Yellow-billed Magpies in central coastal California had significant variation in laying date of first egg and clutch size within and among years. The length of the breeding season was shorter than for other magpies, but timing of breeding varied widely among years. Early breeders laid larger clutches and, for successful nests, fledged more young than later breeders. Breeding coincided with peak food abundance and breeding early, relative to a particular year's food peak, resulted in higher reproductive success. However, variation in annual reproductive success was not correlated with precipitation or temperature. First-year and mixed-age pairs laid later and smaller clutches than older pairs. First-year females fledged fewer young than older females. Heavier females laid eggs earlier than lighter females. Number of breeding attempts was the most important determinant of reproductive success among years.
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