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Most studies of vertebrate breeding success have been unable to pinpoint the cause for a simple and almost universal pattern: its steady increase during an animal's lifetime. Yet potential causes of increased breeding success have profound implications for the evolution of reproductive behaviour. An age-related increase might come about through (1) higher breeding investment by older animals, (2) breeding improvement through learning, (3) better reproductive coordination between familiar mates or (4) accumulated benefits of site familiarity. We used mixed models to investigate age-related predictors of hatching and fledging success in a population of common loons, Gavia immer, that contained many known-age breeders and has been under continuous study for 30 years. Hatching success of breeding pairs increased between the female's first and second years on territory but did not increase with further female experience. In contrast, pair hatching success increased in proportion to the cube root of the males' residency on a territory, a pattern that persisted beyond 15 years. Hatching date was earlier for pairs that had spent at least 1 year together and in which the female was young but had spent at least one prior year on the territory. Fledging rate of hatched young was greater among pairs that nested early but decreased with female age, an apparent indication of female reproductive senescence. Overall, breeding pairs fledged more young when the breeding male had many years of experience on the territory and the breeding female had been on territory for at least 1 year and was of moderate age. Our findings highlight the persistent influence of both site familiarity and chronological age on reproductive fitness.


NOTICE: this is the author’s version of a work that was accepted for publication in Animal Behaviour. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Animal Behaviour, volume 205, in 2023.

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