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2010 SOUTHEASTERN NATURALIST 9(4):731–742
Local and Landscape Habitat Selection of Nesting Bald
Eagles in East Texas
Sarah T. Saalfeld1,* and Warren C. Conway1
Abstract - Throughout their range, Haliaeetus leucocephalus (Bald Eagle) have
experienced dramatic population increases, and breeding productivity has returned
to levels observed prior to the impacts of DDT. To effectively manage growing Bald
Eagle populations, habitat and anthropogenic characteristics influencing nest-site
selection need to be quantified at multiple spatial scales. In this study, we examined
local and landscape characteristics and anthropogenic features influencing nest-site
selection by Bald Eagles in 3 National Forests in east Texas. On a local scale, Bald
Eagles placed nests in large super-canopy coniferous trees, with nest sites surrounded
by shorter and smaller trees than random sites. Bald Eagle nest sites were best predicted
by basal area on a local level and distance to nearest human habitation on a
landscape level, as determined by logistic regression. We suggest that conservation
efforts for Bald Eagles in east Texas should include allowing forests to mature and
reducing disturbance around large water bodies to conserve and create suitable nesting
habitat on public and private lands.
During the last 25 years, breeding productivity of Haliaeetus leucocephalus
L. (Bald Eagle) has returned to levels observed prior to the
impacts of DDT (dichloro diphenyl trichloroethane; US Fish and Wildlife
Service 2007a) and US populations have increased dramatically. Currently,
Bald Eagles nest in all of the contiguous United States and Alaska and
have been removed from the endangered species list (US Fish and Wildlife
Service 2007a). Nests are usually constructed in large super-canopy trees
in close proximity (i.e., ≤3 km away) to foraging areas, such as rivers,
reservoirs, and coastal wetlands (Buehler 2000). However, development
and logging activities tend to remove or fragment suitable nesting habitat
(McGarigal et al. 1991). Beyond habitat-based impacts, human activities,
including temporary recreation disturbances and permanent structural
development near large water bodies, increases human-eagle interaction
frequency, resulting in more human-induced eagle disturbances (Grubb
and King 1991, McGarigal et al. 1991, Therres et al. 1993). Such disturbances
near nests temporarily agitate, disturb, or flush individuals (Fraser
et al. 1985, McGarigal et al. 1991). Although individual eagle responses to
such disturbances vary (US Fish and Wildlife Service 2007a), Bald Eagles
generally select nest sites away from human presence and associated
1Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State University,
Nacogdoches, TX, 75962. *Corresponding author - firstname.lastname@example.org.
732 Southeastern Naturalist Vol. 9, No. 4
disturbances (Andrew and Mosher 1982, Anthony and Isaacs 1989, Buehler
et al. 1991, Fraser et al. 1985, Livingston et al. 1990), even when those nest
sites are located further from foraging areas (i.e., large bodies of water; Andrew
and Mosher 1982, Anthony and Isaacs 1989, Fraser et al. 1985, Wood
et al. 1989). Despite this general preference for selecting undisturbed nest
sites, in cases of dense Bald Eagle populations, nests do occur in areas with
both human development and frequent human disturbance, illustrating that
freedom from human disturbance may not be a necessity for nest placement
(Millsap et al. 2004).
Bald Eagle nest-site selection patterns have been examined throughout
their range, establishing that although Bald Eagles generally nest in close
proximity to large water bodies, nest placement can vary regionally (e.g., on
the ground along the Pacific coast, in large deciduous trees in the Midwest,
in large coniferous trees in the Southeast and Northwest, and in mangroves
in southern Florida; Murphy 1965), potentially due to nest-tree availability.
However, Bald Eagle nest-site selection has not been extensively studied in
the southeastern United States, except in Louisiana (Harris et al. 1987) and
Florida (Curnutt and Robertson 1994, McEwan and Hirth 1979, Millsap et
al. 2004, Wood et al. 1989). Regions throughout the Southeast may vary in
nesting habitat availability, nest-site characteristics, and development pressure
intensity or extent. For example, in east Texas, public lands (i.e., US
Forest Service managed National Forests) contain the best available habitat
for nesting Bald Eagles and support most (>56%) of the eagles nesting in
east Texas (Ortego 2005; S.T. Saalfeld et al., unpubl. data). These areas are
dominated by pine plantations surrounding large reservoirs, which combine
to provide suitable nesting and foraging habitats (S.T. Saalfeld et al., unpubl.
data). Therefore, Bald Eagle nest-site selection patterns within east Texas
may be more dependent on timber management than in other regions.
