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Population status of Bald Eagles breeding in Washington at the end of the 20th century

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From 1980-98 the population of Bald Eagles (Haliaeetus leucocephalus) nesting in Washington increased (P < 0.001) at an exponential, annual rate of 10% as adult eagles reoccupied habitat vacated during the period of widespread persecution and DDT use. Further indications of population health were linear increases in the rates of nest occupancy, productivity, and nest success. Productivity and nest success of eagles affected by contaminants along Hood Canal and the Washington side of the Columbia River estuary also increased during the study period but remained below statewide averages. By 1998, the population was widely distributed, with 89% of pairs nesting west of the Cascade crest, and 11% east of the crest. There were indications that the population stabilized from 1993-98, when statewide occupancy rates decreased (P = 0.040), and productivity and nest success stabilized. Modeling predicts that a statewide population of 733 breeding pairs at carrying capacity would, after 25 yr, provide an equilibrium population of 4913 eagles. Stability of the statewide population of Bald Eagles seems to be less dependent on productivity rates than on adequate numbers of replacement adults, as maintained through high survival.
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161
THE JOURNAL OF RAPTOR RESEARCH
A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC.
V
OL
.36 S
EPTEMBER
2002 N
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.3
J. Raptor Res. 36(3):161–169
q
2002 The Raptor Research Foundation, Inc.
POPULATION STATUS OF BALD EAGLES BREEDING IN
WASHINGTON AT THE END OF THE 20TH CENTURY
J
AMES
W. W
ATSON
,
1
D
EREK
S
TINSON
,K
ELLY
R. M
C
A
LLISTER
,
AND
T
HOMAS
E. O
WENS
Wildlife Program, Washington Department of Fish and Wildlife, 600 Capitol Way North, Olympia, WA 98501 U.S.A.
A
BSTRACT
.—From 1980–98 the population of Bald Eagles (Haliaeetus leucocephalus) nesting in Washington
increased (P
,
0.001) at an exponential, annual rate of 10% as adult eagles reoccupied habitat vacated
during the period of widespread persecution and DDT use. Further indications of population health were
linear increases in the rates of nest occupancy, productivity, and nest success. Productivity and nest success
of eagles affected by contaminants along Hood Canal and the Washington side of the Columbia River
estuary also increased during the study period but remained below statewide averages. By 1998, the pop-
ulation was widely distributed, with 89% of pairs nesting west of the Cascade crest, and 11% east of the
crest. There were indications that the population stabilized from 1993–98, when statewide occupancy rates
decreased (P
5
0.040), and productivity and nest success stabilized. Modeling predicts that a statewide
population of 733 breeding pairs at carrying capacity would, after 25 yr, provide an equilibrium population
of 4913 eagles. Stability of the statewide population of Bald Eagles seems to be less dependent on pro-
ductivity rates than on adequate numbers of replacement adults, as maintained through high survival.
K
EY
W
ORDS
:Bald Eagle; Haliaeetus leucocephalus; breeding;population status;productivity;recovery;Washington.
Status poblacional del a´ guila calva en reproduccio´ n en el estado de Washington a finales del siglo 20
R
ESUMEN
.—Desde 1980–98 la poblacio´n de a´guilas calvas (Haliaeetus leucocephalus) en anidacio´n en Wash-
ington ha aumentado (P
,
0.001) en una tasa exponencial del 10% debido a la reocupacio´ n del habitad
vacante durante el periodo de persecucio´n directa y uso de DPT. Algunos indicadores adicionales de una
poblacio´ n saludable fueron el incremento linear en las tasas de ocupacio´ n de nidos, su productividad y
el e´xito de anidacio´n de las a´guilas afectadas por los contaminantes a lo largo del canal de Hood y el
costado del estuario del Rı´o Columbia el cual tambie´n aumento durante el estudio pero que permanecio´
por debajo de los promedios del estado. En 1998, la poblacio´ n estaba ampliamente distribuida, con 89%
de las parejas anidando en el oeste de Cascade Crest y 11% al este. Hubo sı´ntomas de que la poblacio´n
se estabilizo desde 1993–98, cuando a nivel del estado, las tasas de ocupacio´ n disminuyeron (P
5
0.040)
y la productividad y el e´xito de anidacio´n se estabilizaron. Un modelo elaborado establece que la poblacio´n
a nivel del estado de 733 parejas en anidacio´n,asuma´xima capacidad de carga, despue´s de 25 an˜os
resultarı´a en una poblacio´n en equilibrio de 4913 a´guilas calvas. Finalmente, la estabilidad de la poblacio´n
a nivel del estado, de a´guilas calvas parece ser menos dependiente las tasas de productividad que de los
nu´ meros adecuados del reemplazo de adultos mantenidos por un alta sobre vivencia.
