Abstract
Host defences against cuckoo parasitism and cuckoo trickeries to overcome them are a classic example of antagonistic coevolution. Recently it has been reported that this relationship may turn to be mutualistic in the case of the carrion crow (Corvus corone) and its brood parasite, the great spotted cuckoo (Clamator glandarius), given that experimentally and naturally parasitized nests were depredated at a lower rate than non-parasitized nests. This result was interpreted as a consequence of the antipredatory properties of a fetid cloacal secretion produced by cuckoo nestlings, which presumably deters predators from parasitized host nests. This potential defensive mechanism would therefore explain the detected higher fledgling success of parasitized nests during breeding seasons with high predation risk. Here, in a different study population, we explored the expected benefits in terms of reduced nest predation in naturally and experimentally parasitized nests of two different host species, carrion crows and magpies (Pica pica). During the incubation phase non-parasitized nests were depredated more frequently than parasitized nests. However, during the nestling phase, parasitized nests were not depredated at a lower rate than non-parasitized nests, neither in magpie nor in carrion crow nests, and experimental translocation of great spotted cuckoo hatchlings did not reveal causal effects between parasitism state and predation rate of host nests. Therefore, our results do not fit expectations and, thus, do not support the fascinating possibility that great spotted cuckoo nestlings could have an antipredatory effect for host nestlings, at least in our study area. We also discuss different possibilities that may conciliate these with previous results, but also several alternative explanations, including the lack of generalizability of the previously documented mutualistic association.
Highlights
Coevolutionary interactions are very common and widespread in nature [1]
The experimental repellence tests which showed that the malodorous cloacal secretion discouraged potential nest predators from consuming food with cloacal-secretion added, as well as the association between experimental parasitism and predation rate reported in the paper, support the expected benefits, which would overcome costs of brood parasitism at high predation rates in the population [6]
In order to evaluate whether the results found by Canestrari et al [6] in northern Spain with the carrion crow can be replicated in other populations and other host species, we here compared predation rates of naturally and experimentally parasitized and non-parasitized nests of both the carrion crow and the magpie in a population located in southern Spain
Summary
Coevolutionary interactions are very common and widespread in nature [1] They are frequently complex and sometimes mutualism and parasitism can exist in the same system. Canestrari et al [6] found in their study area that great spotted cuckoos benefit their carrion crow host in contexts of high predation risk. Cuckoo chicks produce a malodorous cloacal secretion when they are grabbed, which apparently deters predators from parasitized host nests. This antipredatory effect would result into more fledglings in parasitized nests compared to non-parasitized nests during breeding seasons with high predation risk. The experimental repellence tests which showed that the malodorous cloacal secretion discouraged potential nest predators (mammals, corvids, and raptors) from consuming food with cloacal-secretion added, as well as the association between experimental parasitism and predation rate reported in the paper, support the expected benefits, which would overcome costs of brood parasitism at high predation rates in the population [6]
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