Scott M. Gende. U.S. Forest ..... Researcher Scott Gende, preparing to measure a captured adult Bald Eagle. ... Glacier Bay National Park, Gustavus, Alas. 67pp.
Perspectives on the Breeding Biology of Bald Eagles in Southeast Alaska Scott M. Gende U.S. Forest Service, Forestry Sciences Lab., Juneau, AK Southeast Alaska (hereafter Southeast) has one of the highest densities of breeding Bald Eagles in North America (Gende et al. 1997). With relatively pristine habitat, low levels of disturbance and minimal contaminant influence, Southeast is a prime area to study factors that naturally regulate population levels, reproduction and general ecology of Bald Eagles. My several years of field research with hundreds of hours of observation of Bald Eagles have led to some perspectives on the breeding biology of eagles in Southeast. I will discuss some of these observations in context of other research that has been done on the life history of Bald Eagles and suggest potential topics for future research. Three distinct stages of reproduction for eagles in Southeast will be discussed: pre-laying, incubation and nestling. The post-fledging period is also an extremely important period as fledglings exhibit the highest rates of mortality during the first year out of the nest (e.g., McCollough 1986, Hodges et al. 1987). Although I have observed adults feeding fledglings for 3 weeks after they leave the nest, I have little insight concerning the postfledging ecology and thus will not discuss this period (1). (The numbers in the text such as (1) refer to text found in the section entitled Potential Research Questions). All related research questions are listed by number at the end of the manuscript.
Pre-laying The pre-laying stage of reproduction is the time when nesting activities resume in the spring (e.g., courtship, talon-locking, nest building) to the time just before females lay their first egg. The pre-laying period cannot be defined as the time breeding pairs arrive on the territories, because the population of adult eagles in Southeast is thought to be non-migratory (Sidle et al. 1986). Some sub-adults will migrate south during the winter but many adults stay and, in fact, exhibit fidelity to their nest sites throughout the winter (Hodges et al. 1987, Kralovec 1994). This does not mean territorial adults will not stray from the territory. On the contrary, eagles are often on the move in the winter, traveling between food patches (e.g., to the winter chum run in the Chilkat River) for up to 10 d at a time (Kralovec 1994). Throughout the winter, pairs will return to the nest territory for brief periods to re-establish their claim. For eagles to be non-migratory, there must be sufficient food during the winter. Sporadic trapping efforts have provided some evidence that food levels in Southeast are sufficient during the winter. For example, in January 1995, Phil Schempf and I trapped an adult female that weighed nearly 7.4 kg, much heavier than normal mass estimates of eagles.
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In early spring, breeding activity resumes and eagles spend more time in their territories. I have seen activities such as talon-locking, nest building and copulation in early March, although the majority of the breeding population doesn't exhibit this behavior until late March or early April. Before egg laying, territoriality does not seem to be rigorously enforced. Adults are still floating between food patches and an intruding adult on a pair's nesting territory is often ignored. Nest-building or maintenance is a major activity for eagles before egg laying. Nests are built up by adding sticks or nest lining (moss or grass). Although nest maintenance is carried out throughout the summer, most of the re-building occurs in the spring. Pairs building a completely new nest often dedicate much of their springtime activities to carrying nesting material and working on the new nest. Rates of nest loss (due to windthrow) has been estimated to be about 6% annually (Hodges 1982). Thus, most of the nest-building activities observed in the spring are for maintenance or building up of old nests.
