Coevolution Between Large Hawk Cuckoos And Its Hosts

Avian brood parasitism,a breeding strategy in which a parasite manipulates a host to raise its offspring,is a classical model system for studying co-evolution,while hosts have evolved different anti-parasitism strategies in the process of antagonizing the parasite which are used in each breeding stage of the hosts.In this study,the large hawk cuckoo(Hierococcyx sparverioides)and its common host,the Elliot's laughingthrush(Trochalopteron elliotii),and two potential hosts which are chestnut thrush(Turdus rubrocanus)and Chinese thrush(T.mupinensis)were selected as the research objects,to explore the differences of their anti-parasitic behaviors in different breeding stages and the coevolutionary relationship between the thrushes and the potential parasites.The main results and conclusions are as follows:1.By comparing the breeding time,nest size,egg size and other life history characteristics of three species,the results showed that the breeding time of two species of thrushes and large hawk cuckoo were synchronous,but because the breeding time of thrush was earlier than that of the laughingthrush host,we suggested that some of the nests of thrushes were not fully utilized by the parasites,which had a certain effect on the low parasitic rate of thrushes.The nest of chestnut thrush was bigger than that of laughingthrush,while the nest of Chinese thrush was smaller than that of laughingthrush.In addition,the eggs of the two thrush species were bigger and heavier than those of laughingthrushes.We assumed that the nest structure of thrush did not affect the young cuckoo to eject the egg out of the nest,but the difference in egg size might cause the parasitic eggs not to hatch effectively and result in reproductive failure.2.In this study,two thrush species and laughingthrush showed different preferences and differences in habitat types and nest site feature selection,with no significant difference in breeding success.Thrushes tended to nest in the forest with higher trees,while the laughingthrush chose the lower spruce nursery as the main nesting habitat,which supported the nest exposure hypothesis.The differences in nesting height and tree species height between thrush and laughingthrush were probably the result of competition among species in the same habitat,which made them occupy different ecological niches.3.In the experiment of egg recognition ability,the rejection rates of chestnut thrush and Chinese thrush to blue model eggs were 41.1%(n=34)and 83.3%(n=18),respectively.The rejection rates of laughingthrush to blue model eggs and white model eggs were 25%(n=8)and 46.1%(n=13),respectively,and there was no significant difference in the rejection rate of non-mimetic eggs among the three species.In addition,through the egg recognition mechanism,the results showed that both chestnut thrush and laughingthrush used the true recognition mechanism,while most of the Chinese thrush had the true recognition mechanism but a few had the discordancy recognition mechanism.We suggested that the egg recognition ability of two potential host thrushes might be preserved due to their history of parasitism with cuckoos,while the common hosts had evolved egg rejection ability due to the current parasitic pressure.There was no direct correlation between host egg recognition ability and current cuckoo parasitism rate.4.The responses of the potential host thrush and the large hawk cuckoo host laughingthrush to parasitic model,predatory model and non-threatening species model were different,but all showed aggressive behaviors,with chestnut thrush being more aggressive than Chinese thrush,which may be related to their anti-parasitic strategies.The aggressiveness of all three species of birds was stronger at the nestling stage than at the incubation stage,but the aggressiveness of all three species of birds to different specimens was not significant at any stage.5.The results showed that the chestnut thrush,which mainly fed on earthworms,could feed the young cuckoos,and did not have the ability to recognize the foreign nestlings.Therefore,the diet of the chestnut thrush was not the main factor affecting its rare cuckoo parasitism.To sum up,we found that the potential host thrush and the common host laughingthrush have some differences in life history and nest site selection,which may have influence on the parasitism rate of thrushes.In addition,we also compared and analyzed the egg recognition ability and the nest defense behavior of the three species.The results showed that all three species had egg recognition ability.Combined with the egg recognition ability and the intensity of the nest defense behavior of the thrushes,they exploited different strategies which supported the strategy restriction hypothesis.In addition,we found that the chestnut thrush could successfully feed the cuckoo chick,which should not be regarded as the main reason to avoid cuckoo parasitism.