[Traduccio´n de Ce´sar Ma´rquez]
For the past 25 years, the population of Bald Ea-
gles (Haliaeetus leucocephalus) breeding in Washing-
ton has been extensively surveyed, researched, and
managed in an effort to recover the species from
1
E-mail address: watson@valleyint.com
state and federal threatened status. In the 1970s,
114 nesting pairs produced a mean of 0.75 young/
occupied territory (Grubb 1976). By 1985, the
population had increased to 227 pairs, but produc-
tivity remained below that of other populations
(McAllister et al. 1986). Surveys since the 1980s
162 V
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ATSON ET AL
.
documented a further increase in the breeding
population (Washington Department of Fish and
Wildlife [WDFW], Heritage Data Base unpubl.
data). The need to reevaluate the recovery status
of the species prompted a review of the population
(Stinson et al. 2001). Here, we report the results
of that assessment for breeding eagles in Washing-
ton, including an analysis of nesting success, pop-
ulation numbers, and distribution. To simulate the
consequences of environmental perturbations on
the stability of the nesting population, we model
population size and structure at carr ying capacity
under various vital rate regimes.
M
ETHODS
During 1980–92, statewide Bald Eagle nest occupancy
was assessed from airplane surveys conducted in early
April, and productivity from helicopter surveys in early
June (McAllister et al. 1986, Watson 1993). From 1993–
98, biologists visited all historic nests each year during
occupancy surveys, but did not conduct comprehensive
productivity surveys. During that period, limited funding
and volunteer efforts resulted in the documentation of
nest success and productivity for a non-random sample
of 28–47% of occupied territories each year. We are un-
aware of any overt biases in the non-random samples due
to changes in sur vey technique (i.e., air vs. ground), dis-
tribution of sites surveyed, or changes in surveyors, that
might have affected parameter estimates.
We estimated three parameters from survey informa-
tion, including (1) nest occupancy—the proportion of
territories with one incubating adult or two adults at the
nest; (2) nest success—the proportion of occupied ter-
ritories producing at least one young; and (3) productiv-
ity—the mean number of young raised to pre-fledging
age (
$
8 wk) per occupied territory. We analyzed trends
of these parameters by fitting them to linear models with
simple linear regression. We determined statewide trends
for (1) all years from 1980–98, (2) 1993–98 only (the
period of nonrandom sampling), and (3) two regional
populations, the Columbia River estuary and Hood Ca-
nal, that experienced depressed productivity during the
survey period (Anthony et al. 1993, WDFW Heritage Data
Base unpubl. data).
Estimates of nest success in raptor populations are sub-
ject to sampling errors when pairs that fail early in the
nesting season may not be discovered and counted, lead-
ing to the overestimation of productivity/occupied site
(Steenhof and Kochert 1982, Steenhof 1987). Because
our surveys were potentially subject to this bias, we used
a second method to calculate productivity recommended
by Steenhof (1987). This method calculates productivity
as the product of the proportion of pairs that bred, the
proportion of pairs that were successful, and the number
of young/successful pair. Each parameter is estimated
from a specific population subsample: proportion of
breeding pairs from a preselected sample that includes
only nests from the population that bred the previous
year; proportion of successful pairs from all nests sur-
veyed twice (i.e., during incubation and pre-fledging);
and young/successful pair from pairs identified in both
early and late sur veys. Proportion of successful pairs is
not a direct computation, but is calculated with the May-
field estimator (Mayfield 1961), which is the daily-nest-
survival rate raised to the power of the length of the
mean period that a nest is at risk of failing (Steenhof
1987). We used 93 d as the mean nest exposure period
(McAllister et al. 1986). We did not determine trends in
productivity estimated by the Steenhof method because
calculations were based on combined parameter esti-
mates that potentially biased sample variances (Steenhof
1987).
We evaluated change in distribution of nesting eagles
during 1980–98 by defining five broad ecoregions; the
Olympic, southwest, and Puget Sound/Islands west of the
Cascade Range, and northeast, and southeast ecoregions
to the east (Fig. 1). The rate of population growth in
each ecoregion was calculated from the number of oc-
cupied territories documented in 1980 and 1998. We
compared density of occupied nests
,
2 km from marine,
lake, and large river shorelines between west and east
ecoregions (Washington Rivers and Marine Shoreline
data base, Wildlife Resource Data Systems, WDFW).
We estimated the number of statewide breeding pairs
expected at carrying capacity by fitting population
growth to a logistic curve based on the number of oc-
cupied territories found each year from 1980–98. The
logistic growth model is a simplistic model that assumes
the population is approaching a steady density; age struc-
ture is not considered, and all individuals are assumed to
have an equal chance to give birth or die (Smith 1974).
