Habitat Structure of Bald Eagle Nest Sites and Management Zones near Juneau, Alaska

Transcription

1 Habitat Structure of Bald Eagle Nest Sites and Management Zones near Juneau, Alaska M. Hildegard Reiser and James P. Ward, Jr. Rocky Mountain Experiment Station, Flagstaff, AZ Knowing how much space and what kinds of forest habitat Bald Eagles need is crucial to making effective decisions about resource management. This study, conducted near Juneau, Alaska, examines specific characteristics of Bald Eagle nest trees, forest structure around the nest tree and forest structure within both the current management zone and a larger area beyond the management zone. As urban and wild lands come into increasingly closer contact and as demands on wildlife and other natural resources increase, effective and sound ecological management of these resources becomes more critical. This is particularly true in Southeast Alaska where timber harvesting and coastal development continue to threaten Bald Eagle (Haliaeetus leucocephalus) populations. No longer can we simply rely upon the vast acreage of Alaska to ensure minimal impacts on North America's largest population of Bald Eagles (King et al. 1972, Hodges et al. 1979). Current minimal standards and guidelines may protect against direct human disturbances to nesting Bald Eagles, but these measures may be insufficient to maintain a healthy landscape, which is necessary for ensuring the existence of future populations (Connor 1979, Sidle et al. 1990). Thus, sound management solutions are currently needed for maintaining eagle populations in Southeast Alaska. Quantitative descriptions of habitat used by Bald Eagles are a prerequisite to establishing effective management strategies. Although general habitat descriptions are reported for active Bald Eagle nests in Southeast Alaska (Hodges and Robards 1982, Sidle et al. 1990), quantitative descriptions of habitat around nest sites used by Bald Eagles are not available. The purpose of this study is to provide an additional understanding of area requirements and characteristics of the habitat used by Bald Eagles for nesting and perching in the vicinity of Juneau, Alaska. In this paper we: 1) quantify and compare characteristics of specific trees eagles used for nesting and perching and 344

2 2) quantify and compare characteristics of habitat found, a) immediately around nest trees, b) within a 100 m radius management zone around nests (currently proposed by the USDA Forest Service for protecting eagle nest sites) and c) outside the management zone but within a 0.5 km area potentially used by eagles during the breeding period. Study Area We studied Bald Eagle habitat use around North Douglas Island, Mendenhall Peninsula and various small islands located in Auke Bay and Fritz Cove in Southeast Alaska (Figure 1). Western hemlock (Tsuga heterophylla) and Sitka spruce (Picea sitchensis) dominated the overstory in this coastal temperate rainforest. The understory trees, particularly along coastal shorelines, consisted of Sitka alder (Alnus crispa sinuata), willow thickets (Salix spp.) and occasionally black cottonwood (Populus balsamifera trichocarpa). Human activity was evident on the study area. Public road use, air transport and residential development occurred on the mainland and on north Douglas Island. The islands were less developed, although we observed trail use along the shorelines. Boating, primarily for fishing, was a frequent activity within coastal waterways of the study area. Most of the habitat used by the eagles was on lands currently administered by the Tongass National Forest or recently transferred to state or city jurisdiction. Some nest (8 of 23) and perch (14 of 33) sites that we examined were located on privately owned lands. 345

3 Methods We characterized areas used by Bald Eagles for nesting by sampling and quantifying habitat within a series of systematic plots (0.05 ha). These plots were placed at and around trees used by eagles (Figure 2). In addition to sampling used habitat, we sampled randomly selected trees and plots in order to quantify any features that eagles may have selected within their domain. Microhabitat and macrohabitat were sampled at 23 Bald Eagle nests randomly selected from a set of 66 nests previously located by the U.S. Fish and Wildlife Service during 1971 to We also sampled the microhabitat at 33 perch sites, randomly selected from 190 observations gathered during this study and 17 trees not used for nesting, but similar in diameter to nest trees and found within 15 m of known nests. Because sampling effort was constrained by available resources, we were not able to examine all of the nests and perches known to us. Though we allocated effort for sampling 23 non-nest trees, we could not find trees that met our "non-nest" criteria at six sites. The purpose of microhabitat sampling was to characterize nest, perch and non-nest trees used by Bald Eagles. We report here on six variables describing tree structure and composition including: 1) tree species, 2) tree crown category, 3) tree height, 4) tree diameter, 5) elevation at the tree's base and 6) overhead canopy closure at the tree (Figure 2A). Tree species and a tree crown category (normal slender, broken top live, unbroken top dead, deformed top, or double top; Hodges and Robards 1982) were recorded for each nest, perch and non-nest tree. Tree height (in m) was measured using a clinometer. Tree diameter at breast height (DBH, in cm) was measured with a steel tape. The elevation (in m) of the tree base above sea level was measured using an altimeter or taken from a topographic map. Overhead canopy closure (in %) was estimated as an average of 4 ocular tube readings taken from each of 4 stations (16 total readings). Stations were located at the base of each tree along 2 perpendicular axes radiating through the center of the nest tree with the first axis established parallel to the main shore closest to the tree. We sampled macrohabitat in order to describe forest stand characteristics within and outside of a 100 m management zone proposed by the Forest Service. Macrohabitat sampling was conducted at 5 plots within the circular management zone (Figure 2B), including 1 plot centered at a Bald Eagle nest tree (Figure 2A) (after B. Noon et al., pers. comm.) and within 4 plots randomly selected from a grid of possible plots situated outside of the management zone but within a 0.5 km area considered to represent habitat 346