Bald Eagle nest with commanding view of Southeast Alaska waters. Photo by Scott Gende. The density of nest structures is very high along saltwater shores in Southeast, as territorial pairs often have more than one nest in their territory. I know of one nesting pair that has five nest structures in their territory. This is a far cry from some populations elsewhere in the country, where a lower density of eagles allow for a larger territory (and
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thus more nests) to be defended. In Wisconsin, for example, some nesting pairs have as many as 12 nest structures in their territory (K. Warnke, Minnesota Coop. Fish and Wildl. Research Unit, Univ. Minnesota, pers. comm.). In Southeast, it is very difficult to determine territorial borders and ascribe nest structures to particular pairs because of the high density of adults and nest structures and the convoluted shoreline. However, from long-term productivity surveys, nest use (but not fate) was found to be related to nest success the previous year, nests that failed were less likely to be used the following year (Gende et al. 1997) (2). Without marked birds, it is difficult to tell if nesting pairs switched nests or are nonbreeding in a given year. Using different nests from year to year seems to be the rule, although some nests are used consistently. I know of several nesting pairs that have used (and successfully fledged young) from the same nest for seven straight years (3). By mid-April, copulation is seen more often for pairs in Southeast. Copulation is thought to precede egg laying by 3-6d (Wiemeyer 1981). The eggs of Bald Eagles are small relative to the size of the adult, averaging approximately 130 g (Hensel and Troyer 1964). In fact, the ratio of egg weight to female body weight for Bald Eagles is one of the smallest of any bird (approximately 3% of female's body weight; Stalmaster 1987), suggesting it is relatively inexpensive energetically for eagles to produce eggs (4). By comparison, the Pine Siskin, a small passerine that nests in Southeast Alaska, lays eggs that weigh approximately 11% of the female's body weight. With an average clutch size of 4 eggs, female Pine Siskins would lay almost half of her body weight during 4 days of laying-quite an investment compared to the Bald Eagle. Not all adult eagles establish territories every year. Estimates of the frequency of nonbreeding adults in Southeast have been as high as 38% (Hansen and Hodges 1985), although other estimates are closer to 25% (P. Schempf pers. comm.). Of those adults that exhibit some form of territorial behavior, not all lay eggs every year. I have observed territorial pairs repair nests, copulate and even sit down into the nest bowl for short stints as if laying, yet fail to lay eggs (5). There is some evidence that habitat manipulation is one factor that may affect the density of pairs that lay eggs. The density of active nests decreased significantly as the proximity to clear cuts increased on Chichagof and Catherine islands in Chatham Strait (Gende et al. 1998).
Incubation Stage In the last week of April or the first week in May, most nesting females can be seen sitting down in nests in incubation posture, signifying that laying has occurred or is about to occur. Although the majority of nesting pairs lay during these two weeks, the range of laying dates include mid-April to early June. This contrasts with nesting populations further north (e.g., the Gulkana River basin in Southcentral Alaska) where the breeding season is more compact and the range of laying dates is shorter (Steidl and Anthony 1995). During the several days before laying, females will often spend some time sitting in and tending to the nest bowl. During a two-year study in Southeast, the average clutch size for eagles in Southeast was 1.94 eggs, with 81% of nests containing a clutch of two eggs, 13% containing a clutch of 1 egg and 6% containing a clutch of 3 eggs (Gende and Willson 1997) (6). These clutch size frequencies are similar to those in nesting
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populations elsewhere in the country (Stalmaster 1987). Eggs are usually laid several days apart, although inter-egg intervals can be up to a week (Wiemeyer 1981). Eagles begin incubating eggs immediately after laying. Although males and females share incubation duties (Cain 1985), I have observed females doing most of the incubation immediately after laying. Once females complete their clutch, the eggs must be incubated almost 24 hours a day. The embryos in eggs left unattended for more than a few minutes in Southeast will probably die of exposure, or possibly predation by corvids. Reflecting this constraint, eagles seldom leave an active nest during incubation. While one adult sits tediously on the nest, the other adult perches nearby. The non-incubating adult forages and, once a fish is caught, will fly back to a tree and eat it completely. On only two occasions have I observed an incubating adult come off the eggs to try to capture a fish. In both instances, the nest was eventually abandoned before the eggs hatched, suggesting that the incubating birds may have been food stressed. Duration of incubation bouts depends on the weather. Eggs are incubated for much longer periods during windy, rainy days, usually by the female (Cain 1985). When the incubating bird wants to switch duties with the foraging bird, it often calls to its mate and stands up on the rim of the nest before flying off. Or the foraging bird will fly onto the rim of the nest before the two switch incubation duties. For successful incubation, therefore, nesting pairs must have good timing and communication. Once chicks hatch, almost all prey caught by either adult is taken directly to the nest. During supplemental feeding experiments, adults (mostly males) would, on occasion, eat part of the fish before bringing it to the nest, although in most instances adults took fish directly to the nest. During one climb to a nest with newly hatched young, I found over 18 uneaten adult herring and several partially eaten walleye pollock. The nest eventually fledged both young. Second, the change in behavior of an incubating eagle is another good indication of when hatching occurs. There are few activities in nature more boring to observe than an incubating eagle. On average, eagles in Southeast will stand and turn the eggs every 75 minutes or so, longer if the weather is cold or wet. During or after hatching, however, eagles will stand and examine the contents of the nest much more frequently, about every half hour just after hatching. Nestlings develop for 10-12 weeks before fledging, which occurs, on average, in midAugust. From the time they hatch to just before fledging, eaglets will gain up to 6 kg (Bortolotti 1986). As eaglets get older, adults spend less time at the nest. Often, when chicks are 6 weeks old or older, adults will catch a fish and simply drop it in the nest, rather than stay in the nest and feed the chicks. Nesting birds will take only food small enough to carry immediately to a safe place to eat, avoiding any major confrontations with pirating eagles that would require the incubating adult to leave the nest (10).
