An international team, led by researchers of the University of Oviedo and the CSIC, and from the University of Valladolid, in collaboration with the University of University of Neuchâtel (Switzerland), has described for the first time how a species of Cuckoo protects the nests of its hosts from predators. This work, that has just been published by the prestigious journal Science, reveals benefitial aspects of brood parasitism and establishes a relation between individuals of different species that improves their chances of reproductive success.

For 16 years, the researchers have been studying and monitoring 741 nests of Carrion Crow (Corvus Corone Corone) and have analyzed their relation with the Great Spotted Cuckoo (Clamator Glandarius), a species of brood parasite of the family of the cuckoo. Their work has been led by Daniela Canestrari, researcher from the Mixed Unit of Research in Biodiversity (University of Oviedo - CSIC) and the Department of Biology of Organisms and Systems of the University of Oviedo, alongside Diana Bolopo, José M. Marcos and Vittorio Baglione, of the University of Valladolid, and Gregory Röder and Ted C. J. Turlings, of the Swiss University of Neuchâtel.

The conclusions of the study reveal that the presence of a fledgling of Cuckoo in the nest may in fact bring benefits for the Carrion Crows, turning the relation from parasitism to mutualism, from which both parties gain something. The field experiments demonstrated that the Cuckoo fledglings protect the nest thanks to a secretion that has a very poignant stench that they produce when they are threatened and that scares predators away- This chemical defense mechanism contributes to the survival of all of the fledglings of the nest, both of Carrion Crown and Cuckoo, by keeping birds of prey and mammals away. Chemical analyses have proven that the secretion made by the Cuckoo fledlings contains numerous acid and toxic compounds that manage to repel predators. This protective measures by the Cuckoos extends during 16 to 18 days, before the fledlings leave the nest. "The conclusions of the study show us that the relation between a brood parasite and its hosts is more complex that what we have always thought, and new oportunities arise to better analyze these interactions", explains Doctor Daniela Canestrari.

Experts led by the University of Oviedo, in collaboration with the CSIC, the University of Valladolid and the University of Neuchâtel (Switzerland) have studied for 16 years in more than 700 nests the relation between the Great Spotted Cuckoo and the Carrion Crow.

The Cuckoo does not expel the fledglings of its host from the nest, but it competes with them for the food that the parents bring to the nest and, frequently, it may kill some of the other fledglings. In this sense, the Cuckoo taxes the host, a typical feature of parasites.

Nevertheless, after analyzing the data gathered during 16 years in 741 nests placed in the surroundings of La Sobarriba in León, the researchers proved that in years with a high density of predators, the nests with parasites suffered the lowest number of losses caused by predation. In these years, the ecological relation between the Cuckoo and the Carrion Crow stops being parasitic (in which the parasite "wins" and the host "loses") and it changes into mutualism (they both obtain something positive from the relation). The field work has also analyzed the influenced that the presence of a parasite has on the effort that the parents have to make to feed the fledglings, showing that a Cuckoo fledgling makes a lower effort in comparison to a Carrion Crow fledgling, due to the smaller size of the parasite.

The reproductive strategy of several species of Cuckoos is widely known and it consists of laying the eggs in the nest of another bird (the host) and let their progeny be raised by adoptive parents. In many cases, looking after a Cuckoo carries a considerable cost for the hoste, since it may even expel the other eggs or fledlings from the nest or monopolize food, provoking death by starvation. Many species have developed strategies to defend themselves against brood parasites, scuh as the ability to recognize and expel the eggs of other species or the chance to actively defend their nest against adult parasites. Nevertheless, some species of hosts, such as the Carrion Crow, do not have any defense against brood parasites and, therefore, their nests may be frequently infected by parasites. The lack of defenses poses an open question for many scientists. The study by Canestrari and her collaborators has proposed that, in the case of the Carrion Crow, the lack of defenses against the Great Spotted Cuckoo is due to the benefitial effect that brood parasitism generates in years with high amount of predators.

The research project has been developed by the internation team with funds of the National Plan for Science of the Ministry of Economics and Competitiveness, and funding given by the Council of Castilla y León.

Publication Information

From Parasitism to Mutualism: Unexpected Interactions Between a Cuckoo and Its Host

Authors:

Daniela Canestrari, (1), (2); Diana Bolopo, (3); Ted C. J. Turlings, (4); Gregory Röder, (4); José M. Marcos, (3); Vittorio Baglione (3), (5).

(1) (2) Department of Biology of Organisms and Systems of the University of Oviedo and Researcher from the Mixed Unit of Research in Biodiversity of the CSIC.

(3) Department of Agroforestry of the University of Valladolid.

(4) Institute of Biology of the University of Neuchâtel (Switzerland).

(5) University Institute for Research on Sustainable Forest Management (University of Valladolid).

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Carrion crow and great spotted cuckoo fledglings demanding food in the same nest.