Аннотация:The breeding plumage of male Pied Flycatchers (Ficedula hypoleuca) varies between conspicuously pied and cryptically pale brownish. Recently, conspicuous males (CM) were found to be more tolerant of cold spring weather than pale males (PM). Singing activity dropped in PM in cold weather, whereas CM performed full-scale advertising behavior within a wide range of temperatures. This asymmetry corresponded to differences in energetics: basal metabolic rate (BMR) in CM tended to be higher than in PM. In PM, moreover, advertising activity under low temperatures (<10°C) was confined to males with high BMRs; those with low BMRs sang actively only in warmer conditions. BMR in CM, however, was not related to ambient temperature. To explain the mechanisms involved, two hypotheses have been proposed: (1) BMR is a stable trait that expresses susceptiblility to cold and/or competitive quality such that temperature-dependent patterns in advertising reflect differences between males in BMR, or (2) contrary to CM, PM have to increase BMR under medium low temperatures at the expense of advertising activity. To test these hypotheses, we caught and temporarily removed wild males of both forms from their breeding areas around Moscow soon after arrival in spring. The BMR of each male was measured twice: on the first night after capture and then three days later after a period in a cold (~5 °C; n=33) or warm (~25°C; n=31) chamber. The experimental treatment increased BMR in both CM and PM, that in birds kept in cold chambers rising higher than in those in warm chambers. The effect of plumage color was pronounced when ambient temperature on the day before capture was taken in account. In birds captured after a warm day, exposure to cold led to a marked increase in BMR in pale but not conspicuous males. The results fit the second hypothesis better that the first, and suggest that medium low temperatures, which are common in spring in the study region, may act as a cold stress factor on the energetic regime of pale males.
