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Recovery of Breeding Bald Eagles on Aberdeen Proving Ground, Maryland

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Abstract and Figures

We conducted annual aerial surveys (1991-2011) for breeding Bald Eagles (Haliaeetus leucocephalus) within Aberdeen Proving Ground (APG), a 350-km2 military installation located along the northwestern shoreline of the upper Chesapeake Bay in Maryland. The population increased exponentially from 1 pair in 1977 to 58 pairs in 2011 with an average doubling time of 5.8 years. This rate was higher than that documented for the broader Chesapeake Bay and is comparable to the highest reported throughout the species range. Annual population increase was highly variable and exhibited no indication of any systematic decline. A total of 646 chicks were produced from 464 breeding attempts during this period. The population has exhibited tremendous forward momentum such that more than 50% of young produced over the 21-year period were produced in the last 6 years. Average success rate was high (79.8%) and reproductive rates exceeded conservation targets in nearly all years. Due to the expansion of urban development throughout the Chesapeake Bay watershed, APG plays an increasingly important role in the recovery and maintenance of the Chesapeake Bay Bald Eagle population.
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March 2014 MARYLAND BIRDLIFE 11
10 MARYLAND BIRDLIFE Vol. 63, No. 1
RECOVERY OF BREEDING BALD EAGLES ON
ABERDEEN PROVING GROUND, MARYLAND
bryAn D. wATTs1,3
eLizAbeTH K. moJiCA1
JoHn T. PAUL2
JAmes J. PoTTie2
1 Center for Conservation Biology, College of William and Mary,
P.O. Box 8795, Williamsburg, Virginia 23187-8795
2 Directorate of Safety, Health, and Environment, United States Army,
Aberdeen Proving Ground, Maryland 21010
3 Corresponding author: bdwatt@wm.edu
AbsTrACT
We conducted annual aerial surveys (1991-2011) for breeding Bald Eagles (Hali-
aeetus leucocephalus) within Aberdeen Proving Ground (APG), a 350-km2 military
installation located along the northwestern shoreline of the upper Chesapeake Bay
in Maryland. The population increased exponentially from 1 pair in 1977 to 58
pairs in 2011 with an average doubling time of 5.8 years. This rate was higher than
that documented for the broader Chesapeake Bay and is comparable to the highest
reported throughout the species range. Annual population increase was highly vari-
able and exhibited no indication of any systematic decline. A total of 646 chicks
were produced from 464 breeding attempts during this period. The population has
exhibited tremendous forward momentum such that more than 50% of young pro-
duced over the 21-year period were produced in the last 6 years. Average success
rate was high (79.8%) and reproductive rates exceeded conservation targets in near-
ly all years. Due to the expansion of urban development throughout the Chesapeake
Bay watershed, APG plays an increasingly important role in the recovery and main-
tenance of the Chesapeake Bay Bald Eagle population.
Keywords: Bald Eagle, Haliaeetus leucocephalus, breeding, Aberdeen Proving
Ground, recovery, Department of Defense
Bald Eagles (Haliaeetus leucocephalus) have likely bred on the land currently occu-
pied by Aberdeen Proving Ground (APG) for thousands of years. However, no assessment
of the population is available prior to the 1930s when the National Audubon Society com-
missioned a survey of a portion of the Chesapeake Bay that included APG (Tyrrell 1936). In
1936, Tyrrell documented nests on Eagle Point, Robbins Point, lower Little Romney Creek
(north of Elm Tree Point), upper Little Romney Creek (near intersection with A-A5 road),
and Bear Point. Stewart and Robbins (1958) documented nests on APG in the 1950s. Abbott
(unpublished eld notes) coordinated Bald Eagle nest surveys from the late 1950s through the
mid-1970s and documented additional nests at the mouth of Canal Creek, Reardon Inlet (near
Westwood Range), Maxwell Point, Swaderick Creek, Leges Point (near Days Point), north
of Ricketts Point, Gum Point, Skippers Point (on Lauderick Creek), Coopers Creek, Back
Creek (near AA-5 road), and three on Spesutie Island (near Locust Point; near Morgan Road;
near Sandy Point). Only four of these historic breeding sites had evidence of Bald Eagle use
when investigated during the early 1960s (Abbott, unpublished data). By the late 1960s, no
occupied Bald Eagle territories were identied for APG.
