Density‐dependence in the survival and reproduction of Bald Eagles: Linkages to Chum Salmon

Abstract

During the late 20th Century, due to decreases in both contamination and persecution, bald eagle (Haliaeetus leucocephalus) populations increased dramatically. Currently, mechanisms regulating eagle populations are not well understood. To examine potential regulating processes in the Pacific Northwest, where eagles are no longer primarily regulated by contaminants or direct persecution, we examined bald eagle reproductive success, breeding populations, winter populations, mortality, and salmon stream use. Wintering and breeding eagle populations in south‐coastal British Columbia (BC) quadrupled between the early 1980s and the late 1990s, and have since stabilized. Density‐dependent declines in reproduction occurred during 1986–2009, but not through changes in site quality. Mid‐winter survival was crucial as most mortality occurred then, and models showed that density‐dependent reductions in population growth rates were partially due to reduced survival. Wintering eagles in British Columbia fed heavily on chum salmon (Oncorhynchus keta) runs, and then switched to birds in late winter, when mortality was highest. Eagles tended to arrive after the peak in salmon availability at streams in BC as part of a migration associated with salmon streams from Alaska to northern Washington. Eagles were most abundant in southern BC during cold Alaskan winters and in years of high chum salmon availability. We suggest that eagle populations in the Pacific Northwest are currently partially limited by density on the breeding grounds and partially by adult mortality in late winter, likely due to reduced late winter salmon stocks forcing eagles to exploit more marginal prey supplies. Larger eagle populations have affected some local prey populations. © 2011 The Wildlife Society.

Nest use dynamics of an undisturbed population of bald eagles

“…Our observed estimate of chick productivity <1 per used nest, and stable occupancy dynamics are in line with populations elsewhere in Alaska (e.g., Steidl, Kozie, & Anthony, 1997; Zwiefelhofer, 2007) where density dependence may play a larger role (Elliott, Elliott, Wilson, Jones, & Stenerson, 2011). Our results strongly suggest that some mandated productivity guidelines may be unrealistic for evaluating successful recovery (Cruz et al., 2018), especially when there is consistent evidence that fecundity is density dependent in eagles (Elliott et al., 2011; Ferrer & Donazar, 1996; Mougeot et al., 2013). …”

Bald eagle (Haliaeetus leucocephalus) population increases in Placentia Bay, Newfoundland: evidence for habitat saturation?

“…Across their range there are similarities and differences in their populations and habitat use. For example, the increasing and high proportion of nonbreeding adults has been observed in Alaska (Hansen andHodges 1985, Zwiefelhofer 2007) and the Pacific Northwest (Elliott et al 2011), in boreal and temperate coastal areas (Placentia and Chesapeake Bay; Jenkins and Sherrod 2005), in central North America (Mougeot et al 2013), and in desert environments (Arizona; Driscoll et al 1999).…”

Landfill Use by Bald Eagles in the Chesapeake Bay Region

“…Juvenile and subadult birds tend to be less successful at foraging and hunting than adults (Stalmaster and Gessaman 1984, Brown 1993, Bennetts and McClelland 1997 and may use different foraging strategies (Stalmaster 1976, Knight andKnight 1983). Immature birds learn foraging skills, including locating food sources (Stalmaster 1976, Elliott et al 2006, and food-stealing (Jorde and Lingle 1988) by watching or following older birds and through experience. The year after hatching is a particularly difficult period for eagles, with post-fledging first-year survival rates averaging 83%, compared to 90% in subsequent years (Hodges et al 1987, Buehler et al 1991, Wood 1992, McClelland et al 1996, Harmata et al 1999, Jenkins et al 1999.…”

“…However, landfills are associated with low-quality food (Belant et al 1993, Smith and Carlile 1993, Annett and Pierotti 1999, increased risk of disease (Monaghan et al 1985, Ortiz and Smith 1994, Nelson et al 2008) and toxicity (Millsap et al 2004), and increased threat of food piracy and injury (Knight andKnight 1986, Elliott et al 2006). Bald Eagles are known to congregate at landfills (Elliott et al 2006). The accessible food in landfills may be especially attractive to immature eagles, whose foraging skills are still developing.…”

“…Landfill sites (n 5 72) were plotted using Google Earth software. Eagles using landfills as foraging sites spend the majority of their time resting (Elliott et al 2006) and frequently move between perches in the surrounding area outside of the landfill and the landfill itself (e.g., Elliott et al 2006). Thus, a 2-km buffer zone was drawn around the border of each landfill, and eagles within this buffer were considered to be present at the landfill.…”