Egg trait variation in a large hawk-cuckoo (Hierococcyx sparverioides) host population of Chinese babax (Babax lanceolatus).

Mutual interaction between brood parasites and their hosts is a well-known model system for studying host-parasite coevolution. Both parties have acted reciprocally, resembling an evolutionary arms race, in which adaptations and counter-adaptations have evolved as a result of host-parasite dynamics, such as the classical cuckoo-host system. Discrimination among parasite and cuckoo eggs and rejection of foreign eggs is regarded as an important anti-parasitism strategy. The Chinese babax (Babax lanceolatus) is a large hawk-cuckoo (Hierococcyx sparverioides) host distributed in southwest China. A previous study shows that the babax is an intermediate egg rejector, and most cuckoo eggs are accepted by the Chinese babax, although a small proportion of hosts reject cuckoo eggs. Interestingly, the large hawk-cuckoo lays non-mimetic eggs in contrast to the uniform blue eggs of babaxes. Because egg coloration is a critical cue used by host species in favor of the recognition of parasitic eggs by hosts, we used a spectrometer to quantify egg color variation to understand the differentiation in discrimination ability between the egg rejectors and acceptors. We found that the chroma of intra-clutch variation of babax eggs was more consistent in egg rejectors than in acceptors. However, no statistical significance was found in inter-clutch variation between these two types of hosts. Our results suggest that hosts lay eggs with a low level of intra-clutch variation without the necessity of a high level of inter-clutch variation simultaneously as predicted by the egg signature hypothesis. This study may further indicate that selection pressures from evolutionarily recent parasites can drive individual-based differences in an anti-parasitism strategy.