INTRASEASONAL MOVEMENT BETWEEN COLONY SITES BY CASPIAN TERNS IN THE GREAT LAKES

Lax-ids nesting in stable habitats tend to occupy the same breeding colony for many years, whereas those that occupy unstable environments shift colony sites frequently if nesting habitat becomes unsuitable (McNicholl 1975, Burger and Shisler 1980). The tendency of larids to return to the colony of previous breeding, providing environmental conditions remain favorable, is referred to as “site tenacity” (Austin 1949). Past studies on intercolony movement in the Great Lakes (Ludwig 1974, Morris and Hunter 1976, Southern 1977, Haymes and Blokpoel 1978, Southern and Southern 1979, Blokpoel and Courtney 1980) have focused on tenacity between seasons, but little is known about the movement of birds between colonies within a single breeding season. I examined seasonal colony site use patterns in Caspian Terns (Sterna cuspia) nesting on five islands in northeastern Lake Michigan to answer the following questions: (1) do individual terns use more than one colony site during a single breeding season; and, if so, (2) what are the factors that influence intraseasonal colony site movement in this population?