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Research Article
Estimating the Residual Value of Alternate
Bald Eagle Nests: Implications for Nest
Protection Standards
BRYAN D. WATTS,
1
Center for Conservation Biology, College of William and Mary and Virginia Commonwealth University, Williamsburg,
VA 23187-8795, USA
ABSTRACT For over 30 years,bald eagle (Haliaeetus leucocephalus) nests and nest treeshave been managed using
a combination of spatial buffers and time-of-year restrictions. Most management standards include the
protection ofnests currently in use as well as alternate nests and treesthat have lost nests. Protection isextended to
alternate nest structures under the assumption that they provide value to the breeding population. However, the
notion that these structures hold enough residual value to warrant the cost of their protection has not been fully
explored. I used nest histories (n>2,250) from a long-term (1962–2011) dataset collected in the lower
Chesapeake Bay to evaluate the relationship between management costs and residual value across the range of
management scenarios currently in use. I used a joint, multistrata, live-recapture/dead-recovery model in
Program MARK to estimate probabilities of annual survival for active and alternate nests and transition
probabilities between active and alternate nests. My primary objective was to assess the residual biological value
of alternate nests and trees relative to the management costs required to protect them. I estimated the per capita
management costs and the residual value of alternate nests and trees. Survival rates were 0.902 0.007
(mean SE) and 0.703 0.017 for active and alternate nests, respectively. Of 1,163 alternate nests, 352 (30.3%)
were determined to be re-used within 5 years. However, the likelihood of re-use declined with time. Most re-
used nests were re-used in the first year (76.4%), with virtually all (98.6%) being re-used in the first 3 years. Only
9.9% (168 of 1,699) of trees that had lost nests were re-used within the first 10 years. Nests wererebuilt in 32%
(equating to 3.1% re-use) of re-used trees in the first year and in 71.4% of these trees in the first 3 years.
Implementation of current national management guidelines resulted in 2.35 nest equivalents of management
cost for each active nest in the population. The residual value and cost functions diverged over time such that the
return on social investment diminishes over the management periods. The cost-to-benefit relationship is
particularly poor when the protection of alternate nests is extended beyond 3 years and when protection is
extended to trees that have lost nests. Ó2015 The Wildlife Society.
KEY WORDS bald eagle, Chesapeake Bay, Haliaeetus leucocephalus, management, nest survival, social cost.
Bald eagle (Haliaeetus leucocephalus) nest sites have been
managed using a combination of spatial buffers and time-of-
year restrictions to comply with the prohibition on take
under both the Endangered Species Act (16 U.S.C. §1531)
and the Bald and Golden Eagle Protection Act (16 U.S.C.
§668-668d) since the 1970s. Human activities considered to
be detrimental to breeding pairs (e.g., residential, commer-
cial, and industrial development; logging; use of toxic
chemicals) are restricted within a primary buffer, and human
activities that are considered to affect the integrity of the
primary buffer (e.g., construction of high-density develop-
ments, multi-story buildings, new roadways) are restricted
within a secondary buffer. Time-of-year restrictions are used
to limit direct human activities (e.g., recreational activities,
logging, mineral exploration, low-level aircraft operations)
within buffer areas that may disturb eagles during sensitive
periods of the nesting cycle. Current national standards
recommend a protection buffer of 200 m (U.S. Fish and
Wildlife Service [USFWS] 2007). In practice, most states
use larger buffer areas ranging to 1,500 m (e.g., Driscoll et al.
2006, Florida Fish and Wildlife Conservation Commission
2008, Gross and Brauning 2010).