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The average Southeast Alaska Bald Eagle clutch size is slightly less than two eggs. Photo by Scott Gende.
Nestling stage Hatching occurs after approximately 35 days of incubation. There are several behavior clues that indicate eggs have hatched. First, when an eagle is observed bringing prey to the nest, hatching has occurred or is about to occur. In captive breeding trials, eaglets have been heard vocalizing at least 12 hours before pipping (Wiemeyer 1981). With hatching occurring 24-48 hours after the onset of pipping (Wiemeyer 1981), adults probably have some clue as to when to start bringing food 36-60 hours before eaglets hatch. This caching behavior is thought to serve as a buffer for periods of food shortage once the eaglets hatch or as a means of reducing sibling competition (Gerrard and Bortolotti 1988). With more beaks to feed, the nestling stage of reproduction would appear to be the
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critical period in terms of food availability. However, this does not appear to be the case for eagles in Southeast Alaska. Until pink salmon become available in late June, eagles seem to be able to supplement their diet with a wide variety of fish. I have often observed 4 or 5 species of fish (including walleye pollock, starry flounders, herring, sand lance and rockfish) cached in the nest at one time in early June (11). Once pink salmon begin milling in near-shore water by late June or early July, they are utilized extensively by eagles (Imler and Kalmbach 1955, Ofelt 1975, Cain 1985). From fish remains in nests and from foraging observations, I think that eagles use pink salmon almost exclusively from the time they arrive in the shallows until after eaglets have fledged. Without pink salmon present, the nestling period would almost surely be more taxing in terms of food availability for adults and likely to be more limiting to nestling survival (12).
Bald Eagles cache food in their nest as seen in this photo. Also, note the size difference of these siblings. Photo by Scott Gende. Another possible reason eagles are not food limited during the nestling stage is that adults are less constrained to the nest once eggs hatch. Adults will spend less time at the nest as chicks develop (Cain 1985), thereby leaving more time to forage (Watson et al. 1991).
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They are also not required to be in the nest around the clock. With no natural predators of large eaglets, adults are free to travel for longer stints to food patches, leaving nestlings to fend for themselves.
Conclusions Although some work has been completed on certain aspects of the reproductive biology of eagles, more work is needed. Studying eagle biology not only lends insight into factors that control nesting success, but also figures prominently in the ecosystem as a whole. Eagles serve as a strong tie between the aquatic and terrestrial habitats and low productivity from eagles may serve as an indicator of problems in the aquatic and terrestrial environment. Understanding the viability of the population of eagles in Southeast hinges on continual understanding of the factors that influence each stage of reproduction. Furthermore, understanding factors that regulate reproduction is needed as conflicts continue to arise over development and extraction of resources utilized by humans and nesting eagles (Gende et al. 1998). Thus, prudent long-term management depends on an understanding of Bald Eagle nesting biology (Willson et al. 1997).