Thus, the model is not subject to changing survival and
mortality rates. When a population grows exponentially,
a linear relationship exists between the number of off-
spring per parent and the sum of the densities of both
generations (Morisita 1965). The slope and intercept of
this regression can be used to calculate the maximum
intrinsic rate of population growth and carrying capacity
as detailed in Caughley (1977) and Swenson et al.
(1986). We determined these two parameters indepen-
dently for eastern and western Washington because of
habitat differences, and summed the numbers of terri-
tories at carrying capacity for eastern and western Wash-
ington to estimate the size of the statewide breeding pop-
ulation at saturation. Because the logistic growth model
did not address habitat limitations to the population,
such as nest site availability, we assessed the reasonable-
ness of the estimates of carrying capacity in light of visi-
ble signs of population stability (i.e., increased incidences
of urban nesting and fatal encounters of territorial adults
with conspecifics), and a subjective estimate of the point
at which the growth would reach an asymptote. At satu-
ration, higher nest density might result in reduced nest-
ing success because of closer distances between adjacent
nesting pairs (Anthony et al. 1994). We used logistic re-
gression to examine the effects of nearest-neighbor dis-
tance on eagle occupancy, activity, and nest success in
1992, when the population showed signs of reaching sat-
uration.
Beyond a certain point, the actual number of nesting
pairs at carrying capacity does not affect population sta-
bility because its true indicator is age and stage structure
at equilibrium (Hunt 1998). Thus, the deviation between
S
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E
AGLE
S
TATUS IN
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ASHINGTON
Figure 1. Distribution of Bald Eagle nests in Washington State among five ecoregions in 1980 (top) and 1998
(bottom).
future and predicted number of nesting pairs at carrying
capacity was inconsequential to models of population sta-
bility. To estimate population structure and stability at
carrying capacity we used a modeling approach based on
Moffat’s Equilibrium (Hunt 1998). Whereas traditional
population modeling emphasizes density-dependent
mechanisms that regulate population growth, modeling
based on Moffat’s Equilibrium focuses on an adaptive
limit to breeding site serviceability that restricts cohort
size per unit area of landscape and consequently limits
the size of the total population (Hunt 1998, Hunt and
Law 2000). Causal regulation is considered modulating.
Model parameters include the number of serviceable
breeding locations (SBLs) at saturation (calculated from
logistic modeling), age-specific survival rates, maximum
longevity, and productivity. We used equations and rou-
tines from Hunt (1998) to calculate age class sizes, floater
to breeder ratios, and total population size at population
164 V
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Table 1. Productivity characteristics of the Bald Eagle population in Washington State from 1980–98. Standard errors
are shown with summary means.
Y
EAR
N
O
.
T
ERRI
-
TORIES
S
UR
-
VEYED
N
O
. (%)
T
ERRITORIES
O
CCUPIED
P
ERCENT OF
P
AIRS
B
REEDING
D
IRECT
a
S
AMPLE
b
P
ERCENT OF
P
AIRS
S
UCCESSFUL
D
IRECT
a
S
AMPLE
c
N
O
.Y
OUNG
/
S
UCCESSFUL
T
ERRITORY
N
O
.Y
OUNG
/O
CCUPIED
T
ERRITORY
D
IRECT
a
S
AMPLE
d
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
154
165
189
231
254
290
301
327
361
424
477
105 (68)
126 (76)
138 (73)
168 (73)
206 (81)
231 (80)
250 (83)
268 (82)
309 (86)
369 (87)
403 (84)
90
97
88
92
95
88
94
93
92
91
93
94
97
90
94
96
88
96
94
93
93
93
64
56
55
49
67
65
73
65
66
63
70
52
37
40
47
58
60
66
54
56
55
61
1.40
1.35
1.35
1.47
1.44
1.50
1.54
1.49
1.50
1.62
1.63
0.90
0.75
0.74
0.86
0.95
0.98
1.11
0.98
0.98
0.99
1.07
0.68
0.48
0.49
0.64
0.80
0.80
0.97
0.75
0.78
0.83
0.92
1991
1992
1993
1994
1995
1996
1997
1998
515
560
588
636
660
709
727
841
445 (86)
468 (84)
493 (84)
547 (86)
558 (85)
594 (84)
582 (80)
666 (79)
91
94
95
93
95
92
95
91
92
94
95
94
95
93
95
93
63
69
63
70
63
64
66
74
52
61
53
65
49
56
50
65
1.57
1.47
1.52
1.49
1.50
1.41
1.53
1.49
0.97
0.99
0.94
1.02
0.90
0.93
0.97
1.10
0.76
0.85
0.76
0.91
0.69
0.73
0.73
0.91
Total 8409 6926 (81
6
1) 93
6
194
6
165
6
155
6
2 1.49
6
0.02 0.95
6
0.02 0.76
6
0.03
a
Direct measurements based on entire population.