Following the rst rediscovered breeding of Bald Eagles on APG in 1977, the Director-
ate of Safety, Health and Environment contacted the United States Fish and Wildlife Service
to initiate consultation under the Endangered Species Act, Section 7(c)(1). This consultation
resulted in studies that lead to the rst Bald Eagle management plan in 1986 and subsequent
revisions in 1995 and 2009 (Paul 2009). These plans established the need and framework for
annual monitoring of the breeding population. Here we provide the results of survey efforts
(1991-2011) and discuss changes in the population relative to the breeding population within
the tidal reach of the Chesapeake Bay.
sTUDy AreA
APG is a 350-km2 United States Department of Defense military installation located
along the northwestern shore of the upper Chesapeake Bay, in southern Harford and eastern
Baltimore Counties, Maryland (Figure 1). Since APG’s establishment in 1917, the Aberdeen
Area has been the site of intensive research and development; large-scale testing of munitions,
weapons, and materiel; and a training school for ordnance ofcers and enlisted specialists.
Due to the nature of its mission, APG is primarily forested and has extensive undeveloped
shorelines. The property is embedded within the Upper Chesapeake Bay Bald Eagle Concen-
tration Area, one of several areas within the Chesapeake Bay where Bald Eagles from along
the Atlantic Coast converge (Watts et al. 2007). Throughout the Bay such concentration areas
have formed within low salinity, tidal-fresh waters where prey availability is high (Watts et
al. 2006). For the resident breeding population, brood provisioning and chick growth tend to
be high in these areas (Markham and Watts 2008) leading to high breeding densities, high
breeding success, and high productivity (Watts et al. 2006).
meTHoDs
Aerial helicopter surveys have been used to survey the entire study area for breeding
eagles (1991-2011). Typically four to six surveys have been conducted between mid-January
and late May to document nests, breeding activity, and productivity. Detected nests were
plotted on topographic maps and given unique codes as names. Each nest was examined
to determine its condition and status. Notes from eld observations were interpreted by the
authors to determine activity status according to national standards. We considered a breed-
ing territory to be occupied if a pair of birds were observed in association with the nest and
there was evidence of recent nest maintenance (e.g., well-formed cup, fresh lining, structural
maintenance). We considered nests to be active if we observed a bird in an incubating posture
or if we detected eggs or young in the nest (Postupalsky 1974). The number of eaglets was
recorded for each nest. Due to the number of ights, we have condence that nesting activity
was well documented.
We dened breeding success as the percentage of occupied nests that contained 1
young, reproductive rate as the number of young per occupied nest, and average brood size
as the number of young per successful nest. We expressed population growth rate using the
(continued)
March 2014 MARYLAND BIRDLIFE 13
12 MARYLAND BIRDLIFE Vol. 63, No. 1
Figure 1. Aberdeen Proving Ground, Baltimore and Harford Counties, Maryland.
average time (in years) required for the population to double in size (tdouble), the intrinsic rate
of increase (r), and the average annual percent increase over the study period. We calculated
average doubling time using the growth equation Nt = N0ert, where Nt is the population size in
2011, N0 is the population size in 1977, e is the base of the natural logarithm, r is the intrinsic
rate of increase, and t is the time interval between population estimates. With this congu-
ration, tdouble = ln(2)/r. We calculated average annual percent increase as (Nt+1-Nt)/Nt x 100.
resULTs
Between 1977 and 2011, the Bald Eagle breeding population on APG increased from 1
pair to 58 pairs. During this period, the population grew exponentially with an average dou-
bling time of 5.8 years. Intrinsic rate of increase (r) was 0.119. Average annual increase was
13.1+ 4.23% (mean + S.E.). The annual population increase, as expressed by a percentage,
was highly variable over the study period and ranged from a low of -20.6% (2005-2006) to
a high of 57.9% (1998-1999). There is no indication over the survey period that this rate has
shown any directional change (R2 = 0.042, F[1,17] = 0.75, p = 0.395).
During the study period, we documented 464 breeding attempts (i.e., active nests) that
produced 646 young (Table 1). Average annualized rates were 79.8 + 3.48%, 1.39 + 0.06%,
and 1.6 + 0.05% for breeding success, reproductive rate, and brood size, respectively. The
population has exhibited tremendous forward momentum such that more than 50% of young
produced over the 21-year period have been produced in the 6 years since 2005.
