Differential maternal investment counteracts for late breeding in magpies Pica pica: an experimental study

Reproductive success in many avian populations declines throughout the breeding season. Two hypotheses have gained attention to explain such a decline: the "timing" hypothesis proposes that deteriorating food availability causes the decline in reproductive success (causal effect of breeding time), whereas the "quality" hypothesis proposes that individuals of lower phenotypic quality reproduce at the end of the breeding season, causing the correlation between breeding time and breeding success. We tested both of these hypotheses in a monogamous single breeder, the magpie Pica pica , by experimentally inducing some pairs to lay a replacement clutch, after removal of the first clutch. The first clutch was left in the nest of another magpie pair (matched by laying date and clutch size), and incubated and raised by these foster parents. In this way we obtained two clutches from the same magpie pair with full siblings raised in conditions of the first and second reproductive attempts. High quality pairs (with laying dates in the first half of the breeding season) reached similar breeding success in replacement clutches as compared to first clutches of the same female. In addition, experimental pairs reared significantly more offspring of similar quality in their replacement clutches as compared to late-season first clutches, thereby suggesting that late season first clutches were produced by pairs of lower phenotypic quality. Results indicate that high quality pairs trade-off clutch size for larger eggs in replacement clutches, which could help magpie pairs to partly compensate for poorer environmental conditions associated with a delayed breeding attempt.