Nest tubular entrance characteristics are not predictive of brood parasitism by Striped Cuckoos on Yellow-chinned Spinetails

Abstract

The interactions between parasitic birds and their host species can result in a wide range of adaptations. For cavity-nester hosts, for instance, nest entrances can reduce parasite access when the latter are larger than the host species, and for at least one group of birds, the weaverbirds, the construction of elongated tubular entrances was hypothesised to reduce interspecific brood parasitism. The function of tubular entrances as antiparasitic defences in closed-nester hosts, however, is one of the less investigated aspects of brood parasitism. Here, we tested the hypothesis that nest tubular entrance can reduce parasitism by the Striped Cuckoo, Tapera naevia, in a population of the closed-nester Yellow-chinned Spinetail, Certhiaxis cinnamomeus. We predicted that if the parasite uses a nest entrance to deposit its eggs, either by entering the nest through the tube or making an egg pass through the tube somehow, then nests with tubes that are longer, narrower, curved, and with low inclination would have lower parasitism probability. Modelling analyses, however, suggested this hypothesis should be rejected, as vegetation cover above nests was the only parameter marginally correlated to parasitism probability. As the parasite was too big to pass through the tubular entrance, and no tunnel enlargements that could evidence attempts to enter were observed, our findings were consistent with a model in which T. naevia may lay their eggs at the rim of the tube entrance, and eggs may roll down towards the incubatory chamber, despite tube length, width, and curvature. Parasitism could be facilitated by the downward inclination of most tubes, but it has to be confirmed by direct observations.