Abstract
Variation in fitness between individuals in populations may be attributed to differing environmental conditions experienced among birth (or hatch) years (i.e., between cohorts). In this study, we tested whether cohort fitness could also be explained by environmental conditions experienced in years post-hatch, using 736 lifelong resighting histories of Greenland white-fronted geese (Anser albifrons flavirostris) marked in their first winter. Specifically, we tested whether variation in age at first successful reproduction, the size of the first successful brood and the proportion of successful breeders by cohort was explained by environmental conditions experienced on breeding areas in west Greenland during hatch year, those in adulthood prior to successful reproduction and those in the year of successful reproduction, using North Atlantic Oscillation indices as proxies for environmental conditions during these periods. Fifty-nine (8%) of all marked birds reproduced successfully (i.e., were observed on wintering areas with young) only once in their lifetime and 15 (2%) reproduced successfully twice or thrice. Variation in age at first successful reproduction was explained by the environmental conditions experienced during adulthood in the years prior to successful reproduction. Birds bred earliest (mean age 4) when environmental conditions were ‘good’ prior to the year of successful reproduction. Conversely, birds successfully reproduced at older ages (mean age 7) if they experienced adverse conditions prior to the year of successful reproduction. Hatch year conditions and an interaction between those experienced prior to and during the year of successful reproduction explained less (marginally significant) variation in age at first successful reproduction. Environmental conditions did not explain variation in the size of the first successful brood or the proportion of successful breeders. These findings show that conditions during adulthood prior to the year of successful reproduction are most important in determining the age at first successful reproduction in Greenland white-fronted geese. Very few birds bred successfully at all (most only once), which suggests that May environmental conditions on breeding areas have cohort effects that influence lifetime (and not just annual) reproductive success.
Highlights
Individual variation in fitness is a feature of vertebrate populations (Gaillard et al, 2000), some of which results from annual variation in conditions experienced during early life (Sæther, 1997), giving rise to ‘cohort effects’ (Lindström, 1999)
To determine whether environmental conditions experienced by cohorts through adulthood and prior to successful reproduction explained variation in AFSR and size of the first successful brood (SFSB), we developed a ‘breeding conditions index’ (BCI) and used annual May North Atlantic Oscillation (NAO) indices as a proxy for environmental conditions experienced during the breeding season
To investigate whether variation in the proportion of successful breeders by cohort (PSBC) was explained by the set of environmental conditions each cohort experienced over its potential reproductive lifetime, we developed a similar index, the ‘cohort breeding conditions index’ (CBCI)
Summary
Individual variation in fitness is a feature of vertebrate populations (Gaillard et al, 2000), some of which results from annual variation in conditions experienced during early life (Sæther, 1997), giving rise to ‘cohort effects’ (Lindström, 1999). Cohort effects are well documented in birds (Van der Jeugd & Larsson, 1998; Krüger & Lindström, 2001; Reid et al, 2003) and mammals (Rose, Clutton-Brock & Guinness, 1998; Coltman et al, 1999; Descamps et al, 2008), where subsequent fitness has been linked to birth year conditions via life history traits. Inter-cohort variation in life history traits can help to explain individual performance in relation to conditions experienced by individuals born in the same year. Prevailing conditions encountered in later life (e.g., population density or weather during the breeding period) will likely contribute to variation in cohort-specific life history traits because different cohorts experience different conditions during their potential breeding lifespan (Reid et al, 2003; Thessing & Ekman, 1994; Reed et al, 2003). In North American red squirrels (Tamiasciurus hudsonicus), silver spoon effects were diluted in cohorts that experienced lower food availability as adults (Descamps et al, 2008)
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