4 used by nesting eagles (Hodges and Robards 1982, Figure 2C). This configuration formed a 200 m x 500 m sampling area which was oriented along the shoreline. Within each plot, we tallied: 1) the number of trees by species, 2) estimated live tree density and basal area for 3 diameter classes (1 to 30 cm, > 31 to 45 cm and > 46 cm) using the point-quarter method (Brower and Zar 1984) and 3) estimated overhead canopy closure (in %) using an average of 4 ocular tube readings taken from stations placed 12.5 m from the plot center along the plot axes. Plot axes were defined in the same manner as described above. Micro- and macrohabitat features were quantified using standard descriptive statistics. Univariate statistical methods were used to quantify differences in habitat features among sampling regimes (i.e., among tree types or among nest sites and management zones). Statistical tests were considered significant at the 5% level. Results and Discussion Tree Characteristics Selection of nest tree species varies over the geographic range of the Bald Eagle. Bald Eagles in the Juneau vicinity appeared to select western hemlock (x² = 4.14, df =1, p = 0.042; Figure 3). Eagles used western hemlock significantly more for nesting than for perching (x² = 9.07, df = 1, p = 0.003; Figure 3). Hodges and Robards (1982) found only 20% (n = 776) of eagle nests in western hemlock in Southeast Alaska; however, relative density of tree species was not available from their study for evaluating tree availability. In coastal areas of the Pacific Northwest, Douglas fir (Pseudotsuga menziesii) and Sitka spruce were predominantly used for nesting (Anthony et al. 1982, Stalmaster et al. 1985). Low use of Sitka spruce may be a reflection of past logging practices. Many of the large Sitka spruce had been logged from the islands located in our study area by the early 1900's (Rakestraw 1981). With the exception of crown top conditions, structural characteristics of Bald Eagle nest trees were similar to previously reported descriptions (Anthony et al. 1982, Stalmaster et al. 1985, Wood et al. 1989). Crown conditions of nest trees were significantly different from perch tree crowns (x² = 14.80, df= 6, p = 0.022), but not significantly different from non-nest tree crowns (x² = 9.16, df = 5, p = 0.10) in our study area (Figure 4). Most nests were located in trees with normal, bushy crowns (40%, n = 9; Figure 4). Bushy crowns may protect the nestlings from the rain and solar radiation. Both perch and non-nest tree crowns were predominantly normal, slender crowned (Figure 4). Crown conditions of eagle nest trees near Juneau differ from those in other geographic areas. The low proportion of trees with broken or dead tops (17%) was dissimilar to the nest tree crown conditions found by Hodges and Robards (1982, 38%) in Alaska, or Grubb (1976, 48%) in Washington. Nest trees tended to be a dominant or co-dominant tree in the surrounding stand. Nest tree heights (=30.0 m) were significantly taller than perch trees (x² = 24.9 m; t = 2.49, df = 54, p = 0.016), while nest trees were found at significantly higher elevations (x² = 11.6 m) relative to perch trees (x² = 4.1 m; t = 3.47, df = 49, p = 0.001; Figures 5 and 6). Bald 347

