Anti-predation And Anti-brood Parasitism Strategies Of Four Shrub Breeding Birds On The Western Sichuan Plateau

Breeding is a very important part of bird life history.The main factors affecting bird reproduction include food resources,nest-site quality,predation,nest parasitism,and so on.Sympatric breeding birds may have intense competition for resources,and in order to avoid such competition,they may show different preferences and vary in nest-site selection,nest characteristics,egg characteristics and reproductive success.In this study,four species of sympatric shrub breeding birds,as hosts or potential hosts of the parasitic cuckoos(Cuculus spp.),namely the Chestnut Thrushes(Turdus rubrocanus),Kessler’s Thrush(T.kessleri),Elliot’s Laughingthrush(Garrulax elliotii)and Grey-backed Shrike(Lanius tephronotus),were used as the research objects to investigate their nest site selection,breeding success,nest defense behavior,anti-predation and anti-parasitism strategies.The main results are as follows:By comparing breeding time,breeding population density,nest-site and egg characteristics,clutch size and other breeding parameters of the four sympatric shrub breeding birds,this study showed that Chestnut Thrushes had the highest breeding population density with a total of 88 nests found,followed by the Elliot’s Laughingthrush,Grey-backed Shrike and Kessler’s Thrush.The breeding time of the four species is from early May to mid-July,and varies in different years.In 2020,the Elliot’s Laughingthrush started to breed the earliest,and the Grey-backed Shrike was the latest,while the Chestnut Thrush started to breed the earliest in 2021,and the Kessler’s Thrush was the latest.All four species of birds were building nests in riverside shrubs,which are mainly alpine willows(Salix cupularis)and sea buckthorn(Hippophae rhamnoides).The nests were all bowl-shaped with an upward opening.The Kessler’s Thrush had the largest nest outer diameter and nest depth and the Grey-backed Shrike had the largest clutch size at 4.39±0.85(n=49).The Chestnut Thrush had the heaviest egg mass at 7.9±0.79 g(n=77),and the Chestnut Thrush had the largest egg length diameter of 31.83±2.25 mm(n=77)and egg short diameter of22.15±1.13 mm(n=77).The nesting success of the four birds varied in different years,with the highest nesting success in 2020 being the Grey-backed Shrike(60.8%;n=23),and in 2021 the Chestnut Thrushes(73.5%;n=34).The dummy experiments of predators and brood parasites were carried out in three species of birds.The results indicated that the Elliot’s Laughingthrush showed the most intense nest defense behavior,and the Chestnut Thrush had a moderate strength nest defense behavior,while the Grey-backed Shrike showed no response to any experimental dummy presented.However,all the three host species could not identify the predator,the parasite,and the control species that were not a threat to them.The playback experiments of calls of predators and nest parasites showed that all the three host species did not respond to these calls.In addition,a behavioral response test to alarm calls of other bird species showed that the Chestnut Thrush did not have any response to heterospecific alarm calls.In the egg recognition experiment,egg rejection rates of the Chestnut Thrush,Kessler’s Thrush,Elliot’s Laughingthrush and Grey-backed Shrike to non-mimetic blue model eggs were 62.5%(n=24),63.6%(n=15),100%(n=17),and 100%(n=20),respectively.There were significant differences in egg rejection between the Chestnut Thrush and Elliot’s Laughingthrush(Chi-square test,χ~2=6.1258,df=1,p=0.013),between the Kessler’s Thrush and the Elliot’s Laughingthrush(Chi-square tests,χ~2=4.5481,df=1,p=0.032),and between the Kessler’s Thrush and Grey-backed Shrike(Chi-square tests,χ~2=5.4279,df=1,p=0.019).Furthermore,there was highly significant difference between the Chestnut Thrushes and Grey-backed Shrike(Chi-square tests,χ~2=7.2647,df=1,p=0.007).In the Chestnut Thrush,there was significant difference in egg rejection between parent birds observing the experimental process and those without observing the experimental process,with the rejection rate of foreign eggs being increased.The egg recognition mechanism experiments showed that all the four host species tested have a true recognition mechanism(template-based recognition),and there was no direct correlation between current cuckoo parasitism rate and the egg recognition ability of the four hosts or potential hosts,as we did not find any case of cuckoo parasitism during the two breeding seasons.In conclusion,the present study found that breeding population density,nest-site and egg characteristics,clutch size and other breeding parameters of the four sympatric shrub breeding birds varied with species and years.Dummy experiments,model egg experiments and play-back experiments showed that the four sympatric shrub breeding birds also varied in their nest defense intensity and egg recognition ability,with all hosts having a true egg recognition mechanism.Our results of nest defense and egg rejection in the Chestnut Thrush and Elliot’s Laughingthrush supported the strategy facilitation hypothesis,while in the Grey-backed Shrike supported the strategy restriction hypothesis.Innovation:1.In this study,nest defense behavior,egg recognition ability and egg recognition mechanism of four species of sympatric shrub breeding birds,as hosts or potential hosts of the parasitic cuckoos,namely the Chestnut Thrushes,Kessler’s Thrush,Elliot’s Laughingthrush and Grey-backed Shrike were investigated and thus made contribution to a better understanding of coevolutionary interactions between parasitic cuckoos and their hosts.2.The Elliot’s Laughingthrush is a host of the large hawk cuckoo(Hierococcyx sparverioides),and the shrikes(Lanius spp.)are main hosts of the common cuckoo(Cuculus canorus),while the thrushes(Turdus spp.)are potential hosts of cuckoos.All the four bird species tested show high egg recognition ability suggesting that,different from the single-cuckoo parasitism system in Europe,the multiple-cuckoo parasitism system in China play a key role in the evolution of egg recognition and nest defense again parasitism.3.This study showed that nest defense intensity and egg rejection varied among the four sympatric bird species,suggesting different hosts use various anti-predation and anti-brood parasitism strategies.Results of nest defense and egg rejection from the Chestnut Thrushes and Elliot’s Laughingthrush supported the strategy facilitation hypothesis,while results from the Grey-backed Shrike supported the strategy restriction hypothesis.