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Direct and Indirect Effects of Peregrine Falcon Predation on Seabird Abundance
Paine, R.T., Wootton, J.T. & Boersma, P.D. (1990) The Auk 107(1):1-9
Peregrine Falcons (Falco peregrinus) have become conspicuous members of the bird assemblage at Tatoosh Island, Washington, since 1983. We have sufficient before and after observations and data to conclude that two species of storm-petrel (Oceanodroma spp.) show no significant population trends (as based on catch per hour in standardized mist nets from 1978-1988) and that numbers of both Cassin's (Ptychoramphus aleuticus) and Rhinoceros (Cerorhinca monocerata) auklets, the primary falcon prey, are probably declining. An important effect of falcons is their consumption of Northwestern Crows (Corvus caurinus) or simply restriction of crow activities. The numbers of Common Murres (Uria aalge) and Pelagic Cormorants (Phalacrocorax pelagicus) that inhabit Tatoosh have increased markedly, probably due to significantly decreased crow-related egg predation. The fledging rate of Black Oystercatchers (Haematopus bachmani) approximately doubled in 1987, when one falcon routinely ate crows. These relationships appear to be independent of the locally abundant Glaucous-winged Gulls (Larus glaucescens). We conclude that the indirect, positive effects are at least as important to seabird abundance patterns as are direct, negative effects. We suggest that more attention be paid to corvid activities near seabird colonies and, especially, that the impact on populations of small alcids or other prey be considered when planning conservation efforts to augment the population of an effective avian predator.

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