COLONIAL DEFENSE BEHAVIOR IN DOUBLE-CRESTED AND PELAGIC CORMORANTS

摘要: COLONIAL DEFENSE BER A VIOR IN DOUBLE-CRESTED AND PELAGIC CORMORANTS DOUGLAS SIEGEL-CAUSEY 1 GEORGE L. H UNT, JR. Department of Ecology and Evolutionary Biology, University California, Irvine, California 92717 USA ABsTRACT.- We eicamine the predictions, based upon hypotheses Coulson (1968), Ham- ilton (1971), Vine that (I) spedes whose colonies are accessible to predators should form tighter groupings have fewer isolated nests than those with reduced accessibility predators, (2) at center be less subject predation edge a colony, (3) both in lowest pressure all nests, (4) individuals nest locations more vigorous sustained antipredator behaviors unlikely come frequently contact predators. To test these we compare intrusions by two Northwestern Crow (Corvus cauri11us) Glaucous-winged Gull (Larus glaucescens), grouped cliff-face nesting Pelagic Cormorant (Phalacrocorax pelagicus) cliff-top Double-crested (P. auritus) on Mandarte Island, British Colum- bia. Both crows gulls preferred visit species, especially level ground. The steeper central location, likely was visitation. Gulls were restricted topography from entering while able land either colony. Predator success high only flat, areas. cormorant species depend habitat deter predator access: utilizes defense regime energetic aggressive behaviors; uses much effective depends as necessary part defense. Received 11 July 1980, accepted 21Ja11uary1981. WE here examine aspects colonial relation their effectiveness protection against intruding Nesting phenology (Nelson 1966, 1970), lo- cation within colony (Coulson 1968), 1967, Nettleship 1972) been identified actual or potential factors deter- mining likelihood particular will predation. amilton (1971) hypothesized clumping prey provides significant predation, (1968) group lesser riisk edge. concerning importance location avoidance, interactions between prey, Double- crested (DCC: Phalacrocorax (PC: P. pelagicus), (Con.ms caur- inus) col- onies Columbia, Canada. In this study, had opportu nity activity effect highly maneuverable (the Crow) relatively clumsy Gull) they attempted steal eggs small chicks nests. Because prefer different habitats consequently structures for colonies, cormorants cliffs (Pelagic Cormorant) areas (Double-crested tightly clumped well loosely ' Present address: Arizona, Tucson, Arizona 85721 USA. Auk 98: 522-531. 1981