In addition to nests supporting eagle breeding activity,
most management guidelines recommend that disturbance
buffers be maintained around alternate nests and nest trees
that have lost nests. Period of protection for these structures
varies state to state. Current national guidelines recommend
protection for 5 years for alternate nests and 3 years for trees
that have lost nests because of storms or other factors
(USFWS 2007). Inclusion of these structures within
management guidelines is an extension of the take provision
made under the assumption that they provide value to
breeding pairs. The magnitude of this value and the extent to
which it changes over time should shape the guidelines
designed for its protection. The latent value of nests and trees
Received: 21 April 2014; Accepted: 4 April 2015
Published: 1 May 2015
1
E-mail: bdwatt@wm.edu
The Journal of Wildlife Management 79(5):776–784; 2015; DOI: 10.1002/jwmg.888
776 The Journal of Wildlife Management 79(5)
depends on the likelihood that they survive to some future
time and if they survive that they will be used. Such
likelihoods have not been broadly explored. Uncertainty in
the appropriate thresholds for management transitions has
resulted in 1) considerable state-to-state variation in the
implementation of management time lags, and 2) a call for
further investigation of the biological benefits of current
recommendations (USFWS 2007, Florida Fish and Wildlife
Conservation Commission 2008).
Many bald eagles nest on privately owned lands, and the use
of management buffers with activity restrictions is a form of
lost-opportunity cost for landowners. Owners are being
asked to forego alternative land uses to benefit eagle
management. Collectively, these costs represent the burden
borne by society to maintain the eagle population.
Developing management strategies that strike the appropri-
ate balance between benefits to eagles and social burden is an
ongoing challenge. Quantifying critical parameters that drive
both the biological benefits of alternate nests and trees that
have lost nests to eagles and the management cost structure
may help to inform the development of protection standards.
I used a long-term (1962–2011) data set from the lower
Chesapeake Bay to estimate survival rates (mortality occurs
when nest is absent) for active nests, alternate nests, and nest
trees; transition probabilities between active and alternate
nests; and re-use probabilities for alternate nests and trees
that have lost nests. These terms have been defined in a
variety of ways throughout the literature (Steenhof and
Newton 2007). Because the objective of this study was to
examine the benefits of management scenarios vis-a-vis
current management recommendations, definitions follow
those specified within the national management guidelines
(USFWS 2007). Accordingly, an active nest is defined as
“a nest that is attended (built, maintained, or used) by a pair
of bald eagles during a given breeding season, whether or not
eggs are laid” (USFWS 2007). Notably, this definition differs
from use in the raptor literature (Postupalsky 1974). An
alternate nest is a nest that is not used for breeding by eagles
during a given breeding season, identified by an apparent lack
of repair work and no observation of attending adults. In
addition to nests, trees that had previously supported nests
are monitored for the construction of new nests.
I used survival rates of nests and nest trees and transition
probabilities between active to alternative nests to estimate
the residual value of nesting structures and to calculate per
capita management costs across a range of management
scenarios. I also examined the tradeoffs between residual
value, defined as the biological benefit to eagles or likelihood
of future use, and management costs, defined as the number
of nests managed per pair, which may be helpful in assessing
the efficacy of standards in current use.
STUDY AREA
This study included the tidal reaches of the lower
Chesapeake Bay and the lower Delmarva Peninsula in
Virginia (Watts et al. 2006). The breeding population within
this location and the broader Chesapeake Bay reached a low
in the early 1970s but has been growing exponentially over
the past 3 decades with an average doubling time of less than
8 years (Watts and Byrd 2002; Watts et al. 2007, 2008).
Nests within the study area have a high turnover rate (Watts
and Duerr 2010) due primarily to wind throw (Watts and
Byrd 2007). The Bay’s wide salinity gradient, shallow water,
and climate have made it one of the most productive aquatic
ecosystems in North America. Bald eagles breed throughout
the estuary from the Atlantic Ocean to the fall line. The fall
line is an erosional scarp where the metamorphic rocks of the
Piedmont meet the sedimentary rocks of the Coastal Plain.
The geologic formations along this boundary frequently
determine the landward extent of tidal influence. Forest
cover within the tidal reach of the Bay has varied dramatically
since European settlement, reaching a low of 50% in the late
1800s, but is now the dominant land cover except in areas
with intensive agriculture (Brush 2001). The Bay supports a
diverse fish community that has been the basis of significant
commercial fisheries (Murdy et al. 1997). The Chesapeake
Bay and its adjacent uplands are under increasing pressure
from residential and commercial development.
