Age and terminal reproductive attempt influence laying date in the thorn-tailed rayadito

Age-specific variation in reproductive effort can affect population dynamics, and is a key component of the evolution of reproductive tactics. Late-life declines are a typical feature of variation in reproduction. However, the cause of these declines, and thus their implications for the evolution of life-history tactics, may differ. Some prior studies have shown late-life reproductive declines to be tied to chronological age, whereas other studies have found declines associated with terminal reproduction irrespective of chronological age. We investigated the extent to which declines in late life reproduction are related to chronological age, terminal reproductive attempt or a combination of both in the thorn-tailed rayadito Aphrastura spinicauda, a small passerine bird that inhabits the temperate forest of South America. To this end we used long-term data (10 years) obtained on reproductive success (laying date, clutch size and nestling weight) of females in a Chilean population. Neither chronological age nor terminal reproductive attempt explained variation in clutch size or nestling weight, however we observed that during the terminal reproductive attempt older females tended to lay later in the breeding season and younger females laid early in the breeding season, but this was not the case when the reproductive attempt was not the last. These results suggests that both age-dependent and age-independent effects influence reproductive output and therefore that the combined effects of age and physiological condition may be more relevant than previously thought.