jim
Farrell surveyed a few hundred miles of transmission line towers
and wooden poles for raven nest sites in the eastern deserts of
California. He found nine nests and one perch site used by
nesting ravens and their families. Two of the nine nests were
occupied by families of ravens that use tortoises extensively for
food. One nest, on a transmission line tower in Ward Valley, had
a total of 87 tortoise shells under the tower with the nest and
under the adjacent five towers. The second nest, on a
transmission line that crosses the Fenner and Piute Valleys in an
east-west direction, had 61 tortoise shells. North of Kelso, a
third raven family used a perch site consisting of eight wooden
power poles for feeding. Nineteen shells were associated with
this perch. Farrell was unable to locate the nest used by this
raven family. Farrell concluded that three out of the ten
nesting pairs of ravens under observation were consuming large
amounts of desert tortoises in the eastern California desert.
Farrell also found several other sites under towers with a few
juvenile shells (Farrell 1989) .

Followup inventories of transmission towers in portions of the
eastern California deserts were undertaken during the spring of
1989 by BLM biologists Phillip Hughes and Bill Montgomery.
Remains of juvenile desert tortoises were discovered beneath or
near 11 different transmission towers (Bill Montgomery, BLM,
field notes; Phillip Hughes, BLM, field notes). Monitoring
conducted during a 1989 "pilot" raven control program documented
additional juvenile tortoise remains underneath raven nesting and
roosting sites (Jim Raley, Quail Unlimited, pers. comm.).

Campbell (1983) described the collection of 136 carcasses of
juvenile desert tortoises ranging in size from 36 mm to 103 mm
carapace length (estimated ages 0 to 7 years) at fence posts on
the Desert Tortoise Research Natural Area in 1980 and 1981.
Berry (1985) reported additional heavy predation rates in the
Natural Area at a raven nest site along the fence and on a study
plot in the late 1970' s and early 1980 's. Remains of seventy-
three juvenile tortoises killed by ravens were found on one
Natural Area study plot between 1978 and 1982. Additional
remains of juvenile tortoises have been subseguently recovered
along the Natural Area boundary fence (Rado 1990) .

Remains of juvenile desert tortoises can be found at the base of
transmission towers, at isolated fence posts, at mining claim
stakes, next to road barricades, under Joshua trees, at the
bottom of wash embankments, on hilltops, and elsewhere in the
deserts of California (Berry 1985; Jim St. Amant, CDFG, pers.
comm. ; John Wear, consultant, pers. comm. ; Jim Raley, Quail
Unlimited, pers. comm.; Ted Rado, BLM, pers. observ.). Examples
of sites with tortoise remains include the Desert Tortoise
Research Natural Area perimeter fence, isolated Joshua trees in
the Desert Tortoise Research Natural Area and immediately west of

22

the Kramer study plot, road barricades west of California City,
the perimeter fence at the Barstow Wooly Sunflower Area of
Critical Environmental Concern, several locations at the southern
end of the U.S. Marine Corps Air Ground Combat Center at
Twentynine Palms, the base of a rocky hill west of Needles, and
below several different transmission towers in the northern
Colorado Desert (Jim Raley, pers. comm. , Roger Twitchell, USMC,
pers. comm., Ted Rado, pers. observ.).

Ravens may also be consuming adult tortoises, although
information is circumstantial. Farrell (pers. comm.) documented
an instance in the northern Colorado Desert where tracks and a
freshly killed adult tortoise indicated that an apparently
healthy animal was attacked and killed by a group of ravens.
Similar attacks on adult tortoises by groups of ravens in the
central Mojave Desert near Kramer Junction have also been
reported (Dr. Dave Morafka, California State University Dominguez
Hills, pers. comm.). Additionally, remains of adult tortoises
under an unidentified raptor nest west of the Desert Tortoise
Research Natural Area have been photographed (Richard Buckberg,
California Energy Commission, pers. comm.).

There is no evidence suggesting that ravens are scavenging large
numbers of dead juvenile desert tortoises killed by disease or
other causes. Until 1988, very few sick or disabled tortoises
were observed on the 27 BLM study plots in the California
deserts. In over 16,000 observations of live tortoises on
California plots, fewer than 0.002% have shown signs of active
diseases or damage. In 1988, two tortoise populations were
discovered with diseases, one at the Desert Tortoise Natural Area
and the other at the Chuckwalla Bench. Large numbers of dead
juvenile desert tortoises have been found under raven perching
and nesting sites in areas where incidence of diseased tortoises
has not been documented. Ravens apparently have preyed on
juvenile tortoises for many years (Miller 1932) , but the examples
of excessive predation are relatively recent and appear to
coincide with the growth of predator populations.

Effects of Raven Predation on Desert Tortoise Populations

Data from permanent desert tortoise study plots provide a sample
of the levels of raven predation occurring on juvenile desert
tortoises in the California deserts (Berry 1985; Berry et al.
1986a, 1986b, 1987; BLM 1988 data). Data collected between 1976
and 1987 are displayed in Table 9. Frequencies are only shown
for sample sizes of 25 or more.

Since the mid- to late 1970' s and early 1980 *s, raven predation
has had significant adverse effects on desert tortoise
populations. Specifically, ravens have: (1) reduced numbers of

23

Table 9. Numbers of small tortoises (<lio mm
carapace length) deaths attributed to common
ravens on 15 desert tortoise study plots in
California*

Desert Region
Site Name

Years Numbers of Deaths
Raven Kills Other

% due to
Raven Kills

Western Mojave Desert

Fremont Valley

Desert Tortoise

Natural Area

Desert Tortoise Nat.

Area Interp. Ctr,

Fremont Peak

Kramer

Calico

Stoddard Valley
Lucerne Valley

Johnson Valley

1976-81

11

26

29.7

1987

2

11

15.4

1978-82

73

19

79.4

1985

16

24

40.0

1977-79

2

5

1980-85

1

0

1980-82

9

11

45.0

1987

44

50

46.8

1978

1

0

1978-81

14

15

48.3

1980

4

2

1986

9

16

36.0

1980

8

9

47. 1

TOTAL

Eastern Mojave Desert

Ivanpah Valley
Goffs

198

202

977-79

3

10

1986

1

14

977-80

2

5

49.5

14.3

TOTAL

29

17.1

Colorado Desert

Ward Valley

Chemehuevi Valley
Chuckwalla Bench
Chuckwalla Valley

1980

1

2

1987

5

27

17.1

1977-82

1

35

2.8

1977-82

13

34

27.6

1980

0

4

1987

0

13

TOTAL

20

115

14.8

*Sources: Berry 1985; Berry et al.
1986a, 1986b, 1987, 1988; 1989
BLM data.

