Use of Artificial Eggs Reduces Double-Crested Cormorant (Phalacrocorax auritus) Fledging

Abstract

Double-crested Cormorant (Phalacrocorax auritus) populations have rapidly grown across the United States. In some locations cormorant populations have become a management concern due to real or perceived effects on local fish populations, altering island vegetation, and displacing other colonial waterbirds. In north-central Minnesota on Leech Lake cormorant populations have been reduced and managed by various removal and population reduction methods, which include fall nest material removal, lethal shooting, and limited harassment. These methods can be time intensive, costly, and can cause cormorants to disperse and renest. This study examines the use of artificial eggs as a new management method to reduce cormorant populations by limiting their fledging success. A portion of viable cormorant eggs from 68 nests were replaced and the nests monitored, and 32 nests were monitored as controls on an island nesting colony. Results demonstrate that cormorant fledging success is reduced by using artificial eggs. The average number of near fledge young per nest that received artificial eggs was 0.7 while control nests that did not receive artificial eggs had an average of 2.5 young per nest. Use of artificial eggs reduces cormorant fledging success and minimizes the chance of adults renesting elsewhere.