COLONIAL DEFENSE BEHAVIOR IN DOUBLE-CRESTED AND PELAGIC CORMORANTS

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