24

juvenile tortoises in the hatchling to eight-year classes, (2)
reduced recruitment of tortoises into the larger and older
size-age classes (e.g., tortoises from 9 to 20 years of age), (3)
altered the size-age class composition of the population to favor
adults, and (4) contributed to overall populational declines from
multiple sources. Examples of the degree and nature of the
impacts are evident at five permanent study plots in the western
Mojave Desert (Figure 2) and at two study plots in the
northeastern Colorado Desert, as well as in other regions.

Briefly, for the purposes of analyzing tortoise population data,
tortoises are divided into six size-age classes based on carapace
length: juvenile 1 (<60 mm) ; juvenile 2 (60-99 mm) ; immature 1
(100-139 mm) ; immature 2 (140-179 mm) ; subadults (180-207 mm) ;
and adults (>208 mm) (Turner and Berry 1984) . The classes
reflect age, degree of shell ossification, vulnerability to
predators, and sexual maturity. In general, tortoises in the
juvenile 1 class are less than three to four years old, those in
the juvenile 2 class are four to seven years old, those in the
immature 1 class are seven to 13 years old, and those in the
immature 2 class range from 13 to 20 years of age. Ages for
tortoises in the smaller size classes vary considerably and
depend on food supply, precipitation, and other factors.
However, most tortoises require 15 to 20 years to reach sexual
maturity, which occurs when individuals reach 180 to 200 mm in
length.

Because ravens prey heavily and selectively on small size-age
classes and are very efficient predators, they have contributed
to significant changes in size-age classes of tortoise
populations throughout the California deserts. In the western
Mojave Desert, where data are available from several study plots,
the pattern is consistent and significant (Berry et al. 1986a,
1986b, 1987, 1988). Table 10 shows pairs of size-age class data
for different years for five study plots. In every case,
frequencies of tortoises in the juvenile and immature 1 size
classes (e.g., tortoises less than 15 years of age) have
declined, while subadult and adult frequencies have increased.
In populations with little or no raven predation, size-age class
frequencies for the juvenile and immature 1 tortoise groups are
generally higher than the 6% to 22% shown in Table 10 for data
collected between 1985 and 1988.

The data on population densities from the study plots underscore
the severity of raven predation on young desert tortoises.
Tortoise population densities have dropped substantially on the
western Mojave Desert plots, with declines ranging from
approximately 30% to 60% between 1979-1982 and 1985-1988 (Berry
et al. 1986a, 1986b, 1987, 1988). Substantial losses to
population densities are occurring in juvenile and immature 1

25

Table 10. Changes in size-age class composition of
desert tortoises at five permanent study sites in
the western Mojave Desert between the 1970 's and
the present*

Study Site

Frequency of Size-Age Class (%)
Year Juvenile/Immature 1 Subadult/Adult

Fremont Valley

Desert Tortoise
Natural Area

Desert Tortoise Nat
Area Interp. Ctr.

Fremont Peak

Kramer Hills

1981

30.0

1987

22.3

1979

15.3

1988

6.2

1979

18.6

1985

13.5

1977

21.9

1985

8.1

1982

38.9

1987

17.5

52.2
64.5

73.1
87.7

67.6
80.6

53.1
89.2

51.4
65.1

*Berry et al. 1986a, 1986b, 1987

size classes. Another indicator of the magnitude of predation is
the dead tortoises found on the plots (Table 9) . High
proportions of young tortoises <100 mm in length are being killed
by ravens. Taken as a unit, this information reveals that raven
predation is having a substantial adverse effect on the desert
tortoise population in the western Mojave Desert.

Of considerable concern is the lack of recruitment of young
tortoises into the adult breeding population. When tortoises are
preyed upon heavily in the hatchling to eight-year cohorts (each
year-class of tortoises is a "cohort") , then few tortoises are
incorporated into the cohorts for nine to 2 0 years and older.
Figure 2 shows a histogram of live tortoises registered at the
Desert Tortoise Research Natural Area Interior Plot in 1979 and
1988. The 1979 histogram depicts a tortoise population
experiencing intensive raven predation pressure; tortoise numbers
were very low in the size-age classes representing 60 to 100 mm
in length (i.e., 3 to 7 years of age). By 1988, the effects of
raven predation pressure were evident in all tortoise juvenile
and immature size-age classes, as well as in young subadults
(i.e., 3 to 20 years of age). The presence of very small

Desert Tortoise Natural Area - 1979

Site 3.

300

200

E

E 100

0)

c

20

16 12

Adults & Subedutts

Immatures & Juveniles

I I I l

8 12 16 20 24

<Q

C

Desert Tortoise Natural Area - 1988

0>

m 300

a

(0

w

(0

a

200 k

Site 3.

100-

20

<s

r^l

» 1 1 1 >

■ m 1 1 1 1 1 1 1 ii i

[num...;.;

9

Adults & Subadults

XHOttKOOOOOOOOMMOOOOttOO

u.

16 12

OOMOOOOMOOOC

-~-L

sS.

I

CI I I I I V I ll ?

Immatures & Juveniles

Z3

8

8 12 16 20 24

Figure 2. Histograms of live tortoises captured at the Desert Tortoise Natural Area
interior study site during 60-day spring surveys in 1979 and 1988. In tre
1979 histogram, note the low numbers of juvenile tortoises 60-100 mm n
length. In the 1988 histogram, note the lack of recruitment of juveni e
tortoises 60-100 mm in length into the larger size classes.

27

tortoises at the bottom of the histogram indicates that females
still lay eggs that are hatching. However, hatchling tortoises
are not surviving to reach the larger size and older age
categories. These histograms illustrate what is happening to
desert tortoise populations in the western Mojave Desert.

