Most studies of birds have reported positive effects of ageing on reproductive traits during the first years of life followed by a decline. We examined the effects of age and laying experience, controlling for confounding factors such as food availability/quality and timing of pair formation in a captive population of Red-legged Partridge Alectoris rufa . We found that all reproductive traits, except for the period between eggs, were highly repeatable within females both within and between years. The repeatability of egg mass was lower than that of clutch size, laying onset or chick traits other than the mass and size increase rates, perhaps suggesting a lower heritability value. As females aged, they showed a tendency to produce marginally lighter eggs (1.3%) and larger clutches, although this latter trend was not signi- ficant. We suggest that the senescence process implies the involvement of hormonal changes that could affect the reduction of egg mass (e.g. low levels of oestradiol). Younger females tended to start laying later than older females, which could be due to a low responsiveness to photoperiodic stimulation of the reproductive system, which would indicate a delay in gametogenesis and steroidogenesis. The mass of last-laid eggs decreased with age of the female. We suggest that physiological changes through the laying sequence could vary with age. Experienced breeders laid larger clutches linked with the advancement of laying onset. For a given age, females with more experience fared better than those with less experience.
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