Habitat management for nesting Bald Eagles is generally reactive, where
management zones ranging from 0.23–1.6 km surrounding nests are created
to minimize human disturbance after nests are constructed (Buehler
et al. 1991; US Fish and Wildlife Service 1987, 2007b). However, Bald
Eagle habitat and nest-site selection in east Texas should be quantified so
that appropriate habitat can be provided for expanding Bald Eagle populations
(Saalfeld et al. 2009). This need is highlighted by rapidly expanding
(numerically and spatially) human populations in east Texas in both highdensity
urban and low-density residential developments (Kjelland et al.
2007, Wear et al. 2004, Wilkins et al. 2000), at rates greater than national
averages (Schuett et al. 2007). As both Bald Eagle and human populations
expand regionally, especially in riparian areas and near large water bodies,
management efforts must focus upon nest-site selection patterns at local and
landscape levels, so as to develop proactive habitat management plans for
future eagle conservation and management. With the development of geographic
information systems (GIS), landscape-scale habitat characteristics
2010 S.T. Saalfeld and W.C. Conway 733
can now be analyzed along with local characteristics to develop habitat
models for a specific region or throughout the range of a species (Buehler
1995, Sánchez-Zapata and Calvo 1999, Sergio et al. 2003). This approach
has been used for many raptors to enhance management and develop conservation
guidelines (Bisson et al. 2002, Buehler 2000, Donázar et al. 1993,
Morán-López et al. 2006, Poirazidis et al. 2004). In this study, we examined
local and landscape characteristics as well as anthropogenic features that
potentially influence Bald Eagle nest-site selection. Further, we determined
which variables were best at identifying Bald Eagle nest sites in 3 National
Forests in east Texas. Because these areas are assumed to be the best available
habitat for nesting eagles, nest-site selection patterns ascertained from
this study will provide baseline selection preferences for future monitoring
and comparison to suboptimal and expansion regions.
Bald Eagle nests were located through aerial surveys by Texas Parks and
Wildlife Department personnel in east Texas during February–April 2005
(see Ortego 2005, Saalfeld et al. 2009). For this study, we used only those
nests located within three US Department of Agriculture, US Forest Service
National Forests (i.e., Angelina National Forest [61,989 ha] in Angelina,
Jasper, and San Augustine counties; Sabine National Forest [65,015 ha] in
Sabine and Shelby counties; and Sam Houston National Forest [65,979 ha]
in Montgomery and Walker counties). All nests were <2.5 km from one of
three reservoirs (measured as the Euclidian distance to the edge of reservoir):
Sam Rayburn Reservoir (46,337 ha; managed by the US Army Corps
of Engineers) on the Angelina River in Angelina National Forest, Toledo
Bend Reservoir (73,491 ha; managed by the Sabine River Authority) on the
Sabine River in Sabine National Forest, or Lake Conroe (8141 ha; managed
by the San Jacinto River Authority) on the San Jacinto River in Sam Houston
National Forest. Forests were dominated by second- or third-rotation Pinus
palustris P. Mill. (Longleaf Pine), P. taeda L. (Loblolly Pine), and P. echinata
P. Mill. (Shortleaf Pine). All three National Forests implement timber
and fire management to maintain diverse tracts of uneven and even-aged
We located 34 of 42 known nests (10 in Angelina National Forest, 19 in
Sabine National Forest, and 5 in Sam Houston National Forest) on foot and
collected ground-validated GPS coordinates using a Trimble ProXR GPS
during August–October 2005, after the breeding season was finished. Eight
nests were not located during ground surveys because of inaccessibility of
some areas after Hurricane Rita (24 September 2005). To avoid pseudoreplication,
we used only the most recently occupied nest when >1 nest was
present per eagle territory, where occupancy was determined from aerial
734 Southeastern Naturalist Vol. 9, No. 4
surveys in February by presence/absence of adult eagles near nests (Saalfeld
et al. 2009). We also located 42 random sites (11 in Angelina National Forest,
22 in Sabine National Forest, and 9 in Sam Houston National Forest)
within 3 km (mean distance to nearest nest) of nest locations using a random
number generator to determine specific GPS coordinates.
