Bald Eagles as Agents of Cormorant Control in Northern Minnesota

Double-crested cormorant populations have rebounded dramatically throughout the Great Lakes region over the last 40 years since DDT and other contaminants were banned.  In many areas, cormorants are actively managed to reduce conflicts with human uses.  Bald eagles have also experienced a meteoric return throughout much of their range in the Great Lakes range as contaminant levels declined.  Bald eagles are primarily known as fish-eaters or scavengers but they are also known to predate on cormorants and other colonial waterbirds.  Recent evidence from the Voyageurs National Park area suggests that bald eagles may be causing local declines in the cormorant population directly through predation and indirectly through disturbance to nesting adults.  Mortalities of four of 27 (15%) transmittered adult cormorants were attributed to bald eagles in 2008-2009, with at least three of the events happening away from cormorant colonies.  More than 40% of all bald eagle nests surveyed annually in June 2009-2011 contained feathers or bones from adult or nestling cormorants, with nests closer to cormorant breeding colonies more likely to contain cormorant remains. Digital game cameras documented repeated disturbance by visiting eagles at one colony in Kabetogama Lake in 2010 and 2011.  Eagles visited the colony 1-3 times/day throughout the breeding season, with most visits happening at dusk or from 0200-1000 hours.  Adult cormorants flushed from the nesting colony when bald eagles would approach or land, exposing eggs and small chicks to predation by co-nesting gulls during crepuscular and daylight hours. Disturbance by bald eagles resulted in adult cormorants being off the nest nearly 8% of time they were monitored, leading to eventual colony abandonment before any eggs hatched in both 2010 and 2011.  We documented repeated eagle disturbance at another colony in Rainy Lake from 2009-2011 that resulted in 100% mortality of pre-fledged chicks in 2009 and 2010.  However, failure of two local eagle nests in 2011 resulted in significant survival of chicks to 3 weeks at the time of this abstract, lending further evidence that direct and indirect effects of eagle predation are a major factor in cormorant population dynamics in the area.