Proximate and Ultimate Aspects of Phenotypic Plasticity in Timing of Great Tit Breeding in a Heterogeneous Environment

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Abstract
Using the theoretical framework of phenotypic plasticity, we studied the timing of breeding in great tits (Parus major), combining proximate questions about its physiological causation and ultimate questions about its fitness consequences. The plasticity observed in the timing of breeding can be explained either as an adaptation to the best time for rearing young or as determined by changing environmental conditions prior to laying. We related the variation in timing of breeding and breeding success to local temperature and food abundance. For the most part, spring temperature (but also caterpillar abundance and low temperature 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 variation 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 were the most successful breeders. The most productive laying date differed significantly among years because unpredictable yearly fluctuations in environmental conditions after the birds started laying still affected the development of caterpillars. In addition to being affected by changes in environmental conditions early in the season, the laying dates 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 variation in environmental conditions will maintain variation in timing. [KEYWORDS: PARUS-MAJOR, REPRODUCTIVE SUCCESS, REACTION NORMS, LAYING-DATE, BLUE TITS, FOOD, SURVIVAL, AGE, HERITABILITY, POPULATION

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