For each nest tree, we used a clinometer to measure total nest-tree
height (m), height to base of crown (m), and nest height (m), and calipers
to measure nest-tree diameter at breast height (DBH; cm). Additionally, we
measured several habitat variables (see Table 1) that could potentially influence
nest-site selection based on published accounts (see Buehler 2000).
Specifically, within a 0.08-ha circular plot centered on each nest tree or
random point, we measured DBH and total height of all trees >10.2 DBH
(cm). In addition, we tallied all trees >10.2 cm DBH by species and placed
them into three DBH size classes (10.2–30.4 cm, 30.5–53.3 cm, and >53.3
cm) following Andrew and Mosher (1982).
For this portion of the study, we used 42 nests (13 in Angelina National
Forest, 24 in Sabine National Forest, and 5 in Sam Houston National Forest).
We generated 42 random sites in each National Forest for landscape analyses
to sample potential nesting habitat within the three National Forests. We
generated random sites using simple random sampling with Hawth’s Analysis
Tools in ArcGIS (Beyer 2004). We restricted random sites to be located
within forested, non-urban areas, <1 km (representative of mean distance
[0.6 km] to reservoirs for nest sites) from reservoirs (i.e., Sam Rayburn,
Table 1. Variables measured at Bald Eagle nests and associated random sites in 3 National
Forests in east Texas, 2005.
Local habitat (0.08-ha plot) (0.5-km and 1.0-km plots) Distance to/patch metrics
Mean diameter at breast height (cm) Total length of roads in Nearest distance to
Mean height (m) plot (km) water > 12 ha (km)
No. of trees with DBH > 10.2 cm Area open water (ha) Nearest distance to
No. of trees with DBH = 10.2–30.4 cm Area woody wetland (ha) water < 12 ha (km)
No. of trees with DBH = 30.5–53.3 cm Area emergent herbaceous Nearest distance to
No. of tress with DBH > 53.3 cm wetland (ha) human structure (km)
Density (# trees/ha) Area deciduous forest (ha) Nearest distance to
Basal area (m2/ha) Area coniferous forest (ha) road < 35 mph (km)
Area mixed forest (ha) Nearest distance to
Area shrub/scrub (ha) railroad (km)
Area herbaceous (ha) Patch area (ha)
Area hay/pasture (ha) Patch perimeter (km)
Area barren (ha) Patch shape
Area developed (ha) Distance to nearest
Area human structure (ha) patch edge (m)
Number of patches
Total edge (km)
2010 S.T. Saalfeld and W.C. Conway 735
Toledo Bend, or Lake Conroe), within each respective National Forest, and
>2.5 km from other nests or random sites. To quantify landscape characteristics,
we used ArcGIS 9.2 to create two circular plots (0.5-km and 1.0-km
radii) centered on nests or random points. These plots correspond to primary
and secondary management zones (primary = 0.23–0.46 km, secondary =
0.46–1.6 km; US Fish and Wildlife Service 1987) suggested for nesting Bald
Eagles. We developed 16 habitat and anthropogenic disturbance variables
which were measured inside each plot, 5 variables measuring Euclidian
distances to assumed foraging areas (i.e., water bodies ≥12 ha) and potential
anthropogenic disturbances, and 4 variables associated with the patch where
eagle nests or random points were located (Table 1). The patch where a
nest or random site was located was defined as the area surrounding a nest
or random point comprised of a continuous land-cover classification (e.g.,
coniferous forest). We selected these variables because of their potential to
influence nest-site selection as suggested in previous studies of Bald Eagles
and other large raptors (see Berkelman 1995; Bisson et al. 2002; Buehler
2000; Donázar et al. 1993, 2002). We obtained land-cover classifications
(i.e., open water, woody wetland, etc.) from 2001 national land-cover data
(Homer et al. 2004, National Land Cover Database 2001). These data provide
relevant, standardized land-cover classifications measured in close
temporal proximity to nest activity. Although four years passed between
land-cover data classification (2001) and data collection (2005), most (i.e.,
>50%) nests included in analyses were initiated prior to 2002 (S.T. Saalfeld
et al., unpubl. data). We used FRAGSTATS (McGarigal et al. 2002) with
2001 national land-cover data to determine land-cover metrics within plots
(i.e., number of patches, total edge, and contiguity) and the individual patch
metrics associated with nest and random points (i.e., patch area, perimeter,
and shape), with patch neighbors defined by the four-cell rule (i.e., patch
neighbors corresponding only to the 4 adjacent cells that share a side with
the focal cell were considered a patch member; McGarigal et al. 2002). Patch
shape provides a measure of patch shape complexity and is calculated as the
patch perimeter divided by the minimum perimeter possible for a maximally
compact patch of the corresponding patch area (McGarigal et al. 2002). We
obtained roads from Street Maps USA for use with ArcGIS 9.2 (ESRI 2005).
