American Avocets (Recurvirostra americana) and Black-necked Stilts (Himantopus mexicanus) are large ground-nesting shorebirds whose eggs and flightless chicks are exposed to a variety of predators during a period of approximately two months. Parents respond to potential predators by ignoring them, by performing a variety of diversionary displays, or by aggressive mobbing. I observed the responses of avocets and stilts to natural predators and to predator models placed near their nests. Responses to mammals were relatively weak, involving mostly aerial circling and terrestrial distraction displays, and mammals rarely were mobbed. Responses to birds were more aerial, with ground displays rarely used, and birds often were mobbed. The response to humans was intermediate between the responses to other mammalian predators and to avian predators, and involved both distraction displays and mobbing. The number of avocets and stilts participating in mobbing eight different avian predators correlated with a subjective ranking of the danger the predators posed to mobbing adults. Mobbing of gulls, raptors, and herons was consistent with the hypothesis that avocet and stilt mobbing behavior is influenced by the relative threat of particular predators to eggs versus chicks. Overall, the observations reported here provide evidence that parent avocets and stilts discriminate among predators on the basis of both the risk to the adults of mobbing the predator and the risk to the eggs or chicks of not doing so.
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Vol. 75 • No. 4