ИСТИНА

The breeding plumage of Pied Flycatcher males varies from conspicuous black-and-white to cryptic brownish. Recently, conspicuous males (CM) were found to be more tolerant to spring cold weather than pale males (PM). PM reduced singing activity in cold weather while CM performed fool-scale advertising behaviour within wide temperature range. This asymmetry corresponded to differences between CM and PM in energetics. Basal metabolic rate (BMR) of CM tended to be higher than that of PM. In PM, advertising activity under low temperatures (<10 C) was peculiar only to males with high BMRs. PM with low BMRs actively sang in warmer conditions. BMR of CM was not related to ambient temperatures (Ivankina et al., 2002). To address the problem, two hypotheses were proposed: (1) BMR is a stable characteristic of a bird and indicates its sustainability to cold and/or competitive quality; the temperature-dependent pattern of advertising reflects differences between males in BMR; (2) on contrary to CM, PM have to increase BMR under medium low temperatures. To test these hypotheses we temporarily removed wild males soon after their arrival to the breeding area in Moscow region. BMR of each male was measured two times: first night after the capture and three days later after its stay in cold (~5 C; n=33) or warm (~25 C; n=31) chamber. The experimental treatment resulted in BMR increase both in CM and PM. BMR of males kept in cold chambers mounted to higher level than that in males from warm chambers. The effect of plumage colour was pronounced when ambient temperature during a day before the capture was taken in account. Among birds removed just after a warm day, cold exposition led to marked BMR increase in pale but not in conspicuous males. The results fit better the second hypothesis and suggest that medium low temperatures, which are common in spring in the study area, may act as a factor of cold stress on energetic regime of pale males.