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Factors affecting individual variation in nest-defense intensity in colonially breeding Black-tailed Gulls (Larus crassirostris)

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The physiological state of parent birds combined with the value of their clutch may affect the intensity of their nest defense. In colonially breeding birds, nest-defense intensity may also be affected by the behavior of neighbors. We investigated individual variation in the nest-defense intensity among colonial Black-tailed Gulls (Larus crassirostris Vieillot, 1818) over 2¬†years. Only 30%–40% of males attacked a decoy of an egg predator (Large-billed Crow ( Corvus macrorhynchos Wagler, 1827)), and the other males and females rarely attacked. Males attacking the decoy had higher levels of plasma testosterone than males that did not attack. Each male’s, but not female’s, nest-defense intensity was consistent throughout the incubation period and also across years. The intensity was not related to egg-laying date, clutch size, or age of offspring. The intensity was likely to be higher when individuals had one or more neighbors, representing higher nest-defense intensity in the year where gulls had larger number of adjacent neighboring nests (5.23 nests), but this trend was not observed in the year where they had smaller number of the neighboring nests (3.73 nests). Thus, in addition to testosterone levels, behavior of neighbors also influences the nest-defense intensity.

Document Type: Research Article


Affiliations: 1: Laboratory of Marine Ecology, Graduate school of Fisheries Sciences, Hokkaido University, 3-1-1 Minato-cho, Hakodate, Hokkaido, 041-8611, Japan. 2: Laboratory of Environmental Zoology, Faculty of Agricalture, Meijo University, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya, Aichi, 468-8502, Japan. 3: Laboratory of Physiology, Graduate School of Veterinary Medicine, Hokkaido University, N18 W9 Kita-ku, Sapporo, Hokkaido, 060-0818, Japan.

Publication date: October 27, 2011

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