Early onset of reduced reproductive performance with age in the treecreeper (Certhia familiaris)

Reductions in reproductive performance with age have been predicted to result from a general deterioration of performance, i.e. senescence. Variation among species in the onset and rate of this deterioration depends on the age-independent extrinsic mortality rate. If few individuals reach a specific age, the strength of selection for mechanisms that retard senescence will be reduced. The aim of this study was to investigate the age-dependent variation in two reproductive traits in a species, the Treecreeper (Certhia familiaris), with a low between-year survival rate. Clutch size did not vary with age, but egg size decreased from the first to the second breeding season. Compared with published age-dependent reductions in egg size, Treecreepers demonstrate the earliest onset of senescence, but they also have the highest total mortality rate, corroborating the predictions from the evolutionary theory of senescence. Production of eggs seems to be demanding for female Treecreepers, as egg size is also positively dependent on ambient temperature, further stressing the vulnerability of this trait for small reductions in female performance