Effects of raven predation on desert tortoise populations are
similar in the northeastern Colorado Desert of California to that
in the western Mojave Desert, although not as severe. Two study
sites in the Ward Valley show low numbers of juvenile 1 and 2
tortoises (Berry et al. 1988, Karl 1988). One study plot
established in 1980 showed a decline in juvenile tortoise
size-age class frequencies from about 23% to 8% between 1980 and
1987. Excessive raven predation has been documented for this
general area.

Without immediate action to counter the losses of young
individuals in the tortoise populations, declines will continue.
Tortoise populations must recruit juveniles and immatures on a
regular basis to maintain themselves. While many other human
uses have severe impacts on tortoises (e.g., Berry and Nicholson
1984, Berry 1986), any management effort to reduce these impacts
will be fruitless unless tortoise populations have the
opportunity to recruit young. Therefore, excessive raven
predation should be reduced.

RAVEN EFFECTS ON OTHER RESOURCES

General Overview

Ravens have damaged crops, livestock, wild game, and structures.
Crop damage has included extensive consumption of pistachios in
the Central Valley of California (Salmon et al. 1986) and pecans
in Oklahoma (Leppla II, 1980). Ravens have been responsible for
killing lambs (Larsen and Dietrich 1970) , calves (Knittle and Orr
1988) , domestic turkeys (Lounsbury 1972) , and chickens (McManus
1935) . Damage by a relatively few birds can be extensive. At
the Fish Springs Hatchery, California, a group of approximately
10-30 ravens have consumed an estimated 100,000 trout, comprising
between 5-10% of total fingerling production (Jimmie Eichman,
California Dept. of Fish and Game, pers. comm.). Damage has
included power outages as a result of raven fecal contamination
of transmission tower insulators in Idaho and Oregon (Young and
Engel 1988) .

Although extensive information is available on raven diet, there
is a paucity of data regarding effects of raven predation on
wildlife resources. Bowles and Decker (1930) commented that
raven predation was exceedingly injurious to waterfowl due to the
"great number of eggs and young birds devoured." Dorn (1972)
observed a raven taking a young robin (Turdus miqratorius) from a

28

nest. Maser (1975) observed a pair of ravens attack a flock of
rock doves (Columbia livia) , and kill one of the group,
apparently for food. Mallory (1977) described a raven hunting
voles (Microtus pennsylvanicus) . Raven predation on sandhill
crane (Grus canadensis) eggs has also been reported (Littlef ield
1986) .

Ravens have been implicated in predation on two federally listed
endangered bird species: the California least tern (Sterna
antillarum browni) (Clark Winchell, Camp Pendleton, pers. com.)
and California condor (Gymnoqyps californianus) (Larry Salata and
Bill Lehman, FWS , pers. comm.). Circumstantial evidence
indicated that ravens destroyed eggs and nests and killed
nestlings in the California least tern colonies at Camp
Pendleton. Ravens have been observed stealing condor eggs on
several occasions during the early 1980s. Additionally, a condor
egg may have been destroyed by parent birds trying to chase
ravens away from the nest (Bill Lehman, FWS, pers. comm.).
Ravens have the capacity to prey on large numbers of individual
animals. Steenhof and Kochert (1982), in a study of the Snake
River Birds of Prey Area in Idaho, noted that ravens visited
their nests to feed young an average of 51 times per day.

Raven Impacts in the California Deserts

Little information has been obtained on raven effects to other
resources in the California deserts. Concentrations of ravens
have been noted around waste grain near Boron in the central
Mojave Desert (John Wear, pers. comm., Ron Rempel, CDFG, pers.
comm.), suggesting that these birds can readily identify and
consume grain crops. Declines in song bird nesting production at
the Harper Dry Lake marsh in San Bernardino County have also
coincided with a large increase in the raven population (Eugene
Cardiff, San Bernardino County Museum, pers. comm.). Raven
predation on nestling birds and bird eggs is believed to be a
contributing factor to the observed declines (Eugene Cardiff,
pers . comm. ) .

EFFORTS TO CONTROL RAVEN POPULATIONS

Synopsis of Bird Control Techniques

Control techniques for birds in general focus on modifying
behavioral patterns or lowering population numbers. Briefly,
these techniques include: use of chemicals to reduce feeding on
crops (aversion) , trapping, shooting, netting, modifying
structures used for nesting and perching, sterilization, hazing
(noise devices) or poisoning. These techniques have been applied
in various circumstances for control of ravens. The following
text provides an overview of these measures as they specifically
relate to raven control.

29

Raven Control Measures

Raven control programs have been instituted to reduce predation
of endangered California least tern chicks at Camp Pendleton in
California (Knittle 1988) , and egg predation of greater sandhill
cranes (Grus canadensis tabida) in Malheur National Wildlife
Refuge, Oregon (Littlefield 1984) . Raven control programs have
also been implemented to reduce raven attacks on livestock in
Oregon (Larsen and Dietrich 1970) and Arizona (Knittle and Orr
1988) , and predation on domestic turkey eggs in Apple Valley,
California (Lounsbury 1972) .

Lethal Control Methods

Control efforts have included poisoning ravens by treating
hard-boiled chicken eggs with the avicide DRC-1339 (Starlicide)
(Knittle and Orr 1988, Knittle 1988, Littlefield 1984) or
injecting Starlicide into meat baits (Knittle and Orr 1988).
Starlicide was developed for control of pest bird species by the
U.S. Fish and Wildlife Service (USFWS 1985), and is acutely toxic
to a narrow range of species, principally starlings, doves, and
crows and their relatives. Game birds and owls are also highly
susceptible (Table 11) . A nonviolent death occurs in one to two
days after ingestion of treated baits, from kidney failure or
central nervous system depression (DeCino et al. 1966).
Exposure potential to other wildlife species from secondary
poisoning is very low (Schafer 1984); monitoring during the 1989
pilot raven control program in the California Desert Conservation
Area did not document any contact with or observed interest in
poisoned eggs (Rado 1990) . Program monitoring results for
control projects employing DRC-13 39 ranged from substantial
decline in the target raven population (Larsen and Dietrich 1970)
to limited reductions of a few "targeted" birds (Knittle 1988) .
Starlicide poisoning has also been used in concert with shooting
(Littlefield 1984) .

