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Local and Landscape Habitat Selection of Nesting Bald Eagles in East Texas
Sarah T. Saalfeld and Warren C. Conway

Southeastern Naturalist, Volume 9, Issue 4 (2010): 731–742

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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. Introduction 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 - 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. Methods 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 forest stands. Local habitat 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). Landscape habitat 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. Landscape habitat 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) Contiguity index 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 Data analyses 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. Results 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). Discussion 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 Local habitat Intercept 2.023 0.656 9.509 0.002 Basal area (m2/ha) -0.075 0.021 12.839 <0.001 Landscape habitat 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. Acknowledgments 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. Literature Cited Andrew, J.M., and J.A. Mosher. 1982. Bald Eagle nest site selection and nesting habitat in Maryland. Journal of Wildlife Management 46:382–390. Anthony, R.G., and F.B. Isaacs. 1989. 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