Potential Research Questions: (1) (The numbered Potential Research Questions relate to the numbers in the preceding text.) Information on the post-fledging ecology of eagles, e.g., movements, diet and general ecology is sorely needed for eagles in Southeast. One interesting aspect would be the relationship between spawning salmon and young-of-year (YOY) eagles. How much YOY eagles rely on spawning salmon and changes in the weight of YOY eagles with and without access to spawning salmon are obvious questions. In addition, documenting movements of eaglets once they fledge and following these birds for 4 or 5 years would provide information on plumage characteristics of known-age eaglets in a high density population. Sub-adult eagles in Southeast, who seem to be tolerated by territorial pairs much more than other adults, may delay adult plumage to enhance survival and food acquisition in a population that may already saturate the habitat (Hansen 1987). (2) It would be interesting to monitor whether nesting pairs attempt to breed each year and which nests are used. (3) A tremendous amount could be learned about the breeding dynamics of eagles in Southeast Alaska by marking a large number of individuals. Monitoring individually marked birds would aid in understanding the number of nest structures in each territory, the extent of nest fidelity between years and rates of turnover for nesting pairs (pair-bond duration). For the latter, I get the impression from watching behavior of pairs that have failed in consecutive nesting attempts that mating for life is not always the rule in Southeast, especially for pairs associated with nests that consistently fail. (4) Although eggs may be energetically inexpensive to produce, clutch formation may be limited by nutrients, such as calcium. Identification of the proximate factors that limit clutch size would aid in understanding which prey items are essential before laying. (5) An important research question is to identify factors that control the density of nesting
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pairs that lay eggs every year. There is much variation in the number of nests that are active every year vs. the number that are successful. How does the variation compare and what factors affect each? What determines whether or not eggs are laid? (6) Determining clutch size for many nests is difficult because nest climbs disturb incubating eagles. Climbs made during our research averaged only 25 minutes (from the time we approached the nest from a boat to the time we motored away). Nevertheless, more data are needed on the annual variation of clutch size, the factors that determine the number of eggs laid and fledging success in different circumstances (for example, nests that fledge 1 young on average have a much different interpretation if the clutch size was 2 vs. 1, a difference of 50% in fledging success). (7) It would be interesting to compare certain aspects of behavior of pairs that successfully breed vs. those that fail in a given year, including the frequency of switching during incubation, nest attentiveness (how long eggs are exposed), length of pair bond and foraging success of pairs during incubation. These clues may provide a clearer picture as to why most nests fail during incubation. (8) More research is needed on the physiological costs of incubation for eagles once the clutch is completed. How much does basal metabolic rate increase as a result of heating and reheating eggs? This, in conjunction with foraging success and weight changes during incubation (See #6), may provide insight as to the threshold at which eagles may abandon the current nesting attempt. (9) It would be interesting to see the daily weight fluctuation of eagles during incubation. Monitoring weight changes year round would also provide insight to physiological stress experienced by eagles during the different seasons. (10) Despite evidence that food is limiting during incubation, it is curious that nesting pairs will not take supplemental food during this period. A series of supplemental feeding experiments using different prey sizes could resolve whether or not prey size is a limiting factor. (11) The wide diversity of prey available to nesting eagles may be a reason that breeding dynamics, including the density of active nests and nesting success, are higher, on average, along saltwater shores vs. freshwater shores (e.g., the Chilkat River Valley). Studying the natural history of prey species taken by nesting eagles in both habitats would aid in understanding not only eagle biology but productivity of coastal vs freshwater habitats.
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Researcher Scott Gende, preparing to measure a captured adult Bald Eagle. Note that Scott keeps control of the sharp talons and is wary of the sharp hooked beak. Photo by Phil Schempf, USFWS. (12) Comparing diet, growth rates and post-fledging survivorship of nesting pairs that do and do not have access to pink salmon (e.g., up a river that has barrier falls, or nests associated with an early run of pinks) would aid in understanding the importance of pink salmon to nesting eagles.