A pilot program to selectively reduce raven numbers in portions
of the California deserts was undertaken during 1989. The
program was developed in response to documented declines in
desert tortoise recruitment as a result of raven predation. The
objective of the program was to boost desert tortoise recruitment
by selective reductions of ravens. Program activities included
identifying areas for control, monitoring control areas, and
killing ravens at these sites using a combination of poisoning
with DRC-1339 and shooting. Longer-term actions will involve
monitoring desert tortoise populations near control sites. Raven
control was focused in three areas: the Desert Tortoise Research
Natural Area (western Mojave Desert, Figure 3), several raven
nesting and perching sites in the general vicinity of Needles
(Colorado Desert), and a landfill at the U.S. Marine Corps Air
Ground Combat Center at Twentynine Palms (central Mojave Desert) .

30

Table 11. Toxicity of Starlicide to various species of
wildlife*

Species

Approximate lethal oral dose
(50% of treated animals, mg/kg)

Amphibians
Frog

Reptiles

Turtle

Birds

Blue-winged Teal
Starling

Redwinged Blackbird
Mourning Dove
Rock Dove
Ground Dove
White-winged Dove
Golden Eagle
American Kestrel
Cooper's Hawk
Northern Harrier
Red-tailed Hawk
California Quail
Common Raven
American Crow
White-crowned Sparrow
House Sparrow
Curve-billed Thrasher
Barn Owl

225**

1,040**

31

3

1

5
17

4.2

4.2
100
320
320
100
320
10
13

1
320
320

3

4

6

8

8-3.2

6-10.0

7

- 1,000

5
33

- 448

2

2

Mammals

Cow

Coyote

Dog

Dog

Laboratory Mice

Deer Mouse

Laboratory Rats

Grey Squirrel

Sheep

10

(no kill)

100

100

(no kill)

71

2, 000

1,800

1, 170

- 1,770

280

1 animal

harmed at

2 00;

1 animal

killed at

400

♦Sources: Schafer, 1984; Environmental Protection
Agency data; Denver Wildlife Research Center data
**Dose not administered orally

31

Approximately 100-110 ravens were killed at the Twentynine Palms
landfill. A total of 96 raven carcasses were subsequently
recovered from this site (Table 12) . Approximately 6-10 ravens
were killed at the Desert Tortoise Research Natural Area.
Program monitoring indicated that ravens readily accept and
consume poisoned baits and risk potential to other species of
wildlife during control is very low (Rado 1990) .

Table 12. Results of raven control at the USMC
Twentynine Palms landfill during 1989*

Date Poisoned eggs
Consumed

May 19

29

May 2 0

22

May 21

8

May 2 2

6

May 2 3

6

May 24

4

May 25

-

Totals 75

Raven carcasses

recove

red

Poisoned

Shot

0

0

0

0

31

0

26

9

13

6

4

2

4

1

78 18

*Source: U.S. Dept. of Agriculture Data

Nonlethal Control Methods

Limited research has been undertaken on more "passive" forms of
raven control. Attempts to dissuade ravens from utilizing
transmission towers as nesting sites in the Mojave Desert have
been unsuccessful. When raven nests have been destroyed, the
birds simply rebuild the nest or move to an adjacent tower (Dan
Pearson, Southern California Edison Company, pers. comm.).
Stiehl (1978) has noted that nest destruction early in the raven
nesting season results in re-nesting by the birds. However,
nests destroyed late in the season are not reoccupied.

Attempts to reduce the number of ravens at Malheur National
Wildlife Refuge utilizing a program of nest destruction and
destruction of raven eggs and broods were unsuccessful at
reducing raven depredation on waterfowl eggs. Study results on
raven behavior during this time indicated that almost 62% of all
bird items collected were obtained from six of 34 raven nests
under study. The researcher also noted that non-nesting area

CANYON

6

• Monitoring Location

ravens observed

0 Monitoring Location
ravens heard

O Monitoring Location
no ravens
A Empty raptor nest
in Joshua Tree

* Dead juvenile tortoise

T31S

LU

cr MDM

^ni'lnt" L°Cati0nS ^ reSU'tS °f raVen monito^g efforts during the
989 .flV . raVen COntro1 Pr°9ram at the Desert Tortoise Research Natural

Area (BLMdata)

•

33

ravens probably preyed heavily on waterfowl eggs; non-nesting
flocks of ravens were observed carrying eggs, and waterfowl nest
destruction was subseguently recorded where a raven flock had
been observed on the previous day (Stiehl 1978) .

Conditioned taste aversion has been undertaken to reduce raven
predation on eider eggs in Iceland (Kristinn Skarphedinsson,
University of Wisconsin, pers. comm.). Eggs were injected with
the emetic carbachol to condition ravens to avoid eating eggs.
This particular project was not successful. Birds routinely
wandered through the area, making conditioning difficult.
Additionally, aversion training for study ravens appeared to be
poor; ravens eating carbachol-treated eggs would repeat this
process within one week (Kristinn Skarphedinsson, pers. coram.) .

However, in a separate experimental study on raven egg predation
at Malheur National Wildlife Refuge, egg aversion conditioning
significantly reduced egg predation relative to untreated eggs at
"control" sites (Nicolaus 1987) . The advantage of this process
over lethal forms of control is that conditioned birds may defend
territories against emmigrants that may prey on "target" prey
(Lowell Nicholaus, Northern Illinois State University, pers.
comm.). Little information is available on the length of time
that conditioned animals will avoid consuming "target" prey
species (Lowell Nicholaus, pers. comm.).

Taste aversion has also been undertaken using two other
compounds, Landrin and Mesurol. Control may be limited in
duration, due to the ability of predator species to develop a
tolerance to the chemical and resume feeding without any ill
effects. Additionally, some of the animals that consume treated
baits may die as a consequence (Woody Hill, FWS, pers, comm.).

Methods to reduce perching and roosting on structures have also
been applied to control ravens. Young and Engel (1988) were able
to prevent fecal contamination of transmission tower insulators
by roosting ravens using a combination of plastic shields and
wooden pegging. However, although insulator contamination was
effectively minimized, ravens continued to use the original
transmission towers as roosts. Different anti-perch wire designs
experimentally tested lowered but did not prohibit continued
perching by ravens (Young and Engel 1988) .

Several anti-perching wire products are commercially available.
These devices are affixed to fence posts or other sites where
problem birds regularly perch. Independent research is lacking
on the effectiveness of these devices (Ed Nittle, Denver Research
Center, pers. comm.).