METHODS
The entire study area has been surveyed systematically for
nesting bald eagles for 50 years (1962–2011) with a
minimum of 2 flights per season (Abbott 1963, Watts
2010). A survey flight is conducted between late February
and mid-March to locate nesting territories. We use a high-
wing Cessna 172 aircraft (Cessna, Wichita, KS) to overfly
the land surface systematically at an altitude of approximately
100 m to detect eagles and nests. The aircraft is maneuvered
between the shoreline and a distance of 1–3 km to cover the
most probable nesting locations. Nests detected are plotted
on 7.5-minute topographic maps and given unique alpha-
numeric codes. Each nest is examined to determine its
condition and status.
I compiled complete nest histories for nests (n>2,250)
included in the 50-year survey. Nests were evaluated annually
from the year that they were coded (coincident with the year
built for most nests) until they were lost. The status of each
nest was classified during annual visits as active, alternate, or
absent. I used a multistate model (Brownie et al. 1993) with
live and dead encounters in Program MARK (Version 6.1,
White and Burnham 1999) to estimate probabilities of
annual survival for active (F
A
) and alternate (F
I
) nests and
transition probabilities (C
AI
,C
IA
) between active and
alternate nests. The conventional live-dead, multi-strata
model includes detection probabilities (p) for live encounters
and reporting probabilities (r) for dead encounters (White
et al. 2006). Because nest locations were known and checked
annually until nests were absent, I fixed both detection and
reporting probabilities to 1. I used program U-CARE
(Choquet et al. 2001) to assess the goodness-of-fit between
the nest history data and the global model. Results indicated
no deviation from model assumptions. I specified model
structure using parameter index matrices and constructed
maximum likelihood models using the logit-link function.
Model fit was described by the deviance (Dev ¼2lnL) and
the number of parameters in the model (K). I assessed
Watts Residual Value of Eagle Nests 777
relative model fit using Akaike’s Information Criterion that
included corrections for small sample sizes (AIC
c
).
I derived annual and overall survival (F) and transition
probabilities (C) from a global model with time dependence. I
calculated overall means and variances by applying the variance
components procedure in Program MARK to the annual
parameters. The annual turnover rate of active nests reflects the
loss of nests (1–F
A
) and the transition of nests to alternate
status (C
AI
) such that turnover rate ¼(1–F
A
)þ(C
AI
). How-
ever, because alternate nests may be used again, the effective
turnover rate (ETR) includes the transition of nests back to
active status (C
IA
) such that ETR ¼(1–F
A
)þ(C
AI
)(C
AI
)
(C
IA
). I calculated annual ETR values from the parameter
estimates from ProgramMARK and used them to derive mean
and variance estimates.
I used survival and transition probabilities estimated in
Program MARK to calculate the costs of management
scenarios. Because actual management costs in conventional
economic terms varies with respect to geography and time, I
standardized the units of cost according to nest equivalents, or
the resources or social burden associated with management
of 1 nest. Costs are scaled to active nests such that a value of
1.5 implies that150 structures would be under managementfor
each 100 active nests. Scenarios considered included extending
protection for alternate nestsfrom 0 to 5 years and for nest trees
that have lost nests from 0 to 3 years following nest loss,
defining a matrix of 24 management combinations.
Under the assumption of a stable eagle population, the per
capita (defined here as per pair) cost of extending protection
to alternate nests includes 5 elements: active nest, per capita
active nests transitioning to alternate status, per capita
alternate nests returning to active status, per capita residual
alternate nests (alternate nests remaining in alternate pool)
remaining from previous years up to the time limit of
management, and the per capita residual alternate nests
(nests in alternate pool brought back to active status) from
previous years returning to active status. I combined these
elements to estimate the per capita nest management cost
(NMC) across the range of nest management scenarios (i¼1
to 5 years) as:
NMC ¼1þwAI
þwAI
wIA
þX
5
i¼1
wAI
fI
i
þX
5
i¼1
wAI
wIA
fI
i
The cost of additional management of trees that have lost nests
includes 2 elements: percapita loss of active nests and per capita
loss of alternate nests. I combined these elements to estimate
the per capita tree managementcost (TMC) across the range of
management scenarios (j¼1 to 3 years) as:
TMC ¼X
3
j¼1
1fA
þwAI
fI
jðÞ
I combined per capita nest and tree management costs to
estimate the costs across the matrix of management scenarios
described above.
