SEASONAL AND ANNUAL VARIATION IN BROOD ATTENDANCE, PREY TYPE DELIVERED TO CHICKS, AND FORAGING PATTERNS OF MALE COMMON TERNS (STERNA-HIRUNDO)

The study took place over 2 years (1990 and 1991) on a concrete breakwater located 1 km offshore on Lake Erie near Port Colborne, Ontario. Ten male Common Tems (Sterna hirundo) in 1990 and 12 in 1991 were radio-tracked by boat or car during the chick-rearing stage. Concurrent behavioural observations of the radio-tagged birds and 23 additional control birds documented the time each sex spent away from or at the nest. The frequency and prey species/size composition of feeds to chicks were recorded. Individuals that carried transmitters had predictable foraging patterns. In peak- (clutch initiation in early May) and late-nesting males (clutch initiation in late June) intra-individual variability was low. Late-nesting males exhibited greater interindividual variability than peak-nesting males. Adults recaptured in 1991 exhibited similar foraging patterns to those they expressed in 1990. There were diurnal, seasonal, and interannual variations in prey delivered to chicks. We suggest that these were due to the temperature tolerances of the prey, rainbow smelt (Osmerus mordax) and emerald shiner (Notropis atherinoides), that controlled their vertical distribution in the water column, and to the presence of large schools of larval fish during the late nesting season.