American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Age-related brood parasitism and egg rejection in magpie hosts”

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Juan Gabriel Martínez, Mercedes Molina-Morales, Marta Precioso, and Jesús Miguel Avilés (May 2020)

We show that individual hosts of a cuckoo species change the expression of their main defensive behaviour as they age

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Breeding for some birds ends up in a disaster: instead of rearing their own healthy chicks, they take care of intruders, the cuckoos, which get rid of any legitimate offspring in the nest and monopolize all the care that host parents should be providing to their nestlings. But hosts of brood parasites are not always parasitized, and if they do, they may not be defenseless, since some individuals are able to recognize and reject cuckoo eggs. If natural selection favors individuals able to avoid parasitism, why do some individuals escape parasitism and/or are able to reject cuckoo eggs, whereas others become victims of the parasites? This article explores how considering the age of individual hosts may help to answer this.

Researchers Juan Gabriel Martínez, Mercedes Molina-Morales, Marta Precioso, and Jesus Avilés (from the Universidad de Granada and Estación Experimental de Zonas Áridas, in Spain) are studying magpies (Pica pica) at a location in South-eastern Spain (La Calahorra, Granada). Magpies are regularly parasitized by great spotted cuckoos (Clamator glandarius), and the team have been following female magpies during their lives for the last 12 years, recording parasitism status and foreign egg rejection of individuals of known-age over their lifetime. They found that the likelihood of being parasitized does not change as females age, although there is a trend that longer-lived females are parasitized less often. However, foreign egg rejection probability does increase with age: even though many females never reject foreign eggs in their nests, most females that do it start rejecting when they are 3 or 4 years old. It is known that magpies may live many years, but most females in the study population are young, and so the majority of them accepted foreign eggs; this suggest that brood parasites exploit younger hosts, benefitting from a lower defensive level.


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 less 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.