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.
The coevolutionary interaction between cuckoos and their hosts has been studied for a long time, but to date some puzzles still remain unsolved. Whether cuckoos parasitize their hosts by laying eggs randomly or matching the egg morphs of their hosts is one of the mysteries of the cuckoo problem. Scientists tend to believe that cuckoos lay eggs matching the appearance of host eggs due to selection caused by the ability of the hosts to recognize their own eggs.In this paper, we first review previous empirical studies to test this mystery and found no studies have provided direct evidence of cuckoos choosing to parasitize host nests where egg color and pattern match. We then present examples of unmatched cuckoo eggs in host nests and key life history traits of cuckoos, e.g. secretive behavior and rapid egg-laying and link them to cuckoo egg laying behavior. Finally we develop a conceptual model to demonstrate the egg laying behaviour of cuckoos and propose an empirical test that can provide direct evidence of the egg-laying properties of female cuckoos. We speculate that the degree of egg matching between cuckoo eggs and those of the host as detected by humans is caused by the ability of the hosts to recognize their own eggs, rather than the selection of matching host eggs by cuckoos. The case of Common Cuckoos(Cuculus canorus) and their parrotbill hosts(Paradoxornis alphonsianus), where it has been shown that both have evolved polymorphic eggs(mainly blue and white), was used to develop a conceptual model to demonstrate why cuckoos should utilize parrotbill hosts by laying eggs randomly rather than laying eggs matching the appearance of host eggs.In conclusion, we found no evidence for the hypothesis that cuckoos lay eggs based on own egg color matching that of the parrotbill-cuckoo system. We argue theoretically that laying eggs matching those of the hosts in this system violates a key trait of the life history of cuckoos and therefore should be maladaptive.
Rates of brood parasitism vary extensively among host species and populations of a single host species. In this study, we documented and compared parasitism rates of two sympatric hosts, the Oriental Reed Warbler(Acrocephalus orientalis) and the Reed Parrotbill(Paradoxornis heudei), in three populations in China. We found that the Common Cuckoo(Cuculus canorus) is the only parasite using both the Oriental Reed Warbler and Reed Parrotbill as hosts, with a parasitism rate of 22.4%-34.3% and 0%-4.6%, respectively. The multiple parasitism rates were positively correlated with local parasitism rates across three geographic populations of Oriental Reed Warbler, which implies that higher pressure of parasitism lead to higher multiple parasitism rate. Furthermore, only one phenotype of cuckoo eggs was found in the nests of these two host species. Our results lead to two conclusions:(1) The Oriental Reed Warbler should be considered the major host of Common Cuckoo in our study sites; and(2) obligate parasitism on Oriental Reed Warbler by Common Cuckoo is specialized but flexible to some extent, i.e., using Reed Parrotbill as a secondary host. Further studies focusing on egg recognition and rejection behaviour of these two host species should be conducted to test our predictions.
