Experimental evidence that age-specific reproductive success is independent of environmental effects

An age-specific improvement in reproductive performance has been reported i n many iteroparous breeders. However, whether this is a consequence of intr insic differences in competence amongst age classes or extrinsic difference s in the environment they experience is unclear since the timing of breedin g within a season generally also differs with age. To disentangle these eff ects, we experimentally manipulated the timing of breeding in shags, Phalac rocorax aristotelis. Old and young individuals thus reared their chicks at the same time both early and late in the breeding season. When breeding in the same environmental conditions, old pairs performed consistently better than young pairs. These data clearly demonstrate that the age-related diffe rences in reproductive performance are not a result of environmental effect s, but rather a consequence of intrinsic differences in brood rearing capac ity.