Literature Cited Bortolotti, G. R. 1986. Evolution of growth rates in eagles: sibling competition vs energy considerations. Ecology 67:182-194 Cain, S. M. 1985. Nesting activity time budgets of Bald Eagles in Southeast Alaska. MS Thesis, University
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of Montana, Missoula, Montana. 46pp. Gende, S. M., M. F. Willson and M. Jacobson. 1997 Reproductive success of Bald Eagles (Haliaeetus leucocephalus) and its association with habitat/landscape features and weather in Southeast Alaska. Can. J. Zool. 75:1595-1604. Gende, S. M., M. F. Willson, B. H. Marston, M. Jacobson and W. P. Smith. 1998. Bald Eagle nesting density and success in relation to distance from clearcut logging in Southeast Alaska. Biol. Conserv. 83:121-126. Gende, S. M. and M. F. Willson. 1997. Supplemental feeding experiments of nesting Bald Eagles in southeastern Alaska. J. Field Ornithol. 68: 590-601. Gerrard, J. M. and G. R. Bortolotti. 1988. The Bald Eagle; haunts and habits of a wilderness monarch. Smithsonian Institution Press, Washington, D.C. Hansen, A. J. 1987. Regulation of Bald Eagle reproductive rates in Southeast Alaska. Ecology 68:13871392. Hansen, A. J. and J. I. Hodges. 1985. High rates of nonbreeding adult Bald Eagles in Southeastern Alaska. J. Wildl. Manage. 49:454-458. Hensel, R. J. and W. A. Troyer. 1964. Nesting studies of the Bald Eagle in Alaska. Condor 66:282-286. Hodges, J. I. 1982. Evaluation of the 100-meter protective zone for Bald Eagle nests in Southeast Alaska. U.S. Dept. Interior, Fish and Wildlife Service, Raptor Management Studies, unpublished report. Juneau, Alas. 11 pp. Hodges, J. I., E. L Boeker and A. J. Hansen. 1987. Movements of radio-tagged Bald Eagles, Haliaeetus leucocephalus, in and from Southeastern Alaska. Can. Field-Nat. 10:136-140. Imler, R. H. and E. R. Kalmbach. 1955. The Bald Eagle and its economic status. U.S. Dept. of the Interior, Fish and Wildlife Service, Circular No. 30. Kralovec, M. L. 1994. Bald Eagle movements in and from Glacier Bay National Park and Preserve. Final Report. Glacier Bay National Park, Gustavus, Alas. 67pp. McCollough, M. A. 1986. The post-fledging ecology and population dynamics of Bald Eagles in Maine. Ph.D. Dissertation. University of Maine, Orono, Maine. 46pp. Monaghan, P. and R. G. Nager. 1997. Why don't birds lay more eggs? TREE 12:270-274. Ofelt, C. H. 1975. Food habits of nesting Bald Eagles in Southeast Alaska. Condor 77:337-338. Sidle, W. B., L. H. Suring and J. I. Hodges. 1986. The Bald Eagle in Southeast Alaska. Wildlife and Fisheries Habitat Management Notes. U.S. Forest Service. R10-MB-9. 29pp. Stalmaster, M. V. 1987. The Bald Eagle. Universe Books, New York. Steidl, R. J. and R. G. Anthony. 1995. Recreation and Bald Eagle ecology on the Gulkana National Wild River, Alaska. Unpublished Final Report to the Bureau of Land Management, Alaska. 71 pp. Swenson, J. E., K. L. Alt and R. L. Eng. 1986. Ecology of Bald Eagles in the greater Yellowstone ecosystem. Wildl. Monogr. 95:146. Watson, J. W., M. G. Garrett and R. G. Anthony. 1991. Foraging ecology of Bald Eagles in the Columbia
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River estuary. J. Wildl. Manage. 55:492-499. Wiemeyer, S. N. 1981. Captive propagation of Bald Eagles at Patuxent Wildlife Research Center and introductions in the wild, 1976-1980. Rapt. Res. 15:68-82. Willson, M. F., S. M. Gende and B. H. Marston. 1997. Wildlife habitat models and land management plans: lessons from the Bald Eagle (Haliaeetus leucocephalus) in Tongass National Forest. Nat. Areas. J. 17:2629.
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