Seasonal Timing of Bald Eagle Attendance and Influence on Activity Budgets of Glaucous-winged Gulls in Barkley Sound, British Columbia

— As raptor populations recover following the banning of organochlorine pesticide use, there may be consequences for prey populations. While Bald Eagles ( Haliaeetus leucocephalus ) actively prey on Glaucous-winged Gulls and their offspring ( Larus glaucescens ), their presence at colonies and roost sites may also influence reproductive success of gulls by impacting activity budgets. Here we investigate changes in Bald Eagle abundance in relation to gull breeding phenology over 6 years at Seabird Rocks (Vancouver Island, Canada) and compare activity budgets of gulls in relation to Bald Eagle abundance. Bald Eagle abundance varied seasonally, peaking during late incubation and hatching of gull eggs. As Bald Eagle presence increased, gulls showed a strong increase in time allocated to vigilance (54%) and frequency of flushing (up to 6 times/hr). These results indicate that Bald Eagle attendance patterns at gull colonies coincide with hatching of gull chicks and can influence time-activity budgets of gulls. Predator-induced changes in gull behavior during the breeding season may influence reproductive success by impacting time and energy budgets and facilitating egg and chick predation. These findings could be important for understanding failure at gull colonies in Barkley Sound and evaluating impacts of increasing Bald Eagle populations in the region. Received 21 December 2005, accepted 23 August 2006 .