Anti-perch wire was experimentally placed on wooden fence posts
along the entire 40-linear mile perimeter of the Desert tortoise
Natural Area during the fall of 1989. Two different anti-perch

34

wire designs were used, Nixalite and Barrier Wire. The wire was
placed by agency personnel, volunteers, staff assistance from
Nixalite of America and Patented Products, Incorporated, and The
Humane Society of the United States. These posts will be
monitored in an effort to determine ef fectivenesss at reducing
raven perching opportunities. Incidence of raven predation on
juvenile desert tortoises at the Natural Area will be obtained
through continued monitoring of two study plots.

Unspecified declines in raven numbers have been noted during
live-trapping (Lounsbury 1972) . The disposition of live-trapped
birds (whether killed or released elsewhere) was not discussed.
Steihl (1978) believed that any live-trapped birds should be
moved minimally 124 miles from initial point of capture before
release. Live-trapping methods include cannon netting, which has
been used successfully for ravens at concentration areas (Dave
Ledig, FWS , pers. comm. ) .

MANAGEMENT ACTIONS

Overview of Planned Tasks

The raven management program consists of two basic components:
(1) short-term application of lethal forms of raven control at
sites where excessive predation threatens tortoise populations;
and (2) longer-term regional management based on results of
initial testing of nonlethal forms of control, research on the
ecology of the raven in the California Deserts, and results of
regional inventories to locate sites where raven predation on
tortoises is occurring. Ravens will be selectively killed at
sites where recruitment rates of juvenile tortoises into adult
age-classes has been severely depressed as a result of predation.
These sites include portions of the western Mojave Desert and
northern Colorado Desert. Killing of birds is necessary on a
short-term basis due to the severe losses of juvenile tortoises
and immediate need to boost recruitment rates to increase chances
of longer-term population survival.

Concurrently with initial site-specific poisoning and/or shooting
of ravens, research shall be conducted on the ecology of the
raven in the California deserts. The focus of this research
effort shall be on obtaining information that may be used to more
effectively manage and maintain raven populations in the desert
while reducing predation rates on the desert tortoise. Baseline
information on total numbers of ravens in the California deserts
shall also be obtained. Current estimates of raven increases are
based on comparisons of relative abundance (e.g. , numbers of
birds observed per unit time) , and not on actual population
numbers. Emphasis of research programs shall be outside of those
site-specific areas where lethal control measures are applied.

35

Several nonlethal forms of raven control shall be experimentally
applied to test effectiveness. Nonlethal techniques that will be
experimentally applied include: conditioned taste aversion; timed
destruction of raven nests; egg addling; placement of anti-perch
devices; live-trapping and off site release of birds; modification
of human waste containment practices; and sterilization. Initial
"focus" of this program shall be in specific areas that will be
limited to that form of experimental control only in order to
determine effectiveness. Where nonlethal forms of raven control
are proven effective, these techniques can be used to substitute
initial control using lethal methods. Over time, raven control
using lethal techniques may be "phased out" or substantially
reduced in favor of one or more nonlethal control measures.

Implementation of program actions are contingent upon inter-
agency cooperation. A "lead" agency is assigned for each
specific task in Table 13. The "lead" agency is responsible for
coordinating project actions with other agencies, completing
necessary paperwork to undertake the project, developing
appropriate experimental design, implementing and monitoring
results of the action, and preparing a written report of results.
Establishment of an interagency Workgroup provides the basic
framework necessary to undertake management plan tasks.
Implementation of planned actions is contingent upon available
staff time and funding by each participating agency.

Actions for implementation of the raven management plan are
summarized below (Table 13) . Management actions are designed to
achieve a specific stated objective. Techniques have been
subdivided into measures addressing research, nonlethal control,
lethal control, and administrative actions.

Planned Actions

The following text details measures that will be undertaken for
the management of the raven in the CDCA. Tasks have been
subdivided into research, nonlethal control, lethal control, and
administrative actions. Management actions are overviewed in
Table 13.

Research Actions

Action 1: Conduct regional inventories of the California

Desert Conservation Area to locate and map raven
nesting, roosting, and perching sites (Call 1978).
Inventories should include both private and public
lands.

Objective; Increase information on the distribution and

nesting habits of the common raven in the
California deserts.

36

Table 13. Management actions, cooperating agencies,
and proposed time frames for initiation.

Action

Agencies*

Date(s)

Research

Inventory California deserts
for raven nesting and roosting
sites

Determine methodologies for
censusing and monitoring raven
populations

BLM, FWS, CDFG**

FWS, CDFG, BLM

1991-92

1990-91

Determine foraging and feeding
behaviors of ravens

Determine seasonal movements
and site fidelity of ravens

Conduct regional inventories to
locate sites of raven predation
on tortoises

Collect information on tortoise
recruitment rates in raven
habitats

CDFG, FWS, BLM
CDFG, FWS, BLM
BLM, FWS, CDFG

BLM, CDFG,
FWS , DOD

1991-92

1991-92

1990-91

Continuous

Nonlethal Control

Limit raven food sources from
roadkills

Modify landfill practices

Modify sewage containment
practices

Caltrans, FWHA,
CDFG, BLM, FWS

Counties, Cities,
SWMB, BLM

Counties, Cities,
RWCB, BLM

Begin 1990
Begin 1990
Begin 1990

Modify raven use of landfills
and sewage ponds by
behavioral disruption
techniques

Modify raven perches and
roosts

Counties, Cities,
SWMB, RWCB, CDFG,
BLM

BLM, CDFG,
DOD, FWS,
ADC

Begin 1991

Begin 1990

37

Table 13. Continued

Action Agencies* Date(s)

