Role of visibility in nesting behavior of Larus gulls.

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Abstract
Larus gulls [L. pipixcan, L. ridibundus, L. maculipennis, L. atricilla, L. argentatus] were studied in England, Argentina and the USA to learn more about the relation among visibility, internest distance and aggression. Four of the species normally nest in marshes with vegetation that varies in density, structure and placement, whereas the 5th species nests in a variety of habitats. Photographs taken with a fish-eye lens were used to measure visibility from gull nests. For all species the nearest neighbors nested in the direction of least visibility, and internest distance directly related to visibility. The relation between the nearest neighbor and visibility varied for all species and was related to the structure of the vegetation. Herring gulls nesting in habitats similar to those of the marsh-nesting species had equivalent relations between internest distance and visibility. The amount of aggression increased when all vegetation was removed in experiments with 3 spp. Visibility affects internest distance and aggression levels, and indirectly affects the size and shape of gull territories.