I conducted a simple, local sensitivity analysis using single
variable substitution to evaluate the responses of manage-
ment costs to variation in survival and transition probabili-
ties. I conducted a sensitivity analysis on the management
scenario currently recommended in the national manage-
ment plan that includes protection of alternate nests for
5 years and trees that have lost nests for 3 years. I assessed
response across a limited range (8%) for survival of active
nests because this parameter was near its limit and across a
wider range (40%) for remaining parameters. I estimated
response slopes using simple linear regression.
The intent of management guidelines for nests and trees is
to protect the latent value of these structures to the
population. Survival is a component of this value because
the structure must be present and available for use to
have value at some future time. Value also includes the
likelihood of use given that the structure still exists. I used
these 2 values collectively to estimate the residual value of 1)
alternate nests and 2) trees that have lost nests. I defined
residual value (RV) as the collective likelihood of future use
for a given time and estimated it as:
RV a¼X
A
x¼a
lx
la
ux
where Ais the maximum observed survival after
abandonment (for alternate nests) or the longest interval
between loss of a nest and rebuilding of a nest (for trees), a
is the time interval for which the estimate is made, l
x
is the
probability of a nest or tree surviving to time x,l
a
is the
probability of a nest or tree surviving to time a,anduis
the probability that a nest will be reactivated (nests)
or rebuilt (trees) during time interval xto xþ1. The term
1
x
/l
a
is the probability that a structure survives to time x
giventhatithassurvivedtotimea. Alternate nests
(n¼1,163) were tracked until none remained to determine
time-specific probabilities of loss, reactivation, or remain-
ing unused. I used data to estimate residual values across
Table 1. Model summary for joint, multistrata, live-recapture/dead-
recovery models fitted to active and alternate bald eagle nests within the
lower Chesapeake Bay (1962–2011). Model parameters include survival of
active nests (F
A
), survival of alternate nests (F
I
), transition probability from
active to alternate nests (C
AI
), transition probability from alternate to active
nests (C
IA
), detection probability (p), and reporting probability (r).
Subscripts (t) and (.) refer to time-dependent and constant, respectively.
Kindicates the number of parameters included in the model and vrefers to
the model weight. AIC
c
refers to model fit using Akaike’s Information
Criterion with corrections for small sample sizes. I assessed relative fit by
comparing each model to the best-fit model (DAIC
c
).
Model AIC
c
4AIC
c
vKDeviance
fA
tfI
tCAI
tCIA
t18,219.7 0.0 0.971 194 17,832.7
fA
:fI
tCAI
tCIA
t18,226.7 7.03 0.029 149 17,918.1
fA
tfI
:CAI
tCIA
t18,237.4 17.75 0.000 146 17,942.0
fA
:fI
:CAI
tCIA
t18,293.7 74.04 0.000 146 17,998.3
fA
tfI
tCAI
:CIA
t18,305.2 85.51 0.000 98 18,107.6
fA
tfI
tCAI
tCIA
:18,373.1 153.45 0.000 101 18,169.5
fA
tfI
tCAI
:CIA
:18,379.0 159.31 0.000 146 18,083.6
fA
:fI
:CAI
:CIA
:18,453.9 234.23 0.000 5 18,443.9
778 The Journal of Wildlife Management 79(5)
the 5-year protection period currently recommended
within national management guidelines. Reconstruction
of new nests within trees that had supported nests
previously was recorded, making it possible to determine
the interval between nest loss and reconstruction.
However, the condition of all nest trees was not
determined annually following nest loss, preventing the
estimation of loss rates for previously used trees. For this
reason, I estimated tree loss rates from the population of
nest trees for the time period that they supported nests.
Because observations of most trees were right-censored, I
used a Kaplan–Meier estimate (Kaplan and Meier 1958).
RESULTS
The model with the greatest support among those tested
was the full time-dependent model (Table 1). The next 2
most parsimonious models estimating active and alternate
nest survival had different combinations of time dependence
for the parameters. The better fit for these models appears
to reflect a shift in the variance of the survival parameters
(Fig. 1) over the study period. The eagle breeding
population was subjected to high contaminant levels during
the 1960s and 1970s, possibly contributing to high nest
turnover rates. Since the early 1970s, the eagle breeding
Figure 1. Annual survival probabilities for active (A) and alternate (B) bald eagle nests monitored in the lower Chesapeake Bay illustrating the reduction in
variance over the study period. Values are means (1 SE).
