Coevolution between the large hawk-cuckoo (Cuculus sparverioides) and its two sympatric Leiothrichidae hosts: evidence for recent expansion and switch in host use?

Abstract

Obligate brood parasites only account for 1% of birds in the world, but utilize a great variety of avian species as hosts. Host switch theory predicts that parasites should shift from one host to another during the long-term arms race with hosts whenever such a shift would be facilitated by similarity in ecology and distribution. However, few studies have been conducted to address this puzzle because it is extremely difficult for humans to witness such host shifts during the long-lasting process of evolution. Here we adopted an alternative way to understand host switch behaviour of brood parasites by comparing egg colour variation, cuckoo egg mimicry and egg recognition capacity between two sympatric hosts, the Chinese babax (Babax lanceolatus) and the white-browed laughing thrush (Garrulax sannio), which are both parasitized by the large hawk-cuckoo (Cuculus sparverioides). The babax lays dark blue eggs whilst the laughing thrush lays white to pale blue eggs, and the large hawk-cuckoo parasitizes them by laying eggs that optimally match laughing thrush eggs according to avian vision. The laughing thrush possesses a greater capacity of egg recognition than the babax because it rejected all non-mimetic eggs while the babax is an intermediate rejecter. Furthermore, all the nest characteristics measured were similar in these two host species with no statistical significant differences. These results are consistent with the hypothesis that the white-browed laughing thrush is the original and main host species that has a longer coevolutionary interaction with the large hawk-cuckoo than the Chinese babax, which is a recent host acquired through a host switch by the hawk-cuckoo. We discuss the possible outcome of the interaction between the large hawk-cuckoo and these two host species, and emphasize that host switch behaviour in brood parasites is more likely an adaptation to expand the range of host species rather than a change in host species favoring an increase in reproductive output.