PROXIMATE AND ULTIMATE ASPECTS OF PHENOTYPIC PLASTICITY IN TIMING OF GREAT TIT BREEDING IN A HETEROGENEOUS ENVIRONMENT

Using the theoretical framework of phenotypic plasticity, we studied t he timing of breeding in great tits (Parus major), combining proximate questions about its physiological causation and ultimate questions ab out its fitness consequences. The plasticity observed in the timing of breeding can be explained either as an adaptation to the best time fo r rearing young or as determined by changing environmental conditions prior to laying. We related the variation in timing of breeding and br eeding success to local temperature and food abundance. For the most p art, spring temperature (but also caterpillar abundance and low temper ature around the expected start of egg laying) as well as the presence of the birds in their breeding territories during the previous winter explained within-population variation in timing. Intraindividual vari ation in timing showed a plastic response to changes in environmental conditions prior to laying, but the plasticities did not differ among individuals. Birds raising their young when food was most abundant wer e the most successful breeders. The most productive laying date differ ed significantly among years because unpredictable yearly fluctuations in environmental conditions after the birds started laying still affe cted the development of caterpillars. In addition to being affected by changes in environmental conditions early in the season, the laying d ates of experienced tits were affected by the difference between their timing and the caterpillars' timing in the previous year. Thus, great tits adjust their timing to the best time for rearing young, but vari ation in environmental conditions will maintain variation in timing.