b
Sample estimate from territories occupied the prior year (Steenhof 1987).
c
Sample estimate calculated by the Mayfield Method (Steenhof 1987) from pairs surveyed twice.
d
Steenhof (1987) estimate of productivity
5
(% breeding from sample) (% successful from Mayfield) (No. young/successful pair).
equilibrium based on a maximum eagle longevity of 25
yr. This was greater than the 16-yr longevity estimated for
eagles from the Greater Yellowstone Ecosystem (Harmata
et al. 1999), but less than the oldest documented Bald
Eagle longevity record of 28 yr (Schempf 1997). Annual
survival rates of adults (0.88), subadults (0.95), and ju-
veniles (0.71), and productivity of 0.86 young/pair, were
used in calculations, and were based on survival and pro-
ductivity of 159 telemetered eagles and 622 occupied
nests from Prince William Sound, Alaska (Bowman et al.
1995), where habitat is somewhat similar to that of coastal
Washington. In any case, our interest was not so much in
determining the accuracy of these statistics, but rather
how changes in their values affected population stability.
We modeled effects of hypothetical environmental per-
turbations on population size and structure by reducing
the number of SBLs, the productivity rate, and age-spe-
cific survival. The barometer of population stability was
the ratio between floating and breeding adults (F:B ra-
tio), with negative ratios indicative of inadequate recruit-
ment and population decline (Hunt 1998).
R
ESULTS
From 1980–98, the annual occupancy rate of
Bald Eagles in Washington averaged 81% and in-
creased linearly (r
5
0.62, P
5
0.005; N
5
8409
surveyed territories; Table 1); productivity aver-
aged 0.95 young/occupied territory (N
5
6926)
and increased linearly (r
5
0.52, P
5
0.024); and
nest success averaged 65% at occupied territories
and increased linearly (r
5
0.50, P
5
0.031). How-
ever, for the 1993–98 sample of territories (N
5
4161), annual occupancy rates declined by 1.3%
per yr (r
5
0.83, P
5
0.040), and there was no
trend in nest success (P
5
0.282) or productivity
(P
5
0.306) at territories that were surveyed non-
randomly (N
5
1397). Between 1980–98 the num-
ber of Bald Eagle territories in Washington in-
creased from 154–841 (Table 1). The number of
pairs that nested each year increased logistically at
a mean rate of 10.1% per yr ([log e] occupied ter-
ritories
5
4.850
1
0.101 yr; r
5
0.98, P
,
0.001).
Sample estimates of statewide eagle productivity
averaged 0.19 young/yr less than direct productiv-
ity measures (Table 1). Much of this difference was
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E
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S
TATUS IN
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ASHINGTON
due to the Mayfield estimator for percent of suc-
cessful pairs, which averaged 10% less than direct
measures from the entire population. The percent
of eagle pairs breeding in the preselected samples
of pairs successful in the previous year averaged
only 1% higher than direct measurements for the
entire population from 1980–98.
Between 1980–98, the Bald Eagle population
nesting on Hood Canal increased from 3–33 pairs,
and the population along the Washington side of
the Columbia River estuary increased from 1–24
pairs. The annual occupancy rate on Hood Canal
(82%; N
5
398 surveyed territories) was similar to
the statewide rate, but lower on the Columbia Riv-
er estuary (69%; N
5
328 surveyed territories).
Productivity parameters of these populations were
below statewide means (Table 1). Hood Canal ea-
gles produced 0.63 young/occupied territory (N
5
323), with 43% of nesting attempts at occupied ter-
ritories successful. Eagles along the Columbia Riv-
er estuar y produced 0.56 young/occupied territory
(N
5
277), and 41% of nesting attempts at occu-
pied territories were successful. Despite the poor
reproductive history of these populations, produc-
tivity increased linearly from 1980–98 on Hood Ca-
nal (r
5
0.55, P
5
0.016) and the Columbia River
estuary (r
5
0.68, P
5
0.001), as did nest success
(Hood Canal r
5
0.59, P
5
0.008; Columbia River
estuary r
5
0.81, P
,
0.001).