5 Eagle nest trees had greater, but not significantly different, diameters than non-nest and perch trees (Figure 7). Overhead canopy cover at non-nest, nest and perch trees was similar (Figures 8 and 9). Plot Characteristics The number of western hemlock per plot was significantly different at the nest, inside and outside of the management zone (Kruskal-Wallace H =12.24, n = 23, 56, 72 plots, respectively, p = 0.002; Table 1). Fewer western hemlock were found near nest trees, an increasing number were counted within the management zone and even more were found outside the management zone. Sitka spruce exhibited the opposite trend, but this trend was not statistically significant. The density of all trees around eagle nests in the Juneau vicinity (498 trees/ha; Table 2) was greater than densities recorded in other geographic areas (e.g., 109 to 166 trees/ha throughout the Pacific Northwest; Anthony, et al. 1982). The coastal areas around the Juneau area were heavily logged since European settlement in 1880 (Rakestraw 1981). Thus, in this dense secondary growth forest, the eagles may be selecting the least dense stands to nest within, or eagles may be selecting large residual trees in patches of forest where the growth of smaller trees is limited (by shading and nutrient competition). Whether or not the eagles are exhibiting a real preference for sparser stands is not clear. Basal areas of pole sized trees, saw timber sized trees and trees of similar size (DBH) to nest trees, were lower immediately around Bald Eagle nests than in stands within and outside the 100 m management zone (Table 3). However, this difference was not statistically significant. Basal area for all tree sizes (79.6 m²/ha) was slightly greater than those recorded from other areas in the Pacific Northwest 57.9 m²/ha to 70.9 m²/ha (Anthony et al. 1982). Overhead canopy closure was similar at Bald Eagle nests, inside and outside of the management zone (Figure 8). Table 1. Number of trees ( ± s.e.) at and around Bald Eagle nests near Juneau, Alaska, summer Sample size signifies number of plots used to estimate the mean. Tree species Nest (n=23) Number of trees Outside buffer (n=72) In buffer (n=56) Sitka spruce 5.9 (l.32) 5.2 (0.60) 4.8 (0.43) Western hemlock 10.0 (1.89) 13.2 (l.13) 15.4 (0.98)* *Indicates a significant difference among the 3 plot types for the 2 tree species. 348

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7 Management Implications A regional landscape approach to raptor management has been used for Bald Eagles in the Pacific Northwest (Stalmaster, et al. 1985) and for accipiters in the eastern U.S. (Falk 1990). This concept is being considered for management of populations of the Northern Spotted Owl (Strix occidentalis caurina) in the Pacific Northwest (Thomas et al. 1990) and for Northern Goshawk (Accipiter gentilis) populations in southwestern U.S. (R. 350

8 Reynolds, pers. comm.). In this approach, the goal is to create management zones that are large enough to maintain the benefits from fully functioning ecological units that are under consideration. For example, in Southeast Alaska, Bald Eagles are concentrated and probably dependent, on forested zones along the coast and fresh water tributaries with salmon runs. Thus, enough landscape should be provided to ensure the habitat needs for an adequate number of Bald Eagles to maintain the population through time. Within a landscape, structural diversity operates at different scales depending on a species perception of its environment (Hunter 1987). In this study, we examined 4 "human-designated" scales of Bald Eagle habitat use: 1) specific characteristics of nest trees, 2) forest structure immediately around the nest tree, 3) forest structure within a currently utilized management zone and 4) forest structure around a larger use area beyond this zone suggested in the literature. At the smallest scale of habitat use, Juneau area Bald Eagles seem to be choosing characteristic trees, an emergent or co-dominant tree within a stand, with a large diameter. This pattern has been observed elsewhere (Anthony et al. 1982, Stalmaster et al. 1985, Wood et al. 1989). Bald Eagle adult and eaglet in Sitka spruce tree nest. Photo by Phil Schempf, USFWS. At the second scale of habitat use, a spatial pattern emerged. The forest immediately around the nest was sparser than the surrounding forest. This may allow for easier observation of approaching intruders and provide easier access to the nest structure. The 351

