In a remarkable departure from the typical brood parasite-host interaction, Canestrari et al. (2014) describe a nesting relationship in a northern Spain study site that yields benefits to both the parasitized host and the parasite- a form of mutualism. The parasite is the Great Spotted Cuckoo (Clamator glandarius) of southern Europe and western Asia which prefers corvids, especially magpies, starlings and the Carrion Crow (Corvus corone- until recently classified as belonging to the same species as the Hooded Crow) as hosts.
C. glandarius does not remove eggs of host species, relying on the hosts to raise its offspring along with their own. Although many host species like the magpie, evict parasite eggs from their nests, this crow does not. Despite lacking this adaptation, however, overall crow reproduction (parasitized and unparasitized nests) at the study site was less reduced than that of the magpie. Furthermore, after hatching, parasitized crow nests were more likely to produce young crows that reached the fledgling stage than unparasitized nests. This surprising pattern occurred in a site in which over 67% of carrion crow nests were parasitized by cuckoos and was not observed where parasitized rates were low. In transfer experiments, cuckoo hatchlings were transferred into nonparasitized crow nests leading to increased survival of crow hatchlings compared to crows in nests that did not receive transferred cuckoo chicks. Transferring crow hatchlings into crow nests provided no such benefit! Also, among nonparasitized nests, adding cuckoo nestlings increased crow nestling survival.
And the explanation for this pattern is? It turns out that cuckoo nestlings secreted a malodorous, acidic substance from their cloaca when handled by investigators. The percentage of cuckoo chicks voiding material varied but reached 90% of chicks older than 4 days. In contrast none of the adult cuckoos that were captured secreted cloacal material. Repellence tests exposing potential nest predators of the study site- a hawk species, crows and feral house cats – to food items treated and untreated with cuckoo cloacal secretions, demonstrated that the cloacal material would be effective in deterring predators from taking nestlings of either species.
The cloacal material contains acids, indoles, phenol and several other compounds known to repel birds and mammals. Interestingly, as the authors’ speculate, the fact that the presence of cuckoos in nests where parasitism rates are high benefits the host may impede the evolution of host adaptations that might counter the negative effects of cuckoos where only a relatively few nests are parasitized.
Canestrari, D., et al., 2014. From Parasitism to Mutualism: Unexpected Interactions Between a Cuckoo and Its Host. Science 343: 1350-1352.