Comparison Of Counteradatations In Four Sympatric Host Species To Defense Against Cuckoo Parasitism

Nest-site choice is the beginning of avian breeding which selected safe and suitable breeding places through evaluated environmental factors like that predation/parasitism risk,climate conditions,food resources,inter-species or/and intra-species competition and human interference.Thus the quality of nest-sites directly affects reproductive success.The confrontation between adaptation and counter-adaptation of avian brood parasites and their hosts is a good example of co-evolution.Avian brood parasitism,such as parasitic cuckoos,brings the hosts a great deal of breeding cost which forced hosts to evolve counter-adaptive strategies throughout different breeding stages.This study investigated the characteristics of life-history and anti-parasitic strategies in four sympatric host species of common cuckoos(Cuculus canorus),namely Oriental reed warbler(Acrocephalus orientalis),blunt-winged warbler(Acrocephalus concinens),reed parrotbill(Paradoxornis heudei)and vinous-throated parrotbill(Sinosuthora webbiana),to explore the co-evolutionary interactions between cuckoos and their hosts.The main results and conclusions are as follows:1.The four sympatric hosts of Oriental reed warbler,blunt-winged warbler,reed parrotbill and vinous-throated parrotbill,all breed within the same reeds which their nests were randomly mosaic distribution.However,there was differentiation in breeding phenology and nest-site microenvironment,so as to reduce competition among species and reach coexistence.Two species of parrotbills were local resident and breeding earlier than two migratory warbler species.The nest sites and nest height of four species were spatially differentiated with which the dominant population,Oriental reed warblers,was significantly higher than the remaining 3 bird species.The main factors that affected breeding success of four breeding species were nest predation,bad weather,and parasitic parasitism.In addition,avian brood parasitism was also an important factor reducing breeding success of Oriental reed warblers.2.Floods and other extreme events have disastrous effects on breeding birds.Nests were located significantly higher in the vegetation after compared to before the flood(0.75?0.26 m vs.1.17?0.13 m,P<0.01),revealing that reed parrotbills demonstrated behavioral plasticity in their nest-site choice.Nevertheless,it was possible that the reed parrotbills could not accurately grasp information about their suddenly changing environment due to the rare and unpredictable nature of flooding events,which may increase their predation rate significantly after the flood(67% vs.25%,P=0.030),leading to ecological trap.3.Oriental reed warbler,blunt-winged warbler and reed parrotbill were all found to be parasitized by the common cuckoo.There was no significant variation of parasitism rate between two years for different hosts(Fisher's exact test: all P>0.05),but the parasitism rate of different hosts was distinct which couldn't completely support for the habitat imprinting hypothesis(Fisher's exact test: P=0.001).Oriental reed warbler was the main host of common cuckoo in the study area with which more than 90% of the cuckoo eggs found in their nests.Furthermore,cuckoo eggs perfectly mimic to Oriental reed warbler eggs includ ing both the background color and spots.Nevertheless,the cuckoo would also parasitize other sympatric host with similar nest-sites,presenting parasitic strategy of a main host and the rest as secondary hosts.4.The attack rate of Oriental reed warblers against common cuckoos was 100% and neighboring individuals participated in up to 87.1%(n=31)of such attacks.Furthermore,the number of hosts attacking common cuckoos was significantly positively correlated with the number of neighbors within 40 m(r=0.389,n=31,P=0.037),indicating social defense behavior.Isolated Oriental reed warbler nests with more distant neighbors were more vulnerable to common cuckoo parasitism.The distance to the nearest neighbor of unparasitized nests was significantly shorter than that of parasitized nests(Wald chi-square=6.541,df=1,P =0.011).5.Results from egg recognition experiment showed that a ll the four hosts had a certain degree of egg recognition ability suggesting a coevolutionary arms race between them and cuckoos.Both Oriental reed warbler and vinous-throated parrotbill could reject the experimental eggs up to 100%,but the rest two hosts only have intermediate egg recognition ability.The difference may be due to different co-evolution stages with cuckoo which leading to different parasitic pressure.All of the four hosts were given priority to choose puncture to reject experimental eggs,and the larger grasp index of Oriental reed warbler allowed egg rejection with grasp.In addition,Oriental reed warbler could recognize and reject the experimental eggs in the nest using the template recognition(true recognition)mechanism.6.Oriental reed warblers not only exhibit strong nest defense behaviors,but that they also employ a variety of defenses intended to expel and attack any intruders.In addition,nest defense behaviors did not change with nest stage in highly aggressive Oriental reed warblers.Assistance from conspecific helpers was common and may increase Oriental reed warblers' attack intensities.We hypothesized that cooperative defense probably evolved in Oriental reed warblers as an anti-parasitic strategy through long-term coevolution with common cuckoos.However,Oriental reed warblers could identify unfamiliar predators,such as sparrow hawks(Accipiter nisus),which does not support the hawk mimicry hypothesis of cuckoos.But the strong aggression of the Oriental reed warbler also makes it more aggressive against the sparrow hawks.In summary,we found that there was differentiation in the characteristics of life history for four sympatric birds in order to make coexistence.Behavioral plasticity in nest-site selection of birds could not be enough to deal with abrupt changes in the environment.Cuckoos may use parasitic strategy of a main host and the rest as secondary hosts to achieve maximum breeding success.Cooperative defense probably evolved in Oriental reed warblers as an anti-parasitic strategy through long-term coevolution with common cuckoos which could reduce parasitism risk effectively for aggressive host,but it still needs further study to see whether the cuckoo has corresponding counter-adaptive strategies.