Survey information between 1991 and 2011 indicates that the breeding population on
APG has exceeded the goal of 1.1 chicks/breeding attempt set by the Chesapeake Bay Bald
Eagle Recovery Plan (Byrd et al. 1990) every year except 1997 and 1998 (Table 1). During
1997 and 1998, recorded reproductive rate was higher than that suggested for maintenance
but lower than the recovery goal. For the 11-year period 1991-2001, reproductive rates for
APG were virtually identical to those recorded for the broader Chesapeake Bay. The average
number of chicks per active nest was 1.4 + 0.05 (mean + S.E.) and 1.4 + 0.09 for the Chesa-
peake Bay and APG respectively. The average number of chicks per successful nest (average
brood size) was 1.8 + 0.03 and 1.6 + 0.05 for the Chesapeake Bay and APG respectively.
These rates are not statistically distinguishable (for both comparisons, df = 19, F-statistic <
3.2, P > 0.05).
DisCUssion
The recovery of the Bald Eagle breeding population on APG has been dramatic. Popu-
lation growth rate has been faster (doubling time of 5.8 vs 8.2 years) than that documented
for the tidal reach of the larger Chesapeake Bay (Watts et al. 2008). The rate is comparable
to other low-salinity reaches of the Bay that represent some of the fastest growing regions
throughout the species range (Watts et al. 2006). With the exception of locations that have
been developed, virtually all of the breeding territories documented during the 1930s, 1940s
and 1950s have now been re-occupied. No specic estimates of the APG Bald Eagle popula-
tion are available prior to the onset of the DDT era. However, given the tremendous forward
momentum currently exhibited by the breeding population, it seems likely that Bald Eagles
will reach nesting carrying capacity within the installation in a relatively short period of time.
(continued)
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Year Occupied Acve Successful Successful Successful Young Young Young
Nests Nests Nests Young /Occupieda /Acvea /Occupieda /Acvea /Successfula
1991 5 5 4b 5 100 100 1.25 1.25 1.25
1992 5 5 4 8 80 80 1.6 1.6 2
1993 9 8 7 11 77.8 87.5 1.22 1.38 1.57
1994 10 9 7 10 70 77.8 1 1.11 1.43
1995 13 13 10d 18 100 100 1.8 1.8 1.8
1996 16 16 14b 23 93.3 93.3 1.53 1.53 1.64
1997 16 13 5c 9 35.7 45.5 0.64 0.82 1.8
1998 8 8 5 6 71.4 71.4 0.86 0.86 1.2
1999 19 19 11c 20 64.7 64.7 1.18 1.18 1.82
2000 19 13 10 18 52.6 76.9 0.95 1.38 1.8
2001 20 20 19 32 95 95 1.6 1.6 1.68
2002 19 18 12d 20 80 85.7 1.33 1.43 1.67
2003 24 23 23 35 95.8 100 1.46 1.52 1.52
2004 29 27 22 32 75.9 81.5 1.1 1.19 1.45
2005 35 35 29 41 82.9 82.9 1.17 1.17 1.41
2006 29 29 28b 41 100 100 1.46 1.46 1.46
2007 31 31 27 42 87.1 87.1 1.35 1.35 1.56
2008 44 37 33 61 75 89.2 1.39 1.65 1.85
2009 46 37 35 69 76.1 94.6 1.5 1.86 1.97
2010 44 41 36 60 81.8 87.8 1.36 1.46 1.67
2011 58 57 45c 85 80.4 81.8 1.52 1.55 1.98
TOTAL 499 464 386 646 – – – – –
AVERAGE 79.8 ± 3.48 84.7 ± 13.4 1.39 ± 0.06 1.38 ± 0.27 1.60 ± 0.05
TAbLe 1. bALD eAGLe PoPULATion size AnD ProDUCTiviTy wiTHin
AberDeen ProvinG GroUnD, mAryLAnD (1991-2011).
a Based on nests with known outcome
b Final outcome of 1 nest not determined and not included in totals
c Final outcome of 2 nests not determined and not included in totals
d Final outcome of <5 nests not determined and not included in totals
(Note: The 1998 survey was incomplete. Occupied Nest = pair of birds and
evidence of recent nest maintenance; Active Nest = eggs or young in the nest;
Successful Nest = number of nests that produced at least one young; Successful
Nest/Occupied Nest = breeding success; Young/Occupied Nest = reproductive
rate; Young/Successful Nest = average brood size.)
A reproductive rate of 0.7 chicks/breeding attempt has been suggested to represent the
threshold for population maintenance for Bald Eagles (Sprunt et al. 1973). Buehler et al.
(1991a) estimated that 1.0 chicks/successful nest (equivalent to brood size) was required
for sustaining breeding populations in the Bay. A reproductive rate of 1.1 chicks/breeding
attempt was set as the recovery goal for the Chesapeake Bay population (Byrd et al. 1990).
