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

Abstract

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.

Keywords: Savannah sparrow; carry-over effects; demography; juvenile survival; reproduction.

Figure 1.

(a) Recruitment rate of fledgling Savannah sparrows (Passerculus sandwichensis) varied across years. On average, 12% of fledglings were recruited (range 0.04–0.19, n = 4870 individuals, n = 1364 nests, n = 27 years). (b) Population density (the number of breeding adults per hectare) varied across years. Savannah sparrow graphic by Shelby Bohn.

Figure 2.

The effects of nestling mass on recruitment in Savannah sparrows (Passerculus sandwichensis) depended on both (a,b) population density (p = 0.04) and (c,d) fledging date (p = 0.03). (a) At low-population densities, recruitment probability did not differ by mass, but as population density increased, mass had an increasingly strong effect, with heavier fledglings more likely to recruit (GLMM; model predictions with s.e. generated using the R package emmeans, at mean values of other continuous predictors and averaged across levels of other categorical predictors). (b) Count of fledglings that were recruited or not by mass, binned by population density tertile. At lower densities, the distribution of fledglings that recruited versus those that did not overlapped more extensively than at higher densities, where recruits had higher mass than non-recruits. The difference in mean mass of fledglings that recruited versus those that did not, represented by vertical lines, was larger at high density. (c) For early-fledging juveniles, recruitment probability did not differ by mass, but among later-fledging juveniles, heavier individuals were more likely to recruit (GLMM; model predictions with s.e.). (d) Count of fledglings that were recruited or not by mass, binned by fledge date tertile. For early fledge dates, the distribution of fledglings that recruited versus those that did not overlapped more extensively than at later fledge dates, where recruits had higher mass than non-recruits. The difference in mean mass of fledglings that recruited versus those that did not, represented by vertical lines, was larger for later fledglings.