FEEDING ECOLOGY OF FORSTER TERNS ON LAKE-OSAKIS, MINNESOTA

At a Minnesota Forster's Tern colony (Sterna forsteri), I investigated the role of seasonal fluctuations of major prey species and measured the feeding rates and types of prey delivered to the colony during 2 c ritical periods: courtship feeding of females before and after egg for mation and feeding of chicks (at various stages). Mean courtship feedi ng rates were 0.7 (1992) and 1.3 (1993) feedings/h, with rates general ly higher during the evening. There was a significant decline in court ship feeding rates after egg laying. Chicks less than 1-2 weeks old we re fed 2.3 fish/h, but this decreased to only 1.4 fish/h at 3 weeks. A s chicks aged size of the terns' prey increased significantly from 3.3 cm (week 1) to 5.2 cm (week 3). The most abundant species of fish sei ned at selected feeding areas was the spottail shiner (Notropis hudson ius) in 1992 and yellow perch (Perca flavescens) in 1993. The main spe cies of fish brought back to the colony were yellow perch, shiners (No tropis spp.), and sunfish (Lepomis spp.). Forster's Terns are primaril y piscivores; when available, yellow perch are easily captured and the refore more often consumed in freshwater areas.