With the exception of 1997 and 1998, the APG population has met or exceeded the produc-
tivity target outlined in the recovery plan in every year that a survey has been conducted. The
broader Chesapeake Bay reached this threshold in 1985 and has exceeded the target in all
subsequent years (Watts et al. 2008). The reproductive rate documented by Tyrrell in 1936
was nearly 1.5 chicks/breeding attempt. The APG population has approached or achieved this
rate in the years after 2005.
APG plays an increasingly important role in the recovery and maintenance of the Chesa-
peake Bay Bald Eagle population. The availability of mature trees suitable for nesting within
1 km of water has become the dominant limiting factor for Bald Eagles in the region. Human
activity is the best predictor of eagle distribution within the tidal portion of the Bay. Indicators
of human activity such as housing and road density, shoreline use, and boating activity have
been related to nest distribution (Watts et al. 1994), shoreline use (Buehler et al. 1991b, Watts
and Whalen 1997), and the likelihood of nest abandonment (Therres et al. 1993) or recoloni-
zation (B. D. Watts, Center for Conservation Biology, unpublished data). Since Bald Eagles
began their most dramatic decline in the 1950s, the human population within the tidal reach
of the Bay has increased by more than 50% (United States Department of Commerce 2010).
A preliminary review of development occurring around eagle nests in the lower Chesapeake
Bay shows that development had occurred in 55% of shoreline areas by the late 1980s (Byrd
et al. 1990). Extensive undeveloped shorelines and associated uplands on APG have allowed
the property to become a signicant stronghold for the breeding population.
APG will continue to serve as an important Bald Eagle breeding location for the foresee-
able future. APG has been actively working to restore the Bald Eagle population within the
installation since the early 1980s. The Army has adopted environmental stewardship as one of
its missions and it is clear that without federal ownership of this land and the demand for the
ongoing mission, the upper Bay would support considerably less habitat for breeding eagles.
The current Bald Eagle management plan (Paul 2009) provides broad directives to protect
signicant eagle habitat and outlines specic measures to reduce disturbance within known
nesting, foraging, and roosting sites. Management efforts continue that are designed to mesh
the needs of eagles with other military missions.
ACKnowLeDGmenTs
The United States Army has supported breeding population surveys since the 1980s. We
thank the long list of observers who have participated in surveys including Craig A. Koppie,
Joseph P. Ondek, Samuel T. Voss, and Jessica Baylor. We also thank the many individuals
who have managed the survey data over time including Amy Deel and Lynda Hartzell. Glenn
D. Therres and Craig A. Koppie have contributed to shaping the survey.
(continued)
March 2014 MARYLAND BIRDLIFE 17
16 MARYLAND BIRDLIFE Vol. 63, No. 1
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Abstract We conducted annual aerial surveys throughout the tidal reach of the Chesapeake Bay, USA, between 1977 and 2001 to estimate population size and reproductive performance for bald eagles (Haliaeetus leucocephalus). The population increased exponentially from 73 to 601 pairs with an average doubling time of 8.2 years. Annual population increase was highly variable and exhibited no indication of any systematic decline. A total of 7,590 chicks were produced from 5,685 breeding attempts during this period. The population has exhibited tremendous forward momentum such that >50% of young produced over the 25-year period were produced in the last 6 years. Rapid population growth may reflect the combined benefits of eliminating persistent biocides and active territory management. Reproductive rate along with associated success rate and average brood size increased throughout the study period. Average reproductive rate (chicks/breeding attempt) increased from 0.82 during the first 5 years of the survey to 1.50 during the last 5 years. Average success rate increased from 54.4% to >80.0% during the same time periods. The overall population will likely reach saturation within the next decade. The availability of undeveloped waterfront property has become the dominant limiting factor for bald eagles in the Chesapeake Bay. Maintaining the eagle population in the face of a rapidly expanding human population will continue to be the greatest challenge faced by wildlife biologists.
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Survival of 39 radio-tagged Haliaeetus leucocephalus in the Chesapeake Bay region was 100% in the first year of life. Mean minimum survival per year of all eagles was 91%, mean maximum survival 98%. A deterministic life-table model predicted a finite growth rate of 5.8% per year; growth rate based on the maximum survival estimates was 16.6% per year. The breeding population actually increased 12.6% per year from 1986-1990. Intrinsic growth rate was 6.9% based on natality and minimum survival data and 19.2% based on maximum survival data. -from Authors