Magpies do not desert after prolonging the parental care period: an experimental study

Parent–offspring conflict theory predicts that offspring desertion would be adaptive for parents when the fitness costs related to investment in the current brood exceed the expected fitness benefits. In several seabirds, a fixed parental care period has been observed, and a preprogrammed fixed parental care period could be a general life-history trait evolved because of parent–offspring conflict. A recent study suggested that this could also be the mechanism by which hosts could discriminate against brood parasitic chicks that need longer care periods, by abandoning a brood when the parental care period passed the one typically needed by their own brood (i.e., discrimination without recognition). Here, we experimentally tested if a fixed parental care period also existed in magpies (Pica pica), the primary host of the great spotted cuckoo (Clamator glandarius). None of the experimentally prolonged magpie broods were deserted, and neither were any of the control or shortened broods. These results suggest that a preprogrammed parental care period is not a general trait in birds and support the idea that brood parasitism could be a selective pressure for optimal (short) parental care periods to evolve, but only in species where brood parasitic nestlings need a longer care period than host nestlings (which was not the case for the great spotted cuckoo). Furthermore, nestlings reared by parents with prolonged parental care period showed a slower development. Increasing provisioning costs, reduced sensitiveness to begging signals, or sexual conflict could cause this result, upholding parental care periods as a challenging matter of research.