Bald eagle populations in the Pacific Northwest have quadrupled and stabilized since the early 1980s. There are now estimated to be more than 50,000 eagles on British Columbia’s coast. Researchers explored current factors regulating eagle populations including reproductive success, breeding and winter populations, mortality and salmon stream use.

Heavily affected in past years by contaminants and persecution, eagle populations now appear to be partially limited by density on their breeding grounds and partially by adult mortality in late winter. Nest success has declined and seems to be slowing population growth of both breeding and wintering birds. Late-winter mortality is likely due to reduced salmon stocks forcing eagles to exploit marginal prey such as other birds. Use of such prey may lead to both increased winter mortality and females in poor body condition at the onset of breeding.

Wintering eagles in British Columbia feed heavily on chum salmon and switch to birds in late winter. Eagles tend to arrive after the peak salmon run, as part of a migration associated with salmon streams from Alaska to northern Washington. Eagles were most abundant in southern British Columbia during cold Alaskan winters and in years of high chum salmon availability.

The levelling of bald eagle population growth may attenuate some of the concerns that large numbers of these predators are impacting reproduction of marine bird species, such murres, cormorants, gulls and herons. Eagles have been reported to significantly affect the breeding success of such species which breed in large colonies by preying on young and causing disturbance and thus allowing other nest predators, crows, ravens and gull species, to take eggs or young.

This study, which helps in understanding changing factors regulating eagle populations, exemplifies the value of long-term monitoring programs and how data collected by “citizen scientists”--in this case, through the Christmas Bird Count--can support the research of professional biologists.

Source:

Elliott, K. H., Elliott, J. E., Wilson, L. K., Jones, I. and Stenerson, K. 2011. Density-dependence in the survival and reproduction of Bald Eagles: Linkages to Chum Salmon. The Journal of Wildlife Management, 75: 1688–1699. doi: 10.1002/jwmg.233

This paper was presented recently at the Salish Sea Conference 2011 (Salish Sea region formerly known as Georgia Basin/Puget Sound) in Vancouver, British Columbia.

• Read more: B.C.'s south coast hosts world's greatest concentration of wintering bald eagles (Vancouver Sun, October 2011)

Contact: Dr. John Elliott, 604-940-4680, Ecotoxicology and Wildlife Health Division