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,
better-condition 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, time of
breeding, and offspring recruitment.