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Bald Eagles of Alaska - Section: 13 Page: 97 - Perspectives on the Breeding Biology of Bald Eagles in Southeast Alaska

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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 10 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 /0 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 10 (Hansen and Hodges 1985), although other estimates are closer to 25 10 (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 /0 of nests containing a clutch of two
eggs, 13 10 containing a clutch of 1 egg and 6 10 containing a clutch of 3 eggs (Gende and
Willson 1997) (6). These clutch size frequencies are similar to those in nesting


Tag: anthony chatham copulate copulation eggs fledged hansen hensel hodges nests siskin siskins stalmaster steidl wiemeyer willson wisconsin



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