Biology and Growth of the Black-naped Tern Sterna sumatrana: An Hypothesis to Explain the Relative Growth Rates of Inshore, Offshore and Pelagic Feeders

Abstract

The Black-naped Tern breeds on tropical and sub-tropical islands throughout the Indian and Western Pacific Oceans. In Australian waters they are non-sedentary. They usually breed during spring and summer. The mean clutch size is about 1.5 and it is incubated for 21 to 23 days. Most Black-naped Terns hunt within two kilometres of their colonies. Young are fed often; the modal interval between feedings was four to five minutes and the mean was 49.6 minutes. They ate fish from at least 15 families and up to 10 cm long. The mean size of prey fed to chicks increased during the course of the breeding season. Nestlings were fed a greater number of small fish than were chicks (> 7 days old) and mates. The size of fish fed to chicks and mates did not differ significantly but the ones eaten by foraging adults differed significantly from those that they fed to their mates. Breeding success of Black-naped Terns was higher at Eagle Island than at One Tree Island. The major causes of mortality of eggs and chicks were predation by gulls and flooding of nesting areas. Growth rates of Black- naped chicks (inshore feeder) were faster than those of offshore and pelagic feeders. We hypothesise that the ability of parents to transport food to their young (Ricklef's power requirements model) in combination with the nutrient composition of their diets explain the non-genetic differences in the growth rates of inshore, offshore and pelagic feeders. We postulate that inshore feeders provision their young with more of the limiting nutrient per unit time by supplying their young with more food than is reauired to meet their enerev needs. Ricklefs model also accounts for the changes in.the use of foraging zones during the cburse of the breeding season.