Population density and timing of breeding mediate effects of early life conditions on recruitment.

Identifying the factors driving juvenile recruitment is crucial for predicting the response of populations to environmental change. Importantly, how early life conditions carry over to influence recruitment may be highly dependent on the context in which they occur. For example, the effects of challenging early life conditions may be more pronounced under high densities or when young are born late in the season. We examined the ecological factors influencing local recruitment spanning three decades in Savannah sparrows (Passerculus sandwichensis) breeding on Kent Island, NB, Canada. The effect of nestling mass on recruitment depended on both population density and fledging date. At low-population densities or early in the breeding season, nestling mass had little effect on recruitment probability. At high-population densities or later in the breeding season, mass had a stronger effect, with heavier individuals more likely to recruit. Lighter fledglings may have lower recruitment under challenging conditions due to lower competitive ability, lower mobility and greater susceptibility to resource limitation relative to heavier fledglings. Our findings have important implications for life-history evolution and selection on body size in a changing world, highlighting the relationships between population density, timing of breeding and offspring recruitment.