Age and Environmental Conditions Affect Recruitment in Greater Snow Geese

Recruitment is an important determinant of fitness and population growth rates, but few studies have examined the effect of environmental stochasticity on this life history trait. Furthermore, most studies have been unable to separate the influence of juvenile survival and age-specific breeding proportions on recruitment. We used a recently developed approach, based on capture-mark-recapture methods, in which local recruitment is analyzed in a multistate model with an unobservable "nonbreeder" state. The data are drawn from a long-term study of a long-lived, arctic-nesting bird, the Greater Snow Goose (Chen caerulescens atlantica), and include marking and recaptures of female goslings and breeding adult females of unknown age between 1990 and 2000. The model considers four parameters: the probability that an individual aged i with no breeding experience starts breeding (ai), juvenile and adult apparent survival (Φ), and capture probability of breeders (p). The flexibility achieved allows us to assess the influence of environmental conditions encountered during early life and at breeding on juvenile survival and the probabilities of starting to breed at a given age. Recruitment was a gradual process (probability of starting to breed at age 2 yr = 0.25 [95% CI, 0.12-0.45]; at age 3 yr = 0.57 [0.20-0.87]) and was completed by age 4 yr (i.e., all remaining immature females started to breed at that age). Juvenile survival was higher in early-hatched than in late-hatched females. Juvenile survival varied considerably among cohorts, but our environmental covariates could not explain these differences. Probabilities of starting to breed were less variable, except in lemming crash years, when they were considerably reduced. Snow cover at breeding or hatch date did not affect probabilities of starting to breed. These results suggest that environmental conditions can have an impact on life histories of birds in seasonal environments, but that variations in juvenile survival probably account for most of the fluctuation in the proportion of birds from a cohort that recruit into the breeding population. Use of multistate models to estimate recruitment increases precision in parameter estimates with the addition of data from adults of unknown age. However, we are still restricted by some assumptions, most notably the absence of temporary emigration.