Foraging and Breeding Adaptations to Different Food Regimes in Three Seabirds: The Common Tern, Sterna Hirundo, Royal Tern, Sterna Maxima, and Black Skimmer, Rynchops Niger

Abstract

Aspects of foraging and breeding ecology of the Royal Tern, Sterna[=Thalasseus]maxima, Common Tern, Sterna hirundo, and Black Skimmer, Rynchops niger, were compared in light of the food resource. Field studies conducted in 1973 and 1974 in the USA on two of Virginia's barrier islands revealed that the three species differed markedly in foraging ranges and habitat use, social feeding tendencies, and colony sizes and distribution. The investigation was designed to examine how these behavioral and ecological differences are related to the different regimes of food patterns. Analysis of beach seine records from inshore (< 50 m) waters were contrasted with haul seine and trawl collections from offshore (> 3 km) waters from both the Eastern Shore of Virginia and the Delaware Bay. Spatial variability (between sampling site variation) was found to be much greater offshore than inshore, indicating greater ‘patchiness’ of fish. Also, the availability of surfacing fish appeared to fluctuate markedly over time. In response to these food regime differences, Royal Terns, which feed further from the colony than common Terns and Black Skimmers, (1) readily formed feeding aggregations, (2) showed large daily variability in feeding the young, and (3) were the most ‘colonial’ in nest—spacing, colonial size and distribution. The strictly inshore—feeding skimmers usually fed alone, show little variability in feeding, and formed the ‘loosest’ colonies of the three species. Common Terns were intermediate in all respects. The differences in the ‘colonial tendencies’ among the species are consistent with the predictions of two earlier models relating breeding assemblages and food distribution.