A notable change in the statewide distribution
of nesting Bald Eagles from 1980–98 occurred east
of the crest of the Cascade Range where the num-
ber of territories increased from 0–59. Fifty-four of
these territories (92%) were located in the north-
east ecoregion, primarily along the upper Colum-
bia, Spokane, and Pend Oreille rivers (Fig. 1). West
of the Cascade Crest, the increase in number of
nesting territories was similar among the Olympic
ecoregion (380%, N
5
54–259), Puget Sound
ecoregion (350%, N
5
90–405), and southwest
ecoregion (292%, N
5
13–51). The increase in
number of occupied territories was greater in
southwest Washington (829%, N
5
7–65), than in
Puget Sound (475%, N
5
61–351) and the Olym-
pic ecoregion (438%, N
5
37–199), a difference
largely due to reoccupancy of vacant nests along
the Columbia River estuary. In westside ecoregions
there was a progressive expansion of nesting pairs
inland to major rivers and lakes along the coast
and Puget Sound (Fig. 1). In 1998, the mean den-
sity of occupied Bald Eagle nests
,
2 km from 6416
km of forested, marine shorelines in western Wash-
ington was 1 nest/10.4 km. In eastern Washington,
density was 1 nest/34.6 km along 1728 km of in-
land waters. We did not detect any relationship be-
tween nearest-neighbor distance and nest occupan-
cy (P
5
0.534), activity (P
5
0.173), or success (P
5
0.650) at 560 territories in 1992.
Logistic population growth modeling based on
the assumption that the population was approach-
ing a steady density, projected an ecological car-
rying capacity of 639 nesting pairs in western Wash-
ington, and a maximum growth rate of 9.5%. The
model yielded a carrying capacity of 94 pairs in
eastern Washington, and a maximum intrinsic
growth rate of 16.7%. The combined total for nest-
ing pairs (733) was used as the statewide number
of SBLs, in our modeling exercise which predicted
a population of 4913 eagles at Moffat’s Equilibrium
(25 yr after the population reaches carrying capac-
ity). The stable population consisted of 1907 sub-
adults and juveniles, 1540 floating adults, and 1466
breeding adults, resulting in an F:B ratio of 1.05.
When other parameters were held constant, F:B ra-
tios of the predicted population were reduced to a
critical level (i.e.,
,
0) resulting in population de-
cline when adult survival declined 17% (0.88–
0.73), or subadult survival declined 22% (0.95–
0.74), or juvenile survival declined 52% (0.71–
0.34), or productivity declined 52% (from 0.86–
0.41 young/pair). In a hypothetical scenario where
productivity and juvenile age classes were primarily
impacted (e.g., nest disturbance, contaminants)
the population declined when productivity rates
and juvenile survival were each reduced by 31%.
However, in a scenario where survival of all age
classes was impacted (e.g., oil spill, prey crash) the
population declined when adult survival was re-
duced by only 7%, subadult survival by 8%, and
juvenile survival by 10%. In a scenario where the
number of statewide SBLs was reduced by 50% and
survival and productivity rates were maintained
(e.g., habitat loss from development), the equilib-
rium model predicted a 50% reduction in the size
of each age class and total population when the
population stabilized, but the F:B ratio remained
at 1.05, a condition conferring a high degree of
population security.
D
ISCUSSION
Population Growth. Exponential population
growth exhibited by the Bald Eagle population in
Washington in the past 20 yr surpassed that within
the contiguous United States as a whole (i.e.,
166 V
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ATSON ET AL
.
384%, N
5
1188–5748 occupied territories; U.S.
Fish and Wildlife Service unpubl. data). Although
intense habitat management and protection of
nest territories in Washington occurred during
that period, including the development of 1150 ea-
gle habitat management plans with state and pri-
vate landowners (WDFW Wildlife Resource Data
Systems unpubl. data), population growth was most
likely a direct consequence of (1) reduced perse-
cution that decimated the population beginning in
the early 1900s (Dawson and Bowles 1909) and (2)
reduced environmental levels of DDT, the insecti-
cide that caused eggshell thinning and embryo
mortality and was believed to have drastically re-
duced eagle populations after 1945 (Stalmaster
1987). Use of DDT was banned in 1972, eight years
prior to our study. Increased rates of nest success
and productivity that we documented would be ex-
pected when contaminants levels declined in eagle
habitats, eagle prey, and ultimately breeding adult
eagles that were also under reduced threats of di-
rect persecution. This would be followed by in-
creased occupancy of vacant nests at historic sites
as more individuals reached maturity and the pop-
ulation increased. We found population increases
even among contaminant-impaired eagle popula-
tions on the Columbia River estuary and Hood Ca-
nal. Although productivity remained below state-
wide means for those populations, it increased
significantly in the past 20 yr. At their present den-
sities, the contribution of these regional popula-
tions to the number of nesting pairs in Washington
is minor (i.e., in 1998 only 4% of nesting pairs in
the state were on the Columbia River estuary, and
5% on Hood Canal), but these populations are
nevertheless important as local bio-indicators of
contaminant levels (Anthony et al. 1993).