Selectively destroy raven ADC, BLM, FWS, Begin 1991

nests CDFG, DOD

Reduce raven nesting success ADC, BLM, FWS, Begin 1991

by egg addling CDFG, DOD

Sterilize nesting ADC, FWS, CDFG, Begin 1991

ravens in areas of high DOD, BLM
tortoise predation

Reduce raven predation rates ADC, FWS, BLM, Begin 1991

on tortoises by conditioned CDFG, DOD
taste aversion

Live-trap and remove ravens CDFG, ADC, FWS, Begin 1991

for release outside of BLM, DOD,
tortoise areas

Lethal Control

Selectively shoot ravens in ADC, BLM, CDFG, Begin 1990

areas of high tortoise FWS, DOD

predation

Kill ravens by poisoning ADC, BLM, CDFG, Begin 1990

Live-trap and kill ravens ADC, FWS, CDFG, Begin in

areas of excessive BLM, DOD 1993***
tortoise predation

Administrative Actions

Establish Interagency Workgroup BLM, FWS, CDFG, 1991

to oversee management direction DOD, ADC,

Counties

Develop implementing agreement BLM, FWS, CDFG 1991
to ensure agency commitments

Modify management measures as All of above Begin 1990

determined by additional data/ (BLM lead)

monitoring

38

*BLM = U.S. Bureau of Land Management
FWS = U.S. Fish and Wildlife Service

DOD = U.S. Department of Defense (includes the Marine Corps
Air Ground Combat Training Center and the Air Force
Flight Test Center)
ADC = U.S. Dept. of Agriculture, Animal Damage Control
FHWA = Federal Highways Administration
CDFG = California Department of Fish and Game
RWCB = Regional Water Quality Control Board
Counties = Counties discussed under individual tasks below
Cities = Cities discussed under individual tasks below
**First agency listed is "lead", and has responsibility of
establishing specific tasks, timeframes, and costs, and
coordinating with other participating agencies.
***Action contingent upon success of livetrapping and release;
would be dropped if release program successful.

Rationale; Information on raven nesting, perching, and

roosting sites has been available largely as a
result of incidental observation and site-specific
surveys. Systematic inventories are reguired to
more effectively manage and monitor this species.

Responsible Agencies; BLM, FWS, CDFG

Action 2; Determine methodologies for censusing and

monitoring raven populations.

Objective; Standardize data collection on the raven between

agencies.

Rationale; Management of the common raven will reguire

several agencies. Duplication of effort will be
minimized and funding best utilized by obtaining
initial interagency agreement on the methodologies
for censusing and monitoring raven populations.

Responsible Agencies; FWS, CDFG, BLM

Action 3; Determine foraging and feeding behaviors of common

ravens, employing both studies of
radio-transmittered birds captured at landfills
and birds captured from roosting and nesting
sites.

Objective; Supplement current life history information on the

ecology of the raven to facilitate long-term
management of populations in the CDCA.

39

Rationale: Little information is available on the home range
size, foraging behavior patterns, and daily and
seasonal feeding behaviors of ravens in the CDCA.
Such information is needed to facilitate
management of the species.

Responsible Agencies; CDFG, FWS , BLM

Action 4; Determine seasonal movements and site fidelity of

ravens.

Objective! Increase understanding of the biology of the raven

in the California deserts. Apply information to
other management actions to manage ravens on a
long-term basis.

Rationale; Little information is available on the daily and
seaonal movement patterns of the raven in the
California deserts. Basic information relating to
seasonal use patterns of juvenile birds (that may
collectively comprise the majority of the desert
raven population) , and nesting site fidelity of
adult birds (that may function as the major source
of predation on juvenile desert tortoises) is
required.

Responsible Agencies: CDFG, FWS, BLM

Action 5: Conduct regional inventories to locate sites of

raven predation on juvenile desert tortoises. Map
locations, and record such data as: location, type
of site, numbers and sizes of tortoise shells
observed, numbers of ravens observed and behavior,
and recommendations for reducing predation rates.
Monitor sites on a regular basis to determine
frequency of predation.

Objective; Determine specific areas of raven predation on

juvenile desert tortoises.

Rationale: Information on raven predation, to date, has been
based on site-specific studies and largely
incidental observations of juvenile tortoise
shells collected desert-wide. Additional
information needs to be systematically collected
and collated to more effectively manage both
tortoise and raven populations.

Responsible Agencies: BLM, FWS, CDFG, DOD

40

Action 6: Collect baseline data on recruitment rates of

juvenile desert tortoises. Establish a high
priority for continued monitoring of tortoise
populations at sites proximate to 1989 raven
control efforts (Desert Tortoise Research Natural
Area and U.S. Marine Corps Air Ground Combat
Center at Twentynine Palms landfill) . Establish
trend plots to monitor effects of refuse
containment practices and other raven control
measures proximate to land fill sites and sewage
treatment facilities.

Objective; Determine effectiveness of raven control for

enhancing recruitment rates of juvenile desert
tortoises into adult age-classes.

Rationale: Long-term results of the 1989 pilot raven control

program and followup control efforts will require
evaluation. Although pilot program efforts were
limited during 1989, results clearly demonstrated
that common ravens may be readily poisoned using
DRC-1339 injected into hard-boiled eggs (Rado
1989) . However, the results of this raven
reduction effort towards enhancing desert tortoise
recruitment rates will require study over a
several-year timeframe. Results of other
nonlethal and lethal forms of raven control
outlined in this management plan will require
similar evaluation.

Responsible Agencies; BLM, CDFG, FWS, DOD

Nonlethal Control Actions

Action 1: Reduce wildlife mortality along major roads

bisecting desert tortoise populations. Provide
maps of specific areas adjacent to State or
Federal highways where raven predation on juvenile
desert tortoises is excessive. Recommend
placement of barrier fencing along such specific
sections of highways to reduce vehicle-related
mortality to wildlife. Also recommend that
Caltrans implement a program of regular collection
of wildlife carcasses from these roads for
disposal and burial.

Objective: Limit raven food availability from road kills;

reduce concentrations of birds in areas of high
desert tortoise predation.

41

Rationale: Ravens are known to utilize roadkills as a food

source (Alten 1971; Conner and Adkisson 1976) . A
program to reduce wildlife road mortality,
emphasizing selective placement of protective
fencing to impede animal movement onto the
pavement and, secondarily, efficient collection
and disposal of road-killed animals, may
significantly reduce food for ravens in especially
sensitive tortoise areas.

RESPONSIBLE AGENCIES: Caltrans, FHWA, BLM, CDFG

Action 2: Modify landfill containment practices in areas

where raven predation on juvenile desert tortoises
is excessive. Contact respective County and
municipal governments and recommend landfill
containment practice changes to reduce
availability of food for ravens at these sites.
Provide pertinent information to facilitate more
effective refuse control.

Objective: Limit availability of food and water to ravens at

raven concentration sites in tortoise habitats.

