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Abstract
It is well known that reproductive performance improves with age in birds. Many hypotheses,
involving factors such as differential survival, delayed breeding, breeding experience,
foraging ability and reproductive effort, have been proposed to explain this pattern.
Although these hypotheses are not mutually exclusive, they can be classified in three
major groups relating to progressive appearance or disappearance of phenotypes, age-related
improvements of competence, and optimization of reproductive effort. However, a closer
examination of the literature reveals that only few studies have rigorously tested
the hypotheses. Future work should focus on carefully designed tests that critically
investigate the hypotheses.