Tethered Raptor Displaces Roosting Cormorants

Double-crested Cormorant (Phalacrocmax auritus) numbers in the Great Lakes of North America have increased dramatically during the past three decades. Current concerns include negative effects of cormorants on aquatic food webs, destruction of nesting habitat and competition with other avian species and odors due to feces. Control of cormorant nesting and roosting has been variably successful and sometimes involves lethal force and/or loud devices. These approaches may be unsuitable in some settings, such as parks or residential areas. A large group of cormorants (up to 2,000 individuals) has used Farr Island, a small island near houses on the North Shore of Hamilton Harbour at the west end of Lake Ontario, Canada, as a post-breeding roosting area. The concentration of birds has resulted in complaints regarding odors from the island. In an attempt to alleviate the strong odor and to examine options to limit cormorant nest site competition with other species, a tethered Steppe Eagle (Aquila nipalensis) was placed on the roost island for twelve days from 9 to 21 September 2003. The eagle was effective at causing roosting cormorants to avoid the island, apparently causing many to move their roost site a short distance to another island nearby. Complete cormorant displacement lasted at least four days beyond the period of eagle presence on the island. Using a tethered large raptor appears to be an effective non-lethal tool for local displacement of roosting Double-crested Cormorants.