Longitudinal analyses of magpie-cuckoo interactions (main results of Mercedes Molina's thesis)

Avian brood parasites and their hosts provide an ideal system for the study of coevolution and for understanding the evolution of defenses and counterdefenses typical in host-parasite interactions. In particular, the great spotted cuckoo Clamator glandarius and its main host in the Iberian Peninsula, the magpie Pica pica, constitute a suitable system that has provided strong support for an effect of brood parasitism on (i) the reproductive success of the host, and (ii) on the evolution of host defensive mechanisms counteracting the negative effects of parasitism. You can find a nice review on this in Soler & Soler (2000), Brood parasite interactions between great spotted cuckoos and magpies: a model system for studying coevolutionary relationships. Oecologia 125: 309-320.

Left, two great spotted cuckoos just hatched in a parasitized magpie nest. Right, a fledgling cuckoo at the nest's rim waiting for its foster parents. Pictures by Merche Molina.

However, longitudinal studies with marked animals addressing issues such as the patterns of host use by parasites, effects of parasitism on lifetime breeding success or hosts' defense mechanisms are very scarce. Mercedes Molina's thesis has tried to start filling this gap using magpies as study species. You can have a look on the papers published in my list of publications. Here is a little abstract of main results.

La Calahorra (Granada). In top of the hill, La Calahorra Castle, first example of  the italian Renaissance architecture in the spanish castles (1509). Behind the village the impressive view of Sierra Nevada after a heavy snowfall. The almond groves surrounding the small village are our main study area. Picture by Juan Gabriel Martínez.

The thesis analyzes through crosssectional and longitudinal studies (based on resampling known individuals in different breeding seasons), the patterns of parasitism in the population, that is, which variables are associated to the probability of being parasitized. Longitudinal studies also allowed us to address questions related to the evolution of defensive mechanisms against parasitism along the life of individual, in particular the expression of the main host defensive behaviour, the discrimination and rejection of odd eggs.

Capturing and marking magpies has been a fundamental part of the study. Left, cage used to catch magpies during nest building. Right, a couple of magpies just ringed. Pictures by Merche Molina.

Results suggest that the pattern of parasitism, at the population level, responds to a probabilistic process based on the spatio-temporal availability of magpie nests and the abundance of cuckoos in the population. In addition, it has been found that the population phenological mismatch between great spotted cuckoos and magpies changes between seasons depending on climatic factors, thus providing an additional explanation to the annual variation found in the probability of parasitism. Also, the longitudinal study reveals a pattern of structured parasitism at the individual level within the population. Indeed, females with a particular combination of traits (nest size, laying date and habitat characteristics) consistently escaped from cuckoo parasitism.

Individually structured parasitism by great spotted cuckoos. Some females are never parasitized along their lives (Molina-Morales et al. 2013, Journal of Animal Ecology 82: 389-398)

The study of the rejection behaviour of magpies throughout their lives shows that some females always accept, others always reject and some others modify their response to model eggs, in all cases switching from acceptance to rejection. Also females tested in their first breeding attempt always accepted the model eggs, even those individuals whose mothers were egg rejecters. A longitudinal analysis showed that the probability of rejecting eggs increased with the relative age of the female, but was not related to the risk of parasitism in the population. All this suggests that the transition towards egg recognition is related to age, being more likely for older females to develop the cognitive and/or mechanical skills necessary for recognition and rejection.

Magpie eggs (numbers 1 to 6), great spotted cuckoo eggs (numbers 7 to 11) and one of the model plaster eggs used in the rejection experiments (12). Picture by Merche Molina.

In general, this thesis shows the importance of performing  studies through the life of hosts, in particular in long-lived hosts, for studying patterns of parasitism and the expression of phenotypic traits and defensive behavior at the population. Results suggest that ontogeny may play a fundamental role in the expression of host defenses. In the future, I would like to look at the proximate mechanisms that, together with age, explain rejection, and to get a more accurate measure of the effect of brood parasitism on magpie breeding success by studying individuals along their lives, from birth to death.

For more details and reading the whole story check out our publications.