Watts Residual Value of Eagle Nests 779
population increased more than 30-fold such that sample
sizes of nests available for estimating parameters have
improved. For both survival parameters, the coefficient of
variation was more than twice as high prior to 1985 (7.6 and
25.2 for active and alternate nests, respectively) compared to
after 1985 (3.1 and 11.8), a period when contaminant levels
were believed to have declined. However, global parameter
estimates for active and alternate nest survival and transition
probabilities between the 2 were robust with relatively low
variance (Fig. 2). Survival rates for alternate nests were
significantly lower than for active nests (t
98
¼9.84,
P<0.001). The loss rate for alternate nests was very
high. Median life expectancy for alternate nests that were
not re-used was 1.7 0.023 (mean SE) years after
abandonment. Of 1,163 alternate nests, only 2 (0.17%)
survived to 6 years post abandonment. The effective
turnover rate for active nests within the population was
0.251 0.023.
The cost of extending protection for both alternate nests
and trees that had lost nests increased with the term of
protection (Fig. 3). Compliance with current national
standards and recommendations that include the protec-
tion of alternate nests for 5 years and trees that had lost
nests for 3 years results in a management cost of 2.35
structures per active nest or 235% increase in NMC. Nests
and trees influence this cost in different ways. Because of
the high loss rate, the cost of extending protection to
alternate nests is somewhat self-limiting. The added NMC
for extending protection of alternate nests for a single year
is 0.37 nest equivalents, whereas the cost of extending
protection from 4 to 5 years is only 0.038. Because the loss
rate of trees is very low (see below), the cost of extending
protections to trees for additional years is not discounted.
Thecostisequaltothelossratesofactiveandalternate
nests. For each additional year of protection, the cost is
0.23 nests per active nest. Management costs associated
with protection were most sensitive to nest survival rates
followed by transition probabilities from active to alternate
status (Fig. 4). Slopes for the response of NMC to
perturbations in model parameters were 0.0271 for survival
of active nests, 0.0141 for survival of alternate nests, 0.0098
for transition probabilities from active to alternate status,
Figure 2. Diagram of survival (F; nest mortality equates to nest loss) and
transition (C) probabilities for active (A) and alternate (I) bald eagle nests
monitored in the lower Chesapeake Bay (1962–2011). I estimated
parameters using a fully time-dependent multistate model in Program
MARK.
012345
Length of nest protection f ollowing abandonment (y ears)
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
Per capit a cos t of management (nes ts)
Nest
Nest + 1 y r Tree
Nest + 2 y r Tree
Nest + 3 y r Tree
Figure 3. Estimated costs for managing bald eagle nests and trees within the lower Chesapeake Bay according to 24 management scenarios. Scenarios
represented include all combinations of managing alternate nests for 0–5 years and trees that had lost nests for 0–3 years. National management guidelines
currently recommend managing alternate nests for 5 years and trees that have lost nests for 3 years. Costs are scaled to active nests such that a value of 1.5
implies that 150 structures would be under management for each 100 active nests.
780 The Journal of Wildlife Management 79(5)
and 0.0008 for transition probabilities from alternate to
active status.
The residual value of nests declined with time since
the season of last activity from 0.0924 for the first year to
0.0208 at the end of the 5-year protection period (Table 2).
The decline reflects the low survivorship of these nests
and a reduction in the likelihood that surviving nests will
be re-used. Of 1,163 alternate nests, 352 (30.3%) were
determined to be re-used within 5 years. However, the
likelihood of re-use declined with time (x
23
¼9.22,
P¼0.028). Most (76.4%) re-used nests were re-used in
the first year, with virtually all (98.6%) being used in the first
3 years.