Rationale: Desert landfills provide a food supply for ravens
and a "concentration" point for loafing and
roosting birds. Food may serve to attract large
numbers of birds into desert tortoise habitats.
Ravens may fly up to 28 miles from a roosting site
to forage for food (Stiehl 1978) . Larger numbers
of birds that utilize expansive desert areas
surrounding each landfill for extended periods of
time may contribute to significantly increased
predation rates on juvenile desert tortoises.

Responsible Agencies: County of Kern, County of San

Bernardino, County of Riverside,
State of California Solid Waste
Management Board, Cities of: Ridgecrest,
Boron, Apple Valley, Victorville,
Landers, Twentynine Palms, Baker, and
Desert Center, BLM.

Action 3: Modify sewage treatment facility practices in

areas where desert tortoise predation rates are
excessive. Contact appropriate civil and State
government agencies with recommendations for
management modifications. Also request
experimental research on measures to determine
most effective means to reduce raven use.

42

Objective; Limit raven food and water availability at raven

concentration areas in tortoise habitats.

Rationale; Sewage treatment sites provide a concentration

point for large numbers of ravens and available
sources of food and water. Concentrating birds
and bird foraging areas may contribute to
excessive tortoise predation on surrounding lands.

Although Knowles et al . (1989) inventoried these
sites, no specific measures to reduce raven use
were provided. Actions that could be employed in
this regard may include placement of a mesh
netting over pond areas where birds habitually
feed or installation of anti-perching wire on
pipes or other structures used to provide access
by feeding birds. Experimental application of
measures to reduce raven feeding opportunities,
focusing at the three sites where consistently
high raven use was documented, is required to
develop appropriate management techniques.

Responsible Agencies; County of Kern; County of San

Bernardino; State of California Water
Resources Quality Control Board; Cities
of: Ridgecrest, Boron, and Baker; BLM

Action 4; Modify raven use of landfill and sewage pond sites

by behavioral disruption techniques.
Experimentally apply at selected sites to
determine effectiveness. Utilize on a widespread
basis if proven effective.

Objective; Prevent continued use of landfills and sewage

ponds by large numbers of birds. Limit
opportunities for access to food supplies.

Rationale; Implementation of measures such as "hazing" (i.e.,

disrupting raven perching and feeding by
intermittent loud noise) and taste aversion
conditioning may be useful tools to discourage
raven use from concentration points such as
landfills. Hazing, in association with modified
trash containment practices to reduce food
availability to ravens, may significantly reduce
raven use of such areas.

Responsible Agencies; County of Kern; County of San

Bernardino; RWCB; City governments
administering landfill and sewage
treatment sites; BLM

43

Action 5; Modify selected raven roosting and perching sites

on artificial structures and natural features.
Experimentally attempt to retard raven perching at
selected sites by application of anti-perching
wire or other devices. Monitor effectiveness of
such measures and apply on a more widespread basis
if monitoring results show techniques are
effective.

Objective; Reduce raven concentration areas/limit raven

predation opportunities on juvenile tortoises.

Rationale: Researchers (Campbell 1983; Berry 1985) have

observed numbers of individual desert tortoise
shells under specific raven nesting and perching
sites. Predation and collection of many tortoises
from nesting sites has also been observed at the
same sites over several-year periods (Campbell
1983; Berry 1985; Woodman and Juarez 1988). This
data indicates that, in certain areas, individual
birds or nesting pairs of birds may be
significantly reducing constituent tortoise
recruitment rates. Restricting perching and
roosting opportunities in these areas may force
birds to modify foraging behavior patterns.

Responsible Agencies: ADC, BLM, CDFG, DOD, FWS

Action 6; Destroy raven nesting sites in areas experiencing
high tortoise predation rates. Initially,
experimentally destroy nests at pre-selected
locations. Attempt to "time" nest destruction
relatively late in the nesting season to increase
likelihood of effectiveness. Monitor "parent"
bird behavior following nest destruction. Apply
on a more widespread basis if initial results
prohibit successful nesting attempts by birds.

Objective: Modify nesting behavior to limit successful

nesting by birds that prey most heavily on
tortoises.

Rationale: Selective nest destruction, appropriately timed,
may prohibit successful nesting by individual
pairs of birds known to prey excessively on
tortoises. Although successful nest destruction
has been noted (Stiehl 1978) , other information
suggests that these measures do not prohibit
ravens from nesting and perching in the same area
(Dan Pearson, pers. comm. ; Young and Engel 1988).

Responsible Agencies: ADC, BLM, FWS, CDFG, DOD

44

Action 7; Experimentally attempt to restrict successful

raven nesting by "parent" birds determined to be
preying on desert tortoises by addling of eggs.
If successful on an experimental basis, apply this
technigue to other nesting sites.

Objective; Reduce successful nesting by ravens that prey

heavily on tortoises. Limit opportunities for
such birds to "teach" offspring to prey on
tortoises.

Rationale; Egg addling (scrambling eggs on a nest by vigorous

shaking, then returning eggs to confuse nesting
"parent" birds) may provide a long-term means of
selectively reducing numbers of individual birds
in the desert raven population that have learned
to efficiently hunt and kill tortoises.
Application of this technigue assumes that
"parent" birds "teach" offspring to hunt for
certain kinds of foods such as tortoises. A prior
attempt to use this technigue for raven control in
Oregon did not significantly reduce raven nesting
success (USFWS 1986) . Experimental application in
the California deserts is therefore necessary.
Replacement of eggs with artificial ceramic eggs
may also be undertaken at that time.

Responsible Agencies; ADC, BLM, FWS , CDFG, DOD

Action 8; Experimentally attempt to reduce raven population

increases in desert areas by sterilization of
birds. Attempt to sterilize birds in areas of
high tortoise predation rates. Monitor raven
population trends and tortoise predation rates by
ravens in areas where sterilization has been
undertaken. Expand sterilization program if
successful .

Objective; Reduce raven population increases in an effort to

limit predation rates by ravens on desert
tortoises.

Rationale; Raven populations are increasing at a rapid rate

in areas experiencing highest tortoise predation
levels. Effective means to reduce reproductive
rates of birds need to be explored. Trimethyl
ether can be applied to grain baits administered
to birds at regular intervals during the breeding
season to prevent successful reproduction. Ravens
need to consume this treated grain at 2-3 day
intervals to be effective (Lowell Nicholaus, pers.

