Cost of Nest Defense in Azure-Winged Magpies

Abstract

I studied costs of nest defense in Azure-winged Magpies Cyanopica cyana in Tokyo, Japan in 1994 and 1995. I compared time devoted to nest defense, frequency of nestling feeding, and number of fledglings of magpies breeding near nests of Japanese Lesser Sparrowhawk Accipiter gularis and magpies breeding far from such nests. Because the hawks attack potential predators, magpies near a hawk's nest did not defend their nest, whereas magpies breeding far from a hawk's nest defended their nest. Feeding frequencies and number of fledglings were greater near a hawk's nest than far from it. Because nest defense incurs a cost in the form of decreased feeding time, Azure-winged Magpies far from a hawk's nest probably provided less food to their chicks and therefore raised fewer fledglings than those breeding close to a hawk.