INTRODUCTION
This study investigates the relationship between feeding behaviour of an avian predator, the Black-fronted Tern, Chlidoniae hybrid albostrialus (Gray 1845), and the abundance of its prey. Emphasis is placed on predator response to seasonal and daily variations in food supply. During spring and summer the terns are mainly insectivorous, feeding in swallow-like fashion on emerging aquatic insects on or above the surface of shingle-bed rivers and streams.
Black-fronted Terns were found appropriate for examining a relationship between predator feeding and prey abundance because foraging was often restricted to one habitat with prey which underwent large diurnal fluctuations in availability. The diet of a predator becomes specialized with respect to prey selection when food is abundant (Emlem, 1966, 1968). Therefore, if a preferred prey species is abundant, the interpretation of prey selection is simplified: the foraging strategy for a specialised diet becomes stereotyped (Gibb, 1958l Baker, 1974). Holling (1959) investigated predation on cocooned sawflies by small mammals and found that in this simplified case predation was affected by only two variables, densities of both predator and prey. He was able to describe the functional response (feeding rate) and numerical response (number) of predators to change in prey density. Responses of Black-fronted Terns to changes in aquatic insect abundance were described in the same way.