As with alternate nests, the residual value of trees that had
lost nests declined with time since nest loss from 0.0961 in
the first year to 0.0472 after 3 years and 0.0014 after 9 years
(Table 3). This decline is due almost entirely to a reduction in
the likelihood of re-use over time. Only 9.9% (168 of 1,699)
of trees that had lost nests were re-used within the first
10 years. Nests were rebuilt in 32% (equating to 3.1% re-use)
of re-used trees in the first year and in 71.4% in the first
3 years. The likelihood of nest reconstruction declined
among years of nest absence (x
28
¼114.9, P<0.001). Loss
of nest trees appears to be relatively low. Over the 50-year
survey, 92 trees were lost out of the 2,226 that were observed
for at least 3 years. Estimated annual survival rate was
0.992 0.015.
-40-30-20-10 0 10 20 30 40
Adjustment in parameter (%)
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
Per capita cost of management (nests)
Active nest survival rate
Alternate nest survival rate
Active to alternate transition
Alternate to active transition
Figure 4. Response of nest management costs (in units of nest equivalents, or the resources or social burden associated with management of 1 nest) to variation
in the critical parameters survival of active nests (range ¼8%), survival of alternate nests (range ¼40%), active to alternate transition probability
(range ¼40%), and alternate to active transition probability (range ¼40%) for bald eagle nests in the lower Chesapeake Bay. I estimated sensitivity to cost
for the management scenario currently recommended by guidelines (manage alternate nests for 5 years, manage trees that have lost nests for 3 years).
Table 2. Fate of alternate bald eagle nests monitored within the lower
Chesapeake Bay (1962–2011). The term “years alternate” follows national
management guidelines where 1 refers to the first year the nest was
classified as alternate. l
x
is the proportion of nests surviving to time x.u
x
is
the time-specific probability of re-use.
Years
alternate
Alternate
nests
Nests
lost
Nests
activated l
x
u
x
Residual
value
1 1,163 0 1 0.0924
2 379 515 269 0.3259 0.2313 0.0609
3 107 211 61 0.0920 0.1609 0.0549
4 29 61 17 0.0249 0.1589 0.0438
5 8 17 4 0.0069 0.1379 0.0208
6 2 5 1 0.0017 0.0833 0
Table 3. Fate of bald eagle nest trees following loss of nests within the
lower Chesapeake Bay (1962–2011). I calculated tree-loss estimates based
on a tree survival rate of 0.992 estimated from nest trees monitored. Tree
re-use is the number of trees where new nests were constructed following
the loss of a previous nest. l
x
is the proportion of trees surviving to time x.
u
x
is the time-specific probability of a new nest being constructed.
Years
absent
Nest
trees
Estimated
trees lost
Trees
re-used l
x
u
x
Residual
value
1 1,699.0 0 1 0.0961
2 1,632.4 13.6 53 0.9608 0.0312 0.0661
3 1,580.3 13.1 39 0.9301 0.0239 0.0472
4 1,539.7 12.6 28 0.9062 0.0177 0.0307
5 1,509.4 12.3 18 0.8884 0.0117 0.0196
6 1,489.3 12.1 8 0.8766 0.0053 0.0146
7 1,467.4 11.9 10 0.8637 0.0067 0.0081
8 1,448.7 11.7 7 0.8527 0.0048 0.0034
9 1,434.1 11.6 3 0.8441 0.0021 0.0014
10 1,420.6 11.5 2 0.8361 0.0014 0
Watts Residual Value of Eagle Nests 781
DISCUSSION
The rationale for extending protection to alternate bald
eagle nests and trees that have lost nests is that these
structures hold value for breeding eagles. Most eagle
management guidelines assume that the value of these
structures declines over time and this assumption is reflected
in the time limit on protections (e.g., USFWS 2007, Florida
Fish and Wildlife Conservation Commission 2008). This
assumption is supported by the current study. Within
the Chesapeake Bay, the value of eagle nests and trees to the
population declines dramatically with a change in status and
continues to decline in the following years. For alternate
nests, this decline is driven by both a low survival rate and a
reduction over time in the likelihood that existing nests will
be used. For nest trees that no longer have nests, this decline
is driven by a low likelihood of re-use that declines over time.
Reduced life expectancy was the primary driver of the
decline in residual value for alternate nests. Loss rates for
alternate nests were more than 3 times higher than for active
nests. This finding supports a previous analysis over a shorter
time period within this population (Watts and Byrd 2007),
and it points to the importance of nest maintenance to nest
survival. Nest maintenance appears to serve not only to
prepare the nest surface for incubation and brood rearing, but
also to repair structural damage and maintain nest integrity.
