Seasonal Decline in Reproductive Performance of Song Sparrows

Seasonal variation in reproductive success and nestling size were studied in a population of Song Sparrows (Melospiza melodia). I tested whether seasonal variation in reproductive success was due to differences among parents or territories, or whether the variation was an effect of time felt by all individuals in the population. Clutch size declined in the population through the breeding season, and the decline was due to some classes of females both nesting later and laying smaller clutches. Late lying and smaller clutches of yearling females were a major contribution to seasonal decline in clutch size, although seasonal decline in clutch size was also observed within age classes of females. Hatching success was roughly 10% lower for second broods than first broods, but did not vary with date that the nest was intiated. Survival of offspring did not vary seasonally during the period that offspring were in the care of their parents. Although nestling size was greater later in the breeding season, the probability of independent offspring recruiting into the breding population was lower for young born later in the breeding season. The seasonal decline in recruitment of offspring was not due to differences in the quality of territories or parents.