Age-related brood parasitism and egg rejection in magpie hosts

When the strength or nature of a host -parasite interaction changes over the host life cycle, the consequences of parasitism can depend on host population age structure. Avian brood parasites reduce hosts’ breeding success, and host age may play a role in this interaction if younger hosts are more likely parasitized and/or less able to defend themselves. We analyzed whether the age of female magpies (Pica pica) hosts is associated with parasite attack or their ability to reject foreign eggs. We recorded parasitism and model egg rejection of known-age individuals over their lifetime, and established whether likelihood of parasitism or egg rejection changed with age or longevity. Parasitism probability did not change with female age and there was a trend for longer lived females to be more likely to be parasitized. However, model egg rejection probability increased with age for each individual female, and longer-lived females were more prone to reject. Most females in the population were young, and the majority of them accepted model eggs, suggesting that brood parasites exploiting younger host individuals are benefitting from a lower defensive level of their hosts. Our results stress that the intensity of selection by brood parasites may be mediated by the age-structure of host populations, a so far neglected aspect in brood parasite-host research.