Effects Of The Prey Quality And Abundance Of Crabs On The Prey Selecetion Of Far Eastern Curlew(Numenius Madagascariensis) And Eurasian Curlew(Numenius Arquata)

To fuel the long-distance migration,shorebirds usually rely on abundant energy-rich prey from a series of important stopover sites.However,the loss and degradation of coastal wetlands has become the leading reason for the decline of shorebirds around the world,especially the Yellow(Bohai)Sea,which is an international conservation important area located on the East Asia-Australia Flyway.In the degraded habitats,comprehensive assessment of the foraging strategies of shorebirds and prey quality is of great significance for understanding the migration strategies,energy accumulation and threats faced by shorebirds.Far Eastern Curlew(Numenius madagascariensis)and Eurasian Curlew(Numenius arquata)are the two largest sympatric waders,which rely on large area of intertidal mudflat and Suaeda salsa saltmarsh in the Yellow(Bohai)Sea as refuel sites.Previous studies have shown that they mainly feed on crabs,and can involve other macrobenthos(i.e.,snails,clamworms(Nereis succinea)in their preys.Furthermore,among the tidal crab species,there are significant differences in prey quality among species,sex and size.It is of great scientific significance to investigate the feeding strategy of these two curlew species and potential ecological differentiation between them.In this study,we employed the behaviour observation of the foraging and pellet analysis to investigate the prey selection of Far Eastern Curlew and Eurasian Curlew in the Liaohekou National Nature Reserve,Liaoning,China.Besides,the potential effect of seasonal and interspecific differences of the crabs'prey quality on prey selection of curlews was also examined.Our results showed that:There was significant different on the prey composition between Far Eastern Curlew and Eurasian Curlew.The Far Eastern Curlew mainly fed on tidal crabs(80.0%),whereas the Eurasian Curlew mainly can feed on clamworm(41.6%)and tidal crab(33.14%).Further investigation showed that both curlew species mainly fed on Helice tridens tientsinensis(95.1%),followed by Macrophthalmus japonicus(4.36%)and Macrophthalmus dilatatum(0.57%).However,there was no significant difference on the prey quality between crabs(H.tientsinensis crabs:0.236±0.056;M.japonicus crabs:0.245±0.095;t=-1.539,df=553.415,p=0.124).Furthermore,there was no significant seasonal difference in composition of the crab species(?~2=3.337,df=1,p=0.077).The curlews tended to prey on female crabs.The proportions of female crabs in prey composition(H.tientsinensis:77.59%;M.japonicus:45.41%)were significantly higher than those in foraging areas(H.tientsinensis:25.76%;M.japonicus:34.10%).Furthermore,the selection of female H.tientsinensis crabs in spring(85.55%)was significantly higher than that in autumn(female sex ratio:69.32%;?~2=153.590,df=1,p<0.0001).However,there was no significant seasonal difference(spring:41.95%;Autumn:55.36;?~2=3.500,df=1,p=0.061)on the prey selection of sex ratio of the M.japonicus.Further analysis of the prey quality between the sexes of the two crab species showed that the prey quality of the female H.tientsinensis crabs was significantly higher than that of the male(male:0.229±055 vs.female:0.249±0.059;t=-3.393,df=388,p=0.001).However,there was no significant differences between male and female(male:0.239±0.101 vs.female:0.257±0.080;t=-1.685,df=347,p=0.093)for the M.japonicus.The curlews preferred to feeding on smaller H.tientsinensis crabs(prey:1.169±0.558 g;foraging site:2.342±1.357 g;t=-39.833,df=4811,p<0.001),but preferred to feeding on the larger M.japonicus crabs(prey:2.621±456 g;foraging site:1.917±1.252 g;t=6.458,df=648,p<0.001).In addition,there were significant seasonal differences in the selected the size of crabs.Size of crabs fed in spring(dry mass-H.tientsinensis crabs:1.160±0.674 g;M.japonicus crabs:2.930±0.469 g)was significantly higher than that in autumn(H.tientsinensis crabs:0.863±0.545 g;M.japonicus crabs:1.684±0.887 g).According to the relationship between prey quality and the size of crabs,we found that the prey quality of H.tientsinensis decreased with the crab size(especially for males),while it was not for the M.japonicus.Our results demonstrated that the prey selection of crab size and sex by the Far Eastern Curlew and Eurasian Curlew fitted the optimal feeding theory.It indicates that the prey quality is the main factor affecting the prey selection of curlews.However,it seem to be the prey selection among different species is not determined by the sole factor of prey quality.We speculate other factors,i.e.,the prey density or availability might be more important on the prey selection among different kinds of prey.We acknowledged that it was difficult to distinguish the species difference of prey selection between curlew species as a result of the pellets of the two curlew species were unable to separate from the morphology.Other more accurate sampling or molecular analysis based on the DNA in the pellets will be necessary to resolve this problem.