Patterns and Mechanisms for Age-dependent Reproduction and Survival in Birds

SYNOPSIS. The majority of bird species exhibit age-dependent survival and reproduction. In almost all cases, first time breeders or young individuals perform at a lower and slower rate than older individuals. This review highlights the importance of age-dependent effects and urges further study of the proximate and ultimate mechanisms involved. Age effects show substantive variation across life history stage, breeding season, year, cohort, habitat types and environmental conditions both within and across taxons. In some populations or years, age effects disappear. Despite the variable patterns, age effects show amazing persistence in a variety of extreme ecological and environmental conditions. Experimental manipulations of food, predation pressure and breeding experience do not usually remove age effects. Age-dependent effects may be maintained or accumulate through the breeding season and profoundly influence fitness, or they may be washed out or reversed by events later in the breeding season. Recently, we have amassed substantive information about the patterns of variation in age-dependent reproduction and survival among individuals, but we have made little progress toward understanding the proximate mechanisms responsible for this variation. Proximate and ultimate processes are inextricably linked, and thus the study of age-dependence is highly relevant to the further development of life history theory. Studies of age-dependent performance have general relevance to population management and conservation issues as age sub-structuring may contribute substantially to annual or inter-populational variation in reproductive success.