The Effect Of Host Egg Recognition On The Evolution Of Egg Size And Egg Color Of Common Cuckoo Cuculus Canorus

Host egg recognition is an important selection pressure for the evolution of egg mimicry in parasitic birds.The Common cuckoo(Cuculus canorus)is an obligate brood parasitic bird.What degree of the egg mimicry to its host,and if the host's egg recognition is the main driving force for the mimicry evolution of the cuckoo egg is an foundation scientific question? In this study,we choose the main host of common cuckoo in China,the Oriental reed warbler(Acrocephalus orientalis)as study species,to explore three main questions:(1)the mimicry degree of the cuckoo;(2)egg color or pattern matching between cuckoo egg and the warbler eggs during parasitism selection;(3)the effect of egg size and mimicry on the egg rejection of oriental reed warbler.The results are as follows:(1)The egg size of both cuckoo and Oriental reed warbler were smaller than other similar parasitism system(P < 0.05),however,the egg size of cuckoo was still larger than Oriental reed warbler in a little,and the Egg Shape Index(ESI)of cuckoo eggs was smaller than that of warblers(P < 0.05).(2)Spectrophotometer reflectance showed the egg color of background and spot of cuckoo egg were perfectly mimetic to Oriental reed warblers in visible light.Only the brightness of light spots was significantly higher than that of warbler(P < 0.05).The background color of the common cuckoo eggs was significantly lower than that of the host in UV light(P < 0.05),dark spots and light spots were significantly highly mimetic to the host in UV light(P > 0.05).The chromic of the background color and dark spots were highly mimetic to the host(P > 0.05).The chromic of the light spots were significantly lower than that of the host(P < 0.05).The egg hues of these two birds were highly similar in background color,dark spots and light spots(P > 0.05).(3)The cuckoo eggs were perfectly mimetic to oriental reed warbler eggs,particularly in spot size,spot density and spot coverage in the blunt and side regions(P > 0.05).However,the spot density and spot coverage were lower than that of warbler in the sharp region(P < 0.05).(4)There were no special matching in the egg size and color between cuckoo eggs and warblers eggs in the parasitized nests(P > 0.05).However,much highlymatching in one of important spot patterns,the spot density was found in the parasitized nests(P < 0.05).(5)The oriental reed warblers showed high egg recognition ability for different model egg,no matter the size and color mimicry.It seems to egg rejection rate was not affected by egg size and egg color,but the larger model eggs were ejected in a very lower rate(ca.39%),mostly due to the fact that large model egg exceeding the warblers' grasp index.In conclusion,we found that there was obvious mimicry in both egg size and egg surface,including color and spot between common cuckoo and oriental reed warbler in this parasitism system.The egg spot mimicry was one of the most important direction of this evolution,which was largely paid less attention,being similar to this study in our experiment design.Although we did not found evident effect of egg size and color mimicry on the egg rejection,but we could not exclude the important impact of rejection on the evolution of egg mimicry.We have to have a throughout thinking about the possible of innate drawback of hard material model egg on the rejection experiments,and encourage the use of 3 D printed model eggs in similar experiment future.