The Hunting Behavior and Success of Forster's Tern

Abstract

An observational field study was made on predation by Forster's Tern. The major question asked was whether a seasonal change in hunting success occurred. Models proposed by Watt and by Holling and Griffiths predict a decline in the number of captures by individual predators as their density increases. Laboratory studies on protozoa have shown such declines in rate of predation, but in these organisms the decline is offset by an increase in efficiency. In the terns, the effectiveness of their attacks as measured by the percentage of captures increased seasonally from spring to winter. However, the overall hunting success in weight units of fish per unit hunting time declined steadily over the same period. The causes of this were possibly changes in their prey population and changes in the density and composition of the tern population. The increased attack efficiency was insufficient to counteract an overall decline in hunting success. A secondary objective of the study was to determine the relationship of handling time to size of fish. It was found that handling time increased proportionally as the length of the fish squared and was a satisfactory index of prey size.