Body Size, Prey Size, and Ecological Segregation in Five Sympatric Tropical Terns (Aves: Laridae)

Abstract

Collection and analysis of food samples permitted comparison of the diets of five species of terns that occur together on Christmas Island, Pacific Ocean. All the terns feed mainly while flying, catching fish and squid chased to the surface by predatory fish. Sterna fuscata and Anous stolidus, which are of equal size, catch prey of similar kinds and sizes, but S. fuscata can exploit feeding areas further from land, and is more numerous. Gygis alba and especially Anous tenuirostris are smaller and eat smaller prey. Gygis eats many fish and squid of kinds not important to A. tenuirostris, and may catch them at different times. Procelsterna cerulea, the smallest species, eats mainly fish larvae and tiny inveltebrates. The three most closely related species, A. stolidus, A. tenuirostris, and Procelsterna, differ mainly in size. The ratios of their weights are 3.8: 2.0: 1, but the ratios of the mean volumes of the largest 2 per cent of their fish prey are 30: ca. 7 : 1. The reasons for the discrepancy are discussed. The value of data on bill length, bill thickness, and body size in assessing the extent of competition for food among bird species is considered. It is argued that the phenomenon of ecological segregation need not indicate the operation of competitive exclusion, and it is suggested that many natural habitats cannot be assumed to contain the greatest number of related species that could coexist there indefinitely if the opportunity to establish themselves arose.