POSTFLEDGING FORSTER'S TERN MOVEMENTS, HABITAT SELECTION,AND COLONY ATTENDANCE IN SAN FRANCISCO BAY

摘要

Relatively little is known about birds during the postfledging period when flighted chicks have leftthe nest and must learn to forage independently. We examined postfledging movements, habitat selection, andcolony attendance of Forster's Terns (Sterna forsteri) radio-marked just before they fledged in San Francisco Bay,California. The proportion of the day spent at their natal colony declined as juveniles aged, from 65% at the timeof fledging to <5% within two weeks of fledging. Accordingly, the distance postfledging terns were located fromtheir colony increased as they aged, from <500 m within the first week of fledging to >5000 m by their fifth week.Time of day also influenced colony attendance, with older terns spending more time at the colony during nighttimehours (20:00 to 05:00) than during the day (06:00 to 19:00), when they were presumably foraging. Home rangesand core-use areas averaged 12.14 km2 and 2.23 km2, respectively. At each of four spatial scales of analysis, post-fledging terns selected salt pond habitats strongly. No other habitat types were selected at any scale, but terns con-sistently avoided tidal flats and uplands. Terns also avoided open bay habitats at the two largest spatial scales, tidalmarsh habitats at the two smallest scales, and sloughs and managed marshes at several scales. Within salt ponds,terns were located closer to salt-pond levees (58 m) than was expected (107 m). Our results indicate that tern chicksdisperse from their natal colony within a few weeks of fledging, with older chicks using their natal colony primar-ily for roosting during the night, and that postfledging terns are highly dependent on salt ponds.