Discrimination of the threat of direct versus tangential approach to the nest by incubating herring and great black-backed gulls.

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Abstract
These experiments examined the ability of incubating herring (L. argentatus) and great black-backed (L. marinus) gulls to discriminate between people walking directly toward their nests and those merely walking tangentially by their nests. Study groups varied in their habitat (open vs. vegetated) and in their previous exposure to human disturbance. Herring gulls responded when the experimenter was at a greater distance from the nest if the approach was directly toward the nest with the person looking at the incubating bird, and gulls in disturbed areas responded sooner than birds in undisturbed areas. In habitats with low visibility (dense bush cover), the gulls could not see the experimenter early enough to show a differential response to the 2 treatments. Black-backed gulls nested only in undisturbed areas and only slight differences were evident in the distance at which they left the nest. Black-backed gulls called more when the experimenter was approaching the nest directly. Gulls habituated to the continual presence of humans by modifying their responses but even habituated birds continued to reassess the potential danger of a nearby human and perceived subtle differences between a direct and a tangenital approach.