Rapid repopulation of nesting habitat by Bald
Eagles was in part related to the tendency of off-
spring to return to natal regions (Wood 1992, Dris-
coll et al. 1999, Harmata et al. 1999). Evidence
from Montana suggests non-breeding male Bald
Eagles exhibit fidelity to geographically small natal
areas that are familiar to them (e.g., Greater Yel-
lowstone Ecosystem population), whereas many fe-
males disperse more widely (Harmata et al. 1999).
In Washington State, we have no data to indicate
that breeding eagles from western Washington
cross the Cascade Mountains and pioneer new ter-
ritories in eastern Washington, although the Cas-
cade crest is no hindrance to movement of winter-
ing eagles ( J. Watson unpubl. data). The more
rapid growth in eastern Washington compared to
the west side suggests carrying capacity for nesting
eagles will be reached sooner in western Washing-
ton. The density of nesting Bald Eagles in eastern
Washington is presently half of that in western
Washington based on available shoreline, but the
amount of difference due to lower prey and nest
tree densities is unknown, as is the density the east
side eagle population may reach at saturation. A
density of 1 nest/11 river km is reported along the
upper Columbia River in southern British Colum-
bia to the north of eastern Washington (Blood and
Anweiler 1994).
Population Equilibrium. The logistic growth
model, our examination of trends in nesting pa-
rameters from 1993–98, and recent occupation of
eagle territories in urban areas all indicate that the
population of breeding eagles in Washington is ap-
proaching saturation. Equilibrium theory predicts
that as competition for the limited number of SBLs
increases within a population, increased interfer-
ence from floating adults for prey and nest sites
should reduce productivity and survival (Haller
1996, Hunt 1998). Indeed, in Washington during
the past 5 yr at least six fatal encounters between
floating adults that attacked breeding adults have
been documented, whereas prior to that time no
similar events were reported ( J. Watson unpubl.
data). The linear decrease in nest occupancy, and
stabilization of productivity and nest success of
Bald Eagles in Washington during the 1990s are
consistent with predicted modulating effects of
floater pressure following population saturation
(Hunt 1998), a phenomenon also documented in
other Bald Eagle populations (Hansen 1987, Bow-
man et al. 1995). Our sur veys of the subpopulation
of Bald Eagles nesting in the San Juan Archipelago
of northwest Washington (i.e., 90 territories) show
the number of occupied territories declined by
,
10% in the years following a peak in 1994 (Fig.
2). This may indicate the range of population de-
cline to be experienced throughout Washington
from the density-dependent effects of floater inter-
ference. The occupancy rate of Washington Bald
Eagles is unlikely to increase from present levels to
high levels such as reported in Arizona (i.e., 90%,
Driscoll et al. 1999), because many of the unoc-
cupied territories have degraded habitat, excessive
levels of disturbance, or may be limited by prey
availability ( J. Watson unpubl. data). Nevertheless,
a small but increasing number of Bald Eagles in
Washington demonstrated surprising tolerance to
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ASHINGTON
Figure 2. Growth of the Bald Eagle population in the
San Juan Islands in northwest Washington. Data for
1962–79 from Nash et al. (1980), and for 1980–98 from
WDFW (unpubl. data).
human activity in the 1990s (Watson et al. 1999)
and established new territories in urban parks,
neighborhoods, and golf courses.
The estimated productivity level of 0.95 young/
occupied territory, the recent decline in nest oc-
cupancy, and stabilization of productivity and nest
success rates, provide further evidence that the
Washington population of nesting Bald Eagles is at
saturation. However, the effects of incomplete,
non-random surveys on estimates of the latter pa-
rameters is uncertain. In some cases Bald Eagle ter-
ritories affected by management plans, and poten-
tially having higher human disturbance levels, were
given survey priority (S. Negri and S. Ament, pers.
comm.), but productivity of such nests has not
been found to be different from the general pop-
ulation (G. Schirato unpubl. data). Early literature
suggested productivity of 0.7 young/nest was nec-
essary for population stability (i.e., Sprunt et al.
1973). If survival is as high as reported elsewhere
for juvenile and adult eagles, mean productivity of
,
1.0 young/nesting pair appears adequate for
population stability (Buehler et al. 1991, Bowman
et al. 1995, Harmata et al. 1999). Our direct esti-
mate of statewide productivity in Washington (0.95
young/occupied territory) is within that range.
Even if the sampling method more accurately re-
flects true productivity of Washington eagles (0.76
young/occupied territory, 20% lower than direct
estimates), either survival rates are high enough to
sustain such rapid population growth, or the Wash-
ington population is being supplemented substan-
tially by immigration from other populations, or
both. We suspect productivity estimates from the
sampling method were unrealistically low, because
in Washington locations of virtually all Bald Eagle
nests were well-documented and nests were highly
visible from the air. This increased the likelihood
of encountering adults to confirm activity even at
failed nests or those where no eggs were laid, so
we believe that few early nest failures were missed.