Nests that do not receive regular maintenance become
fragile and are more vulnerable to wind throw. The south
Atlantic Coast experiences 35–40 temperate and tropical
storms annually with enough intensity to affect coastal
habitats (Dolan et al. 1988). Because eagles often build nests
in trees exposed to high winds, fragile nests are susceptible to
being dislodged. Median life expectancy of alternate nests
that were not reoccupied was 1.7 years after last maintenance.
For surviving alternate nests, the likelihood of re-use also
declined with time. Although it is not possible to
differentiate between categories within the survey data,
alternate nests likely fall along 2 different tracks, including
those that are accurately classified as alternates and those that
are misclassified. Bald eagles leave nests for many reasons,
including the loss of nests to competitors or predators.
Within the Chesapeake Bay, great horned owls (Bubo
virginianus) and raccoons (Procyon lotor) take over a
significant number of nests annually (B. D. Watts and
M. A. Byrd, College of William and Mary and Virginia
Commonwealth University, unpublished data). Such nests
are almost never re-occupied by eagles, even after the other
species have left. Tree mortality is a second frequent cause of
a change in status from active to alternate. Because many nest
trees extend above the surrounding canopy, they are
frequently killed by lightening strikes. Most pairs will
move within 2 years following the death of a nest tree.
Clearly, nests that truly change status between active and
alternate have little residual value for the population. Nearly
94% of pairs that re-occupied alternate nests did so within
the first 2 years. These nests likely were occupied by pairs that
either did not make a breeding attempt that year or made a
breeding attempt that the survey did not detect (i.e., failed
attempt prior to the first survey flight). Misclassified nests
have the same value as those classified as active. From this
perspective, one of the greatest benefits of extending nest
guidelines is that they protect against false negatives. Based
on the pattern of re-occupation within this study area, this
benefit appears to be mostly concentrated in the first 2 years.
The residual value of trees declined following the loss of
nests but was maintained by the very high estimate of
survival. Loss of trees was estimated to be below 1%/year,
suggesting that these structures represent a resource
available to pairs for a long period of time. Notably, the
survival estimate used here was based on trees while they
supported eagle nests and as such enjoyed associated
protections. Loss of trees in this cohort was mostly due to
natural forces. Trees that lose nests and protections likely
have higher loss rates because of logging and other
anthropogenic activities. Even though tree survival was
high, re-use rates were low. Less than 10% of trees were re-
used within 10 years of nest loss. Although re-use
probabilities were concentrated within the early years, only
7% of trees were re-used within the recommended 3-year
protection period.
Choices in length of protections extended to nesting
structures have cost implications. Extension of protections
for alternate nests to 5 years requires 1.64 nest equivalents of
cost for every active nest in the population. Extension of
protection for trees that have lost nests to 3 years would
require an additional 0.69 nest equivalents of cost. On the
landscape level, these differences translate into substantial
costs. In 2011, the bald eagle breeding population within the
study area was 730 pairs (B. D. Watts and M. A. Byrd,
unpublished data). The average land surface area within
management buffers recommended by the national guide-
lines (radius of 200 m and discounting buffer overlaps) for
this study area is 11 ha (B. D. Watts, unpublished data). The
choice of extending protection to trees that have lost nests
results in an increase of land under management from 13,156
to 18,469 ha.
The cost structure for management scenarios is sensitive
to the underlying forces that drive shifts in activity status for
both nests and nest trees. Turnover and loss rates of
nests are high within the study area (Watts and Byrd 2007,
Watts and Duerr 2010). Clearly these parameters vary
across the wide breeding range of this species, but it is not
clear where the Chesapeake Bay fits within this range.
The life expectancy of active nests is 5 years in southern
Florida, USA (Curnutt and Robertson 1994), and
Saskatchewan, Canada (Gerrard et al. 1983), compared
to 13 years in Alaska, USA (Stalmaster 1987). Estimates of
nest turnover rates vary from 22.4% in Florida (Broley
1947) to 28.2% in Maine (Todd 1979) to more than 30% in
interior Alaska (Steidl et al. 1997). However, no studies
have reported on survival rates of alternate nests, trees that
have lost nests, transition probabilities between active and
alternate nests, or re-use probabilities for alternate nests
or trees that have lost nests. These are the parameters
required to evaluate the relative costs and benefits of nest
management standards.
