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Colonial Defense Behavior in Double-Crested and Pelagic Cormorants
Douglas Siegel-Causey and George L. Hunt, Jr.
Vol. 98, No. 3 (Jul., 1981), pp. 522-531
Published by: American Ornithologists' Union
Stable URL: http://www.jstor.org/stable/4086119
Page Count: 10
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We examine the predictions, based upon the hypotheses of Coulson (1968), Hamilton (1971), and Vine (1971), that (1) species 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 caurinus) 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 Columbia. 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.
The Auk © 1981 American Ornithologists' Union