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Ornithological Science
Abstract
Abstract
Since parent birds are hypothesized to adjust the level of their nest defense against predators so as to enhance the survival of their offspring, parental nest defense is expected to increase in intensity in relation to increasing clutch size. Empirical studies of bird species, in which clutch sizes have been manipulated artificially, however, have produced results contradicting such expectations, partly, it is thought, because of various errors in experimental design, such as a lack of nesting habitat control, or providing too short a time for parents to assess the value of the manipulated clutch, Hence, further evidence needed to be gathered to clarify whether the basic hypothesis is adequately supported. We manipulated the clutch size of Blacktailed Gulls Larus crassirostris and observed the responses of male parents to a crow decoy in controlled nesting habitats. The intensity of defense was not affected by the clutch size. Opportunities for future reproduction, or constant individual levels of aggressiveness of seem to best explain the observed intensity of nest defense in these long-lived Black-tailed Gulls, rather than the value of the current clutch.
We thank Kazuki Kosugi, Masahiko Sato, Rie Sato, Nobuaki Takahashi, Junko Kaji, Takazo Kawabata, Toru Nishijima, Kanako Nishijima, Hideo Takahara, Takayuki Fukuda, Takashi Hirotsu and Kanako Toge for their support and assistance in the field. Many thanks are due to Motohiro Ito, Yusaku Arikawa, Yumi Kobayashi, Keiko Kato and Fumi Hirose for their assistance in making dummy eggs. We appreciate the improvements in English usage made by Phil Whitford and Daniel M. Brooks through the Association of Field Ornithologists' program of editorial assistance. Hokkaido Souya sub-prefectural office gave us permission (#36-1-5) to work on the Black-tailed Gulls and to cut back the vegetation in the study area on Rishiri Island.
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