782 The Journal of Wildlife Management 79(5)
The residual value and NMC functions diverged over time
such that the return on social investment diminishes over the
management periods. The disparity in survival rates between
alternate nests and trees that have lost nests resulted in
different cost trajectories and associated returns. Because the
loss rate of alternate nests was high, the cost of extending
protections for additional years was relatively low and nearly
self-regulating. Extending protections from 3 years to 5 years
resulted in a cost increase of only 6%. However, based on re-
use probabilities, this 6% is being invested to capture less
than 1.5% of nests that were re-used. Because the estimated
survival of nest trees is so high, costs for extending
protections increase at a constant rate. Extending protections
of trees that had lost nests from 2 years to 3 years resulted in a
cost increase of 15.7%. Based on the probability of re-use,
this 15.7% increase in NMC is being invested to capture
1.8% of trees expected to be re-used. Calculated for the study
population in 2011, to capture this 1.8% requires 2,272 ha to
be placed under management.
MANAGEMENT IMPLICATIONS
The conflict between private property rights and the
protection of imperiled species has become a common topic
of debate within the conservation community (e.g., Dwyer
et al. 1995, Goldstein 1996, Bean and Wilcove 1997). As
outlined by Innes et al. (1998), this debate centers around the
opposing mandates defined under current endangered
species law and the Fifth Amendment to the United States
Constitution. For wildlife management practitioners, the
question of how to strike a balance between benefits to target
species and the burden imposed on society has become an
increasingly complex problem. From the outset, the objective
of bald eagle management guidelines on both national and
state scales has been to protect eagles while minimizing the
burden to private landowners. Within the Chesapeake Bay,
implementation of current national guidelines results in 2.35
units of management for every active nest (i.e., more than
doubling the social burden). Much of this cost is due to the
later years within the protection period extended for nest and
nest trees, even though captured benefits to the eagle
population are very low for these additions. It seems prudent
to shorten the period of protection within the Chesapeake
Bay breeding population. However, nest re-use or loss rates
may vary across the breeding range. The current uncertainty
in the cost-benefit relationship would argue for caution in
reducing guidance recommendations for most populations.
ACKNOWLEDGMENTS
Financial supported for the Virginia annual bald eagle survey
has been provided by the Virginia Department of Game and
Inland Fisheries, the U.S. Fish and Wildlife Service, the U.S.
Department of Defense, the U.S. Army Corps of Engineers,
the Virginia Society of Ornithology, and The Center for
Conservation Biology. Funding was provided, in part,
through Federal Aid in Wildlife Restoration grants and
contributions to the Chesapeake Bay and Endangered
Species Fund. Many individuals and organizations have
contributed to the success of the Virginia Bald Eagle
survey. J. Abbott, F. Scott, and M. A. Byrd have provided the
backbone of this long-term survey. Several biologists from
the Virginia Department of Game and Inland Fisheries have
provided oversight and logistical support, including R.
Duncan, K. Terwilliger, R. Fernald, D. Bradshaw, D. Schwab,
K. Cline, and J. Cooper. K. Cline, C. Koppie, R. Lukei, A. C.
Markham, E. K. Mojica, and B. Paxton have assisted with
surveys or provided data. I thank the many private citizens and
government employees who have provided information and
assistance over the years. Several pilots have provided expert
flying services, including S. Beck, Captain C. “Fuzzzo”
Shermer, and M. Crabbe. Data management and analysis for
this investigation were supported by Constellation Energy. I
thank Y. Abernathy for support and assistance and M. D.
Wilson, E. K. Mojica, M. Watts, and M. A. Byrd for fruitful
discussions about bald eagle nest management. C. Turrin
assisted with manuscript preparation. I thank T. Grubb, J.
Lyons, and associate editor M. Bechard for helpful comments
on an earlier draft of this manuscript.
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Associate Editor: Marc Bechard.
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