Feeding Behavior of Several Aerial Foragers after an Extended Rainy Period

Abstract

Introduction It is generally known that climate, especially temperature, plays an important role in the activities of birds (Kendeigh, 1934; Koskimies, 1950; Lack, 1956). Numerous instances of mass mortality of birds during unfavorable weather have been documented (Forbush, 1927, 1929; Kendeigh, 1934; etc.). If not extreme, unfavorable climatic conditions may influence breeding activities of birds by delaying the beginning of breeding, extending the interval between laying of successive eggs, or prolonging the incubation or nestling period. Although an unfavorable climate may lengthen the nestling period of many songbirds by only a few days, Lack and Lack (1951) have shown that the nestling period of European Swifts (Ap apus) may vary by up to three weeks depending on climatic conditions. Birds specialized for feeding in the air are placed in a more unfavorable position than most songbirds during extended cool and wet periods because of the restricted availability of insect prey and the low foraging flexibility of most aerial foragers (Robins, 1970). Unfavorable weather of varying length is encountered by aerial foragers virtually every breeding season but little is known of the behavior of the adults during these periods. Anderson's (1965) description of a mass mortality of swallows and flycatchers indicated that their response to cold weather may consist of little more than huddling together in protected areas. Forbush (1927, 1929) noted the same response when he described Ending masses of dead swallows and bushels and wheelbarrow loads of dead Chimney Swifts (CChaetgra pelagica) at the bases of several chimneys during an extended cool, wet period in Massachusetts. Ramsey (1970) showed experimentally that Chimney Swifts lower their body temperature when placed in a cold environment and, indeed, go into an apparent state of torpor. During migration when temperatures were low, European Swifts have been observed roosting on walls like swarms of bees (Lack, 1956:127). In other instances European Swifts, presumably non-breeding birds, have been reported to travel several hundred miles to avoid storms (Lack, 1956:152). Some North American aerial foragers may increase