We digitized all other cover types (i.e., human structures and water bodies)
using 1-m resolution, 2004 National Agriculture Imagery Program (NAIP)
digital orthophoto quarter-quadrangle aerial photographs (Texas Natural
Resources Information System 2004). Because human structures were digitized
based upon aerial photographs, the degree of use or occupancy of these
structures was unknown. Although these structures likely do not provide
equal rates of disturbance to nesting Bald Eagles, we assume that presence
of these structures likely provides some disturbance to nesting eagles and
could influence nest-site selection.
736 Southeastern Naturalist Vol. 9, No. 4
We calculated tree density (No. of trees/ha) and basal area/ha from habitat
data collected in plots around nest trees and associated random points.
To determine potential differences in nearest nest distances among National
Forests, we used an analysis of variance (PROC GLM; SAS Institute
2002). For local and landscape habitat analysis, we used stepwise logistic
regression with nest sites coded one and random sites coded zero (PROC
LOGISTIC; SAS Institute 2002) to determine variable(s) most predictive of
a nest site. Variables were permitted to enter and remain in the logistic model
at a 0.05 significance level, with correlated variables restricted from entering
the same model.
Bald Eagles in this study nested solely in coniferous trees (28 in
Loblolly Pine, 4 in Longleaf Pine, and 2 in Shortleaf Pine) ranging from
24.7–42.7 m tall (x̅ = 34.4 m, SE = 0.7), 48.0–94.0 cm in DBH (x̅ = 70.2
cm, SE = 2.0), and 10.1–29.0 m height to crown base (x̅ = 21.5 m, SE =
0.9). Nests were built on average 26.9 m off the ground (SE = 0.6, range =
18.6–32.0 m) and <13 m from top of tree (x̅ = 7.5,SE = 0.4, range = 2.1–
12.8 m). On average, nest trees were 13 m taller than the mean height of all
surrounding trees with >10.2 cm DBH within 0.08-ha plots surrounding
nests. Overall, 53% of nest trees were the tallest trees within the 0.08-ha
plots surrounding nests, 29% were the second tallest, and 18% had two or
more trees taller than the nest tree. Random plots (i.e., 0.08-ha plots) had
a similar range of tree sizes (DBH: 10.2–99.06 cm, height: 12.2–41.1 m)
as nest sites (DBH: 10.2–85.1 cm, height: 10.7–42.7 m), but on average,
random plots contained more trees (x̅ = 6 trees, range = 0–16 trees) with
characteristics necessary for nest placement (i.e., DBH ≥ 48 cm and height
≥ 24.7 m) than nest sites (x̅ = 4 trees, range = 1–12 trees). Nests averaged
0.6 km (SE = 0.9, range = <0.1–2.5 km) from foraging areas (i.e., major
reservoir) and 3.1 km (SE = 0.2, range = 1.6–7.0 km) from the next nearest
nest. We did not detect any differences (F2,39 = 1.32, P = 0.279) in mean
distance from the next nearest nest among National Forests (Angelina National
Forest: x̅ = 2.7 km, SE = 0.5 km; Sabine National Forest: x̅ = 3.1 km,
SE = 0.2 km; Sam Houston National Forest: x̅ = 4.0 km, SE = 1.0 km).
