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.

Methods
We used 27 years of detailed demographic data on a population of Savannah sparrows (Passerculus sandwichensis) breeding on Kent Island, NB, Canada, to investigate the ecological factors influencing local recruitment of juveniles. All individuals breeding within the study area are marked with a unique combination of color bands and a USFWS aluminum band. Each year, we record the identities of all adults breeding on the study site. We find and monitor all nest attempts every year to produce reproductive data including hatching date; number of eggs, nestlings, and fledglings; and fledging or failure date of the nest. All nestlings and unbanded adults are banded and measured. Assignment of territories and mating status (monogamous or polygynous) are based on field observations of marked individuals.

This dataset consists of all individuals marked as nestlings on the study site. Individuals were considered to have recruited if they were resighted or recaptured as breeding adults on the study site in any year.