Influence of age on reproductive performance in the Seychelles warbler

Abstract

I studied age-related breeding performance of the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis) on Cousin Island, Seychelles, during 14 years. The annual number of young that fledged is significantly related to territory quality and number of helpers in the breeding group. Accounting for these factors and for the partner's breeding experience, annual production of fledglings by breeding birds increases from 2 to 5 years and decreases beyond 5 years of age. Age-related changes in reproductive success within breeding individuals, paired with the same experienced partner and occupying the same breeding territory with similar amount of food from 2 to 8 years of age, show that Seychelles warblers have higher hatching success and produce more hatchlings and fledglings as they become older. This is probably not a response to decreasing residual reproductive value caused by decreasing life expectancy as they become older. Primiparous warblers produce the same number of fledglings as multiparous warblers of the same age, have a similar life expectancy as multiparous warblers, and occupy territories with similar amounts of resources available for reproduction. As all warblers have similar access to food before reproduction and similar foraging efficiency, the low reproductive success in younger warblers cannot be ascribed to differences in environment but to the bird's ability to breed successfully (e.g., a result of previous helping and/or breeding experience). Effects of senescence on reproduction begin to occur from age 6 for both sexes. From that age, eggs have lower hatching success, but fledging success remains the same. Birds that start breeding at a young age on a given quality territory produce more fledglings in their lifetime than birds that delay breeding. Many old birds never fledge young of their own. They have not been able to breed because of a shortage of breeding territory vacancies.