Management‐Induced Reproductive Failure and Breeding Dispersal in Double‐Crested Cormorants on Lake Champlain

Abstract

We studied breeding dispersal of double‐crested cormorants (Phalacrocorax auritus) associated with management practices that suppressed their reproduction on Lake Champlain in the northeastern United States. We implemented an experiment on one colony by spraying corn oil on cormorant eggs in portions of the colony and leaving other portions untreated. Gulls (Larus spp.) consumed cormorant eggs during the oiling process, but we reduced and then eliminated predation levels after the first year of the study. We used mark‐recapture techniques within the experimental framework to measure rates of breeding dispersal for cormorants from the experimental colony and an unmanaged colony in Lake Champlain. Egg oiling increased the movement rate to the unmanaged colony by 3% during the year with no egg predation by gulls. When gulls depredated cormorant eggs at high rates during egg oiling, movement to the unmanaged colony increased by 20%. When cormorants are managed to reduce population sizes, methods that limit dispersal away from the managed colony may be most effective. Such methods would mitigate effects to nontarget populations and allow for a greater portion of the metapopulation to be managed.

Energetic considerations for managing double-crested cormorants on Lake Champlain

“…Energetic requirements of cormorants were greater on Four Brothers Islands than on Young Island. The unintended consequence of managing cormorant productivity on Young Island was dispersal of cormorants (Duerr et al, 2007) to a site where energy demands were greater and, thus, total fish consumption by cormorants on Lake Champlain increased. An expected consequence of oiling cormorant eggs is a reduction in activity levels and energy demands of adults whose eggs were oiled because they do not have to expend energy to provision nestlings.…”

“…The second colony was Four Brothers Islands, New York, owned by The Nature Conservancy and managed by its Adirondack Chapter. The two study sites are 35 km apart; further descriptions are given in Duerr et al (2007).…”

“…Throughout this time, Young Island was managed experimentally such that about half of nests were treated with corn oil to prevent hatching (Duerr et al, 2007). Cormorants on Four Brothers Islands were not managed during the period of our study.…”

“…They began oiling eggs to curb population growth and limit associated destruction of vegetation and displacement of other colonial species on this island (Garland et al, 1998). Management activities on Young Island led to increased breeding dispersal from Young Island south 35 km to Four Brothers Islands, where cormorant numbers were not controlled (Duerr et al, 2007). Diets of cormorants from these colonies differ (DeBruyne et al, 2012).…”

Double-crested cormorants and urban wilderness: conflicts and management

“…While wildlife management can be considered a science, the efficacy of cormorant management is far from certain, and is criticized by scientists and advocacy groups alike (CDI 2008a;Cuthbert et al 2002;Korfanty et al 1997). Because double-crested cormorants are so widespread, management can result in birds abandoning one colony for another (Duerr et al 2007) or alternatively, an actively reduced colony may repopulate over time with cormorants from other colonies. According to Korfanty et al (1997: 140), "[a]uthorized techniques (e.g., shooting adults, hunting seasons, harassing with flare guns, and oiling eggs) have not been successful in controlling [cormorants] over larger geographic areas or in the longer term."…”