45

comm.). However, the technique is untested on
ravens and requires further study.

Responsible Agencies; ADC, FWS, CDFG, DOD, BLM

Action 9; Experimentally attempt to reduce raven predation

rates by conditioned taste aversion usinq "dummy"
tortoises treated with carbachol or similar
emetic. Monitor feedinq behavior of test birds
after experimental treatment. If successful,
apply technique on a widespread basis.

Objective: Modify selective predation pressure from ravens

throuqh conditioned taste aversion.

Rationale: Conditioned aversion traininq may provide a

cost-effective method of reducinq predation rates
on juvenile desert tortoises. If successful, this
technique has the added advantaqe of maintaininq
conditioned birds in areas that may otherwise be
quickly re-occupied by emmiqratinq ravens that
prey on tortoises. Additionally, conditioned
birds may "teach" offsprinq or mates to avoid
tortoises.

Responsible Agencies: ADC, FWS, BLM, CDFG, DOD

Action 10: Experimentally attempt to reduce raven predation

rates on desert tortoises by live-trappinq and
removal of birds. Live-trappinq techniques
include: cannon-nettinq, leq-hold traps, and
drop-in traps (U.S. Fish and Wildlife Service
1983; Enqel and Younq 1989b; Bloom 1987). If
successful, employ methods on a widespread basis.

Objective: Reduce raven numbers in areas where excessive

tortoise predation has been documented to increase
recruitment rates of juvenile tortoises into adult
aqe-classes.

Rationale: Live-trappinq has been used for ravens on a

limited basis in the California deserts.
Lounsbury (1972) mentioned reducinq raven numbers
in response to predation on domestic turkey eggs
in Apple Valley by live-trapping. However,
disposition of captured birds was not specified.
Ravens have the capability to travel larqe
distances (Steihl 1978; Kate Enqel, University of
Wisconsin, pers. comm. ; Dick Steel, Southwestern
Missouri State University, pers. comm.). Behavior
patterns of captured and released birds also
requires monitorinq, since release may result in

46

increased predation on desert tortoises or other
sensitive wildlife species within a previously
unimpacted site.

Responsible Agencies; CDFG, ADC, FWS, BLM, DOD

Lethal Management Techniques

Action 1; Selectively reduce raven populations in areas of

excessive tortoise predation by shooting birds.

Objective; Reduce raven predation rates on juvenile tortoises

by killing birds where excessive rates of
predation on tortoises have been documented.

Rationale; Nonlethal forms of raven control are largely

experimental. Shooting can be an effective means
of selective raven control at locations where
limited numbers of birds may be significant
contributors to tortoise declines.

Responsible Agencies; ADC, BLM, CDFG, FWS, DOD

Action 2 ; Reduce raven numbers in areas of excessive

predation by poisoning birds. Poisoning will be
applied on a site-specific basis within the
California Desert Conservation Area. DRC-1339
(Starlicide) will be used. This avicide will be
injected into hard-boiled eggs.

Objective; Increase tortoise recruitment rates by reducing

levels of predation by ravens.

Rationale; Raven numbers can be reduced using DRC-1339; risk

to other wildlife species using this technique is
very low (Rado 1989) . Conversely, alternative
nonlethal measures of raven control are largely
untested. Ravens that have their behavior
patterns disrupted by restricting nesting or
perching opportunities or by limiting roosting
areas may simply move to other areas and prey on
juvenile tortoises.

Responsible Agencies; ADC, BLM, CDFG, FWS

Action 3; Selectively reduce raven populations in areas of

excessive juvenile desert tortoise predation by
initially live-trapping, then killing birds.
Implement this action if initial attempts to
relocate ravens successfully fail.

47

Objective; Reduce raven numbers in areas where excessive

tortoise predation has been documented to increase
recruitment of juvenile tortoises into adult
age-classes.

Rationale; Live-trapping of ravens may prove an effective
means of capturing birds. However, subsequent
release of captured birds may not reduce overall
raven predation rates. Released birds may simply
begin killing and eating tortoises at points of
release, or may impact other sensitive wildlife
species.

Responsible Agencies; ADC, FWS, CDFG, BLM, DOD

Administrative Actions

Action 1; Establish interagency Workgroup to oversee raven
management program direction, review information,
coordinate with other agencies/groups, solicit
funding for implementation of specified management
measures, and distribute information/data. The
Workgroup shall meet regularly to discuss raven
management actions.

Objective; Increase efficiency of raven management program in

the CDCA.

Rationale; Implementation of management plan actions will

require a coordinated effort by several agencies.
The plan area covers roughly one third of the
State of California. Management actions are large
in scope and may be difficult to fund and
implement. Several agencies maintain
jurisdictional authorities over lands or
permitting authorities over actions that require
management. Increased coordination between these
agencies will facilitate plan implementation.

Responsible Agencies; BLM; FWS; CDFG; DOD; ADC; County of San

Bernardino, County of Kern, County of
Riverside, County of Los Angeles, County
of Inyo.

Action 2; Prepare an Implementing Agreement that shall

provide specific direction and agency commitments
for carrying out planned management tasks between
Workgroup agencies. Representatives of each
agency shall sign this agreement.

48

Rationale: Implementation of management measures specified in

this plan will require a long-term commitment in
terms of funding, personnel, and administrative
time. A binding document that specifies roles of
each agency towards raven management is needed to
ensure that this commitment is maintained.

Responsible Agencies; BLM, FWS, CDFG, County of San

Bernardino, County of Kern, County of
Riverside, County of Los Angeles, County
of Inyo.

Action 3; Monitor both raven status and effectiveness of

management actions at reducing excessive predation
rates on juvenile desert tortoises while
maintaining viable raven populations in the
California deserts. Adjust management measures as
necessary to ensure that primary and secondary
objectives of the management plan are met.

Rationale: Implementation of some of the preceding plan

measures may be ineffective at accomplishing plan
management goals. Additional actions may also be
required. Plan changes may be desired based on
additional information derived from monitoring or
life history information obtained on the common
raven through implementation of plan actions.

Responsible Agencies: BLM, FWS, CDFG, DOD, ADC, RWCB, SWMB,

County and City governments.

49

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U. S. GOVERNMENT PRINTING OFFICE: 1990 — 787-079/39152

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