Population Stability. Predictive models based on
equilibrium theory provided a prioritization of
population parameters for their relevance to main-
taining population stability during hypothetical en-
vironmental perturbations. While the eventual size
of the Bald Eagle population in Washington will be
limited by the number of SBLs, maintaining an ad-
equate ratio of floating to breeding adults is the
ultimate determinate of population stability (Hunt
1998). Ideally, the population of floating and
breeding adults could be surveyed simultaneously
on a periodic basis to assess population stability. In
Washington, floating adults may spend up to 40%
of the year in Canada and southeast Alaska from
June–November ( J. Watson unpubl data). Surveys
conducted in spring in Washington could allow an
accounting of breeders on territories and provide
an estimate of floating adults, but might be im-
practical because of costs. Therefore, the most im-
portant emphasis for maintaining the eagle popu-
lation is to maximize survival, and prevent or
ameliorate environmental factors that result in di-
rect mortality (e.g., shooting) or indirect mortality
(e.g., lead poisoning) of adults, and secondarily
subadults, during their 3-yr transition to adult-
hood. The ratio of floating to breeding adults was
least sensitive to changes in rates of productivity
and juvenile survival, so these are the least impor-
tant parameters to population stability. Dramatic
declines in eagle productivity or juvenile survival
(i.e., 50%) would have to be experienced to pro-
duce the same effects as small declines in the sur-
vival of older birds (e.g., 7–10% for adults). This
corroborates Grier’s (1980) conclusion that popu-
lation dynamics of Bald Eagles depend more on
survival than reproduction. Reproduction has
more often been the parameter monitored to de-
termine Bald Eagle status because it is a sensitive
indicator of contaminant problems and it is also
easier to monitor than eagle survival (Harmata et
al. 1999). The equilibrium model suggests that de-
termining a minimum number of SBLs needed to
maintain population stability in Washington
168 V
OL
. 36, N
O
.3W
ATSON ET AL
.
should be based on what number is necessary to
provide an overall reserve of nonbreeding adults
adequate to buffer fluctuations in density-indepen-
dent mortality factors (e.g., weather, electrocu-
tions, oil spills). The optimum number of SBLs in
Washington State, however, must be determined af-
ter consideration of aesthetic values of Bald Eagles;
the public may, for example, desire to protect
more territories than necessary for population sta-
bility. Current management of breeding Bald Ea-
gles in Washington as directed by state legislation
is to manage all territories equally on state and pri-
vate land regardless of habitat quality. Our popu-
lation model suggests the ultimate need to con-
serve the population is to protect the quality
breeding habitats for a target number of territo-
ries, whether greater or less than the 733 projected
territories, and thus ensure a stable number of
breeding locations into the foreseeable future. Pri-
oritization of existing territories for protection
based on their distribution, the condition of habi-
tat, threats to the habitat, and proximity to forag-
ing areas is an objective of Bald Eagle recovery in
Washington (Stinson et al. 2001).
A
CKNOWLEDGMENTS
We thank G. Hunt for introducing us to the equilibri-
um model. He and K. Steenhof provided insightful dis-
cussions and comments on population estimates and
modeling. C. Dykstra and F. Isaacs provided excellent
comments that improved an earlier manuscript. Biolo-
gists with the WDFW conducted the majority of surveys
throughout the years of this study including S. Ament,
D. Anderson, J. Bernatowicz, E. Cummins, T. Cyra, M.
Davison, L. Hofmann, L. Leschner, A. McMillan, P. Miller,
R. Milner, S. Negri, G. Schirato, L. Stream, M. Zahn, and
S. Zender. Other organizations contributed substantial fi-
nancial or survey support including the U.S. Fish and
Wildlife Service, U.S. Forest Service, the Weyerhauser
Company, and Puget Power. We especially thank biolo-
gists R. Anderson, G. Walter, M. Murphy, M. Stalmaster,
E. Taylor, and U. Wilson for their survey contributions.
Data base assistance was provided by J. Stofel, and graph-
ics support by J. Talmadge.
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Received 13 November 2001; accepted 19 April 2002
Associate Editor: Marco Restani
... Bald Eagles are largely piscivorous birds with greater than 50% of their diet consisting of fish throughout most of their range. In Washington, one study indicated that Bald Eagle diets consisted primarily of fish (78%), followed by birds (19%), and then mammals (3%) (Watson 2002). Most prey is captured alive (73%), but Bald Eagles also use carrion (15%) and pirate food from other species (12%) (Watson 2002, Watson et al. 1991. ...
