Studies on terns with particular reference to feeding ecology

Abstract

Four species of sea terns in the genus Sterna were studied primarily to determine how they differed in strategies used to exploit available food, and to identify factors that influenced their success in obtaining it. Where the four species coexisted during the breeding season, interspecific differences were found in feeding dispersion, dive height, immersion time, use of 'contact-dipping' and size of prey taken. These variations should alleviate competition for food, and therefore facilitate coexistence. The food-robbing behaviour exhibited by Roseate Terns on Coquet Island was examined. The intensity and success of attacks were correlated with the density of "host" terns. Fish size affected the likelihood of attack, and of robbing success. Several variables were found to influence rates of fish capture at sea. Feeding rates contained both diurnal and tidal patterns, the latter corresponding with tidal variations in the rate of fish capture in the littoral zone. In Africa, first-winter Sandwich Terns were less successful than older birds at catching fish, suggesting that foraging skill is acquired gradually. An assessment of weather factors influencing fishing success, and growth rates of chicks indicated that windspeed and/or sea surface disturbance increase to levels that optimise fishing ability of inshore-feeding terns, after which their effects become detrimental. The inimical effects of paralytic shellfish poisoning on the breeding success of terns in 1968 are described. Analysis of the composition of tern eggs demonstrated significant differences between the terminal egg and the prior egg(s) of any laying sequence. The adaptedness of terns to a variable food supply is discussed with reference to other predator-prey systems. Hypotheses for the causation and function of deferred maturity in birds and other animals are assessed in relation to the slow acquisition of foraging skills by Sandwich Terns and some other seabirds.