On a local scale, stepwise logistic regression selected basal area of all
trees with >10.2 cm DBH within 0.08 ha (negative model estimate coefficient) as the best predictor of Bald Eagle nests (P < 0.001; Table 2). The
logistic regression model correctly predicted (i.e., probability > 0.5) nest
sites based on basal area of all trees with >10.2 cm DBH within 0.08 ha with
68% accuracy, and the Hosmer-Lemeshow goodness-of-fit statistic indicated
that the model fit the data well (P = 0.998).
On a landscape scale, stepwise logistic regression selected nearest
distance to human structure (km; positive model estimate coefficient) as
the best predictor of a Bald Eagle nest (P = 0.001; Table 2). The logistic
2010 S.T. Saalfeld and W.C. Conway 737
regression model correctly predicted (i.e., probability > 0.5) nest sites
based on nearest distance to human structure (km) with 67% accuracy, and
the Hosmer-Lemeshow goodness-of-fit statistic indicated that the model fit
the data well (P = 0.493).
Bald Eagles in east Texas selected nest trees with specific structural
characteristics (e.g., coniferous, large diameter, super-canopy, and close to
foraging areas), similar to other regions (Maryland: Andrew and Mosher
1982, Oregon: Anthony and Isaacs 1989, Columbia River estuary: Garrett et
al. 1993, north-central Florida: McEwan and Hirth 1979, Florida: Wood et
al. 1989). At a local scale, Bald Eagle nests were located in trees surrounded
by shorter and smaller trees than were available in random areas. These results
are consistent with the hypothesis that eagles select nest trees with an
unobstructed view and flight path from the nest (Anthony and Isaacs 1989,
McEwan and Hirth 1979, Wood et al. 1989). However, it should be noted that
random plots were not centered on a potential nest tree. Because a large tree
centered within a plot can influence the composition of surrounding trees,
there may be a potential bias in random sites as related to nest sites. However,
the majority of random locations (i.e., 86%) contained ≥1 tree with the
necessary characteristics for nest placement (i.e., DBH ≥ 48 cm and height ≥
24.7 m). Therefore, we conclude that minimal bias occurred between random
and nest sites because random sites contained the necessary tree characteristics
for nest placement, but were not selected.
Avoidance of humans during nesting has been well documented in Bald
Eagles (Andrew and Mosher 1982, Anthony and Isaacs 1989, Buehler et al.
1991, Fraser et al. 1985, Livingston et al. 1990), where nests are typically
placed further from human habitation and disturbance. Although evidence
is mounting that Bald Eagles are able to acclimate to regular disturbances
in some locations (Millsap et al. 2004), this study corroborates their tendency
to avoid human presence, as greater distances to nearest human
structure resulted in a higher probability of nest-site selection. Specific
mechanism(s) causing eagles to avoid such areas is unknown, but could
be a result of being repeatedly flushed off nests or simply instinctually
avoiding areas with human presence (Buehler et al. 1991). It is suspected
Table 2. Model results from logistic regression of local and landscape habitat of Bald Eagle
nests and associated random sites in 3 National Forests in east Texas, 2005.
Parameter Estimate SE χ2 P-value
Intercept 2.023 0.656 9.509 0.002
Basal area (m2/ha) -0.075 0.021 12.839 <0.001
Intercept -1.094 0.408 7.181 0.007
Nearest distance to human structure (km) 0.001 0.000 9.657 0.002
738 Southeastern Naturalist Vol. 9, No. 4
that increased human presence near nests could result in increased energy
expenditure, nest abandonment, and reduced reproductive success and productivity
(see Buehler 2000, Fraser et al. 1985, McGarigal et al. 1991);
however, few studies have been able to document changes in productivity
and/or reproductive success with varying degrees of human disturbance
(Fraser et al. 1985, McEwan and Hirth 1979, Schirato and Parson 2006). In
this study, we assumed that foraging opportunities were consistent among
locations within reservoirs; however, this is likely not the case. Therefore,
it should be noted that selection of nesting habitat could also be directly
impacted by foraging opportunities. Regionally, Bald Eagle populations
are increasing exponentially (Saalfeld et al. 2009), and likely will continue
to do so until limited by habitat and/or foraging opportunities. We suspect
that selection of nest sites is based upon the most limiting feature in the
landscape (i.e., distance from human habitation on the landscape level).