... In Washington, one study indicated that Bald Eagle diets consisted primarily of fish (78%), followed by birds (19%), and then mammals (3%) (Watson 2002). Most prey is captured alive (73%), but Bald Eagles also use carrion (15%) and pirate food from other species (12%) (Watson 2002, Watson et al. 1991. Bald Eagles that overwinter in Washington are particularly dependent on chum salmon and other salmon species in the fall and early winter (Stinson et al. 2007), and rely more heavily on waterfowl in mid-to late-winter (Elliott et al. 2011); carrion is also consumed during winter (Stalmaster 1987, Watson 2002. ...
... Most prey is captured alive (73%), but Bald Eagles also use carrion (15%) and pirate food from other species (12%) (Watson 2002, Watson et al. 1991. Bald Eagles that overwinter in Washington are particularly dependent on chum salmon and other salmon species in the fall and early winter (Stinson et al. 2007), and rely more heavily on waterfowl in mid-to late-winter (Elliott et al. 2011); carrion is also consumed during winter (Stalmaster 1987, Watson 2002. ...
... Since 1995, 74% of occupied territories produced at least one young. Success rates in many parts of North America have ranged between 60% and 65%, including the Pacific Northwest (Anthony et al. 1994, Watson et al. 2002 and the Rocky Mountains (Swenson et al. 1986, Kralovec et al. 1992. In Alaska (Stiedl et al. 1997) and Arizona (Driscoll et al. 1999) only half of nesting pairs produced young. ...
... Productivity in the Rocky Mountain states has ranged from 1.0 to 1.2 young per nesting pair (Swenson et al 1986, Kralovec et al. 1992. Reproductive rates in the Pacific Northwest were 0.9 young per occupied nest (Anthony et al. 1994, Watson et al. 2002. In Alaska, productivity (0.8 yg/pair) was well below that in the Chesapeake Bay (Stiedl et al. 1997). ...
... Since 1995, 74% of occupied territories produced at least one young. Success rates in many parts of North America have ranged between 60% and 65%, including the Pacific Northwest (Anthony et al. 1994, Watson et al. 2002 and the Rocky Mountains (Swenson et al. 1986, Kralovec et al. 1992. In Alaska (Stiedl et al. 1997) and Arizona (Driscoll et al. 1999) only half of nesting pairs produced young. ...
... Productivity in the Rocky Mountain states has ranged from 1.0 to 1.2 young per nesting pair (Swenson et al 1986, Kralovec et al. 1992. Reproductive rates in the Pacific Northwest were 0.9 young per occupied nest (Anthony et al. 1994, Watson et al. 2002. In Alaska, productivity (0.8 yg/pair) was well below that in the Chesapeake Bay (Stiedl et al. 1997). ...
... Since 1995, 74% of occupied territories produced at least one young. Success rates in many parts of North America have ranged between 60% and 65%, including the Pacific Northwest (Anthony et al. 1994, Watson et al. 2002 and the Rocky Mountains (Swenson et al. 1986, Kralovec et al. 1992. In Alaska (Stiedl et al. 1997) and Arizona (Driscoll et al. 1999) only half of nesting pairs produced young. ...
... Productivity in the Rocky Mountain states has ranged from 1.0 to 1.2 young per nesting pair (Swenson et al 1986, Kralovec et al. 1992. Reproductive rates in the Pacific Northwest were 0.9 young per occupied nest (Anthony et al. 1994, Watson et al. 2002. In Alaska, productivity (0.8 yg/pair) was well below that in the Chesapeake Bay (Stiedl et al. 1997). ...
... Since 1995, 74% of occupied territories produced at least one young. Success rates in many parts of North America have ranged between 60% and 65%, including the Pacific Northwest (Anthony et al. 1994, Watson et al. 2002 and the Rocky Mountains (Swenson et al. 1986, Kralovec et al. 1992. In Alaska (Stiedl et al. 1997) and Arizona (Driscoll et al. 1999) only half of nesting pairs produced young. ...
... Productivity in the Rocky Mountain states has ranged from 1.0 to 1.2 young per nesting pair (Swenson et al 1986, Kralovec et al. 1992. Reproductive rates in the Pacific Northwest were 0.9 young per occupied nest (Anthony et al. 1994, Watson et al. 2002. In Alaska, productivity (0.8 yg/pair) was well below that in the Chesapeake Bay (Stiedl et al. 1997). ...
... After years of decline, bald eagle (Haliaeetus leucocephalus) populations throughout North America rebounded in the latter part of the twentieth century following tightened protection, reduction in the use of lead shot by hunters, and the outlawing of pesticides such as DDT (Hipfner et al., 2012;Watson, Stinson, McAllister, & Owens, 2002). This recovery has provided one of the great success stories of the conservation movement (Millar & Lynch, 2006). ...
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