The Role of Bald Eagles in Declines of Waterbirds in Puget Sound/Straight of Georgia

Robert Butler
Pacific Wildlife Foundation
$25,100

Reports of declines in the abundance of seabirds, waterfowl and shorebirds in the Puget Sound/Strait of Georgia coincide with an increase in numbers of bald eagles (Haliaeetus leucocephalus) in the region. The objective of this proposed research is to examine the relationship between these declines and changes in eagle abundance. Eagles, like other predators, can affect the abundance of their prey in two ways: they can kill their prey or their presence can cause prey to move away from the danger posed by eagles. Both factors can result in a decline in abundance in a region. The predation hypothesis predicts a real decline in abundance as an outcome of predation whereas the danger hypothesis results in an apparent decline as birds move to new areas for safety.  

Eagles depart the south coast from late summer until fall for salmon spawning rivers in Alaska and northern British Columbia. We propose that this temporal change in predation danger brought about by seasonal movements of eagles has affected when and where seaducks and seabirds choose to settle on the coast. We also contend that the timing of the danger from eagles returning to the coast is dictated by the strength of salmon spawning runs. Hence, there is a three way interaction between salmon, eagles and their prey. This information is important to resource agencies that need to know if declines of bird populations in the region are natural adjustments or if they have an anthropogenic root cause.

We will test the predation hypothesis using standard allometric equations for eagles, by calculating the number of birds the eagle population would remove to sustain them each year and compare it to the rate of decline of nesting seabirds and seaducks. We will test the danger hypothesis by comparing the seasonal distribution of eagles and their prey both within historical data and in the field; and by examining danger avoidance behaviour by waterbirds in the field through both direct observations and with a series of field experiments with an eagle model. Our data base includes 7 years of regular monthly surveys of eagles, seabirds and seaducks compiled by Bird Studies Canada on 200 beaches in British Columbia, radio and satellite data on the seasonal movements of seaducks in the Strait of Georgia and the Pacific Coast, data on timing of molt by seaducks, and seabird colony survey data in the Strait of Georgia.