However, if habitat within this region would become limiting, distance to
nearest human structure might not remain the most important variable selected
for by nesting eagles in this region.
In east Texas, National Forest lands provide >11,000 ha of potential
habitat for nesting Bald Eagles (S.T. Saalfeld, unpubl. data), but public lands
account for only 8% of all forested lands regionally (estimate in 2006; Texas
Forest Service 2007). Assuming the average territory size for this region
ranges from 1–3 km2 and food availability is not limiting, we estimate that
these 3 National Forests can support approximately 42–114 active nests in
any given year (S.T. Saalfeld, unpubl. data). While this is nearly 3 times as
many nests as were documented in 2005, more than 50% of all nests in the
region occur on National Forests, and nesting habitat will likely become
saturated here first. Therefore, long-term regional conservation efforts for
nesting Bald Eagles should encourage conservation of potential suitable
nesting habitat on private lands. However, recent changes in ownership
patterns of both private non-industrial and industrial forest lands have resulted
in ownership fragmentation and dramatic declines in average parcel
size during the last 15 years (Wilkins et al. 2000). Similarly, fragmentation,
development, and conversion of forested lands bordering National Forests
is becoming a genuine conservation concern (Radeloff et al. 2005). In sum,
neither the long- nor short-term impacts of these dramatic changes in land
ownership, changes in land management practices, and alterations in habitat
structure on nesting Bald Eagles can be predicted at this time. However, as
eagles in this study selected nesting habitat away from human structures,
changes in land-use practices and increased development have the potential
to negatively impact nesting Bald Eagles within this region.
This study clearly provides some guidance and outlines future concerns
for management strategies for nesting Bald Eagles within this region that
may have range-wide application. Although we analyzed local and landscape
habitat variables separately, we suggest that habitat management should
focus upon both levels, as models that combine both levels will likely have
2010 S.T. Saalfeld and W.C. Conway 739
greater success at predicating nest sites than when considered separately.
Site-specific local timber management practices performed during the nonbreeding
season could be used to enhance and/or create Bald Eagle nesting
habitat near (i.e., less than 3 km) large water bodies. Specifically, by protecting
large, isolated super-canopy coniferous trees within shorter stands of uneven
heights, necessary tree characteristics for nest placement near foraging areas
could be obtained. Once suitable nest sites are available, however, reducing/
preventing disturbance around potential nest locations will be important
for enhancing and maintaining eagle productivity. If Bald Eagles continue
to increase regionally, nesting habitat may eventually become limited on
public lands. Even if eagles become acclimated to human presence regionally
(Millsap et al. 2004), continued revaluation of private lands based upon
non-agricultural values will not likely ameliorate the expansion and distribution
of low-density residential developments regionally (Wear et al. 2004),
particularly surrounding other potentially suitable reservoirs not managed
by public agencies, thereby limiting available nesting sites. If Bald Eagle
populations become habitat limited by saturating optimal nesting habitat
on public or private lands, nesting productivity may plateau or decline (see
Saalfeld et al. 2009). Such metrics will be key elements for determining if
changes in habitat-selection patterns and tolerance of human disturbance
impact reproductive success over long temporal scales.
We thank all of the individuals that assisted with data collection at Texas Parks
and Wildlife Department, US Fish and Wildlife Service, US Forest Service, and
Stephen F. Austin State University during the time this research was conducted. We
specifically thank John T. Steele for initial work on this project and Fred LeBlanc
of The Woodlands Operating Corporation for financial support. We also thank Jeff
Reid, Bill Bartush, David Plair, Ricky Maxey, Chris Gregory, Brent Ortego, and the
field staff of the Angelina, Sabine, and Sam Houston National Forests for advice and
logistical support. Finally, we thank Christopher Comer and Daniel Scognamillo
for comments on an earlier version of this manuscript. The Arthur Temple College
of Forestry and Agriculture, Stephen F. Austin State University, Texas Parks and
Wildlife Department, US Forest Service, and US Fish and Wildlife Service provided
financial and logistical support for this research.
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