9 forest structure appears relatively uniform away from the nest tree. It is a multi-layered and dense forest. The denser forest stands around a nest site could provide screening from more distant disturbances and neighbors and provide some measure of protection from the wind. Trees situated in open conditions are subject to windthrow (P. Schempf, pers. comm.). We did not notice any significant differences in forest structure between the third and fourth scale of habitat use, the two management zones. However, use of habitat by eagles at the fourth scale would include a greater number of perches, likely increasing the chance of foraging success. We found that nests averaged 0.65 km apart along the shorelines in the Juneau vicinity. In a concurrent study (Reiser and Ward, in prep), radio-marked Bald Eagles utilized an area approximately 0.55 km wide by 1.25 km length of shoreline. This area is twice as large as the expanded management zone (the fourth scale of habitat use) that we examined. Thus, the scale of the landscape used to manage Bald Eagles in the Juneau vicinity and possibly Southeast Alaska should be expanded to ensure the habitat needs of this species. Other components to consider in a habitat management strategy for the Bald Eagle include the distribution of perch trees and the distance of perch sites to the nest trees (Reiser and Ward, in prep). Moreover, the availability of prey may influence perch site selection, requiring additional adjustments in the size and placement of management zones. Finally, the reproductive history of each eagle nest site should be evaluated in relation to habitat structure and prey availability to provide information for linking habitat and population management. Acknowledgements We wish to thank P. Schempf, M. Jacobson and B. Conant, U.S. Fish and Wildlife Service and A. Doyle, U.S. Geological. of Agriculture, Forest Service, for their valuable input and support of this project. We also gratefully acknowledge the logistical support and general assistance provided by C. Johnson, S. Gilbertson and B. Grochow, the City of Juneau Land and Parks and Recreation Offices. This work would not have been possible without the labor and devotion of C. Blair, intern and the many School for Field Studies (SFS) students, who collected data as part of a Bald Eagle Ecology course during the summer Financial support of this project was provided by the SFS, Beverly, Massachusetts. Literature Cited Anthony, R. G., R. L. Knight, G. T. Allen, B. R. McClelland and J. I. Hodges Habitat use by nesting and roosting Bald Eagles in the Pacific Northwest. Trans. North Am. Wildl. Nat. Resourc. Conf. 47: Brower, J. E. and J. H. Zar Field and laboratory methods for general ecology (2nd ed.). Wm. C. Brown Publ., Dubuque, Ia. 226pp. Connor, R. N Minimum standards and forest wildlife management. Wildl. Soc. Bull. 7:

10 Falk, J. A Landscape level raptor habitat associations in northwest Connecticut. M.S. Thesis, Virginia Polytech. Inst. State Univ., Blacksburg. 116pp. Grubb, T. G A survey and analysis of Bald Eagle nesting in western Washington. M.S. Thesis, Univ. Washington, Seattle. 87pp. Hodges, J. I. and F. C. Robards Observations of 3,850 Bald Eagle nests in Southeast Alaska. Pages In: W. N. Ladd and P F. Schempf, eds. Proc. of a symposium and workshop on raptor management and biology in Alaska and Western Canada, February 1981, Anchorage, Alas. U.S. Dept. Inter., Fish Wildl. Serv., Anchorage, Alas. 335pp. Hodges, J. I., J. G. King and F. C. Robards Resurvey of the Bald Eagle breeding population in Southeast Alaska. J. Wildl. Manage. 43: Hunter, M. L., Jr Managing forests for spatial heterogeneity to maintain biological diversity. Trans. North Am. Wildl. Nat. Resour. Conf. 52: King, J. G., F. C. Robards and C. J. Lensink Census of the Bald Eagle breeding population in Southeast Alaska. J. Wildl. Manage. 36: Rakestraw, L. W History of the United States Forest Service in Alaska. U.S. Geological. Agric., For. Serv., Alaska Hist. Comm. 221pp. Sidle, W. B., L. H. Suring and J. I. Hodges, Jr The Bald Eagle in Southeast Alaska. U.S. Dept. Agric., For. Serv., Wildl. Fish. Habitat Manage. Notes. R10-MB pp. Stalmaster, M. V., R. L. Knight, B. L. Holder and R. J. Anderson Chapter 13. Pages In: E. R. Brown, ed. Management of wildlife and fish habitats in forests of western Oregon and Washington. U.S. Geological. Agric., For. Ser., Pac. Northwest Reg. Thomas, J. W., E. D. Forsman, J. B. Lint, E. C. Meslow, B. R. Noon and J. Vemer A conservation strategy for the Northern Spotted Owl. Interagency Sci. Comm. to Address the Conserv. of the Northern Spotted Owl. Portland, Oreg. 458pp. Wood, P. B., T. C. Edwards, Jr. and M. W. Collopy Characteristics of Bald Eagle nesting habitat in Florida. J. Wildl. Manage. 53: Appendix 1. Non-nest (random), nest and perch tree variables Tree species Crown condition (Categories are normal slender, broken top & alive, unbroken top & dead, deformed top & alive, doubled top & alive) Nest height (in m) Elevation above water (in m) from the base of the tree Diameter at breast height (DBH) (in cm) Overhead canopy cover Nest, inside buffer, outside buffer forest stand variables Number of trees by species Density (trees/ha) of trees Basal area (m ²/ha) of trees Overhead canopy closure (in %) 353