The available prey of seabirds nesting on tropical oceanic islands can be considered to be distributed in 2 dimensions; their feeding area thus gives an estimate of the volume of available prey. This is greater in pelagic feeders and in migrants than in inshore-feeding and resident species. If prey abundance determines population size of a predator, pelagic feeders should be more abundant than inshore feeders and migrants more numerous than resident speices. Analysis of the seabird communities of 9 tropical oceanic islands confirms both these predictions. Migration is a more effective booster of population size than the pelagic habit, suggesting that limiting mortality occurs outside the breeding season. The pelagic habit has long been viewed as requiring numerous adaptations to cope with its inherent drawbacks; here, the evolutionary advantage of the larger population sizes is emphasized. [Diomedea nigripes, D. immutabilis, Phaethon rubricauda, P. lepturus, Sula dactylatra, S. leucogaster, S. sula, Fregata minor, F. ariel, Hydroprogne caspia, Thalasseus bergii, Sterna lunata, S. fuscata, S. anaethetus, S. sumatrana, Anous stolidus, A. tenuirostris, Procelsterna cerulea and Gygis alba were discussed.].