Occurrence Of Cooperative Breeding In Azure-winged Magpie Colonizing The Tibetan Plateau

Cooperative breeding refers to a breeding unit containing extra individuals rather than parents that care for the young,with the alloparental care being the typical character of cooperative breeding.Ever since molecular-based parentage analysis was introduced into avian research,more and more cooperatively-breeding species have been reported.Many hypotheses try to demonstrate what factors drive the occurrence of cooperative breeding,including kin selection hypothesis,ecological constraint hypothesis,and phylogeny hypothesis.But up to date,they have not reached a consensus.In this study,the azure-winged magpie,Cyanopica cyana,of which a new population began to colonize the northeast Tibetan Plateau about ten years ago.To address what factors are correlated with the dynamics of cooperative breeding in this high-altitude population of the azure-winged magpie,I tested 1)the variation of annual proportion of cooperative groups with their colonization history;2)whether dominant breeders in cooperative groups adjusted their primary and secondary offspring sex-ratios according to the identity of helpers;3)how dominant females allocated their reproductive rights between different types of males.The major findings are listed below.First,the proportion of cooperative groups within this high-altitude population changed with the colonization year as a quadratic curve.That is,the proportion of cooperative groups was low at the onset of colonization,which increased before it reached the top level and then decreased.The identity of helpers also changed with the colonization process and the proportion of female helpers significantly increased with the colonization year.This result indicates that when vacant niches are occupied gradually by enlarged population in the colonization,even females fail to obtain chances of independent breeding and become helpers.It thus provides new evidence for the ecological constraint hypothesis.Second,dominant females adjusted the primary offspring sex ratio according to the body condition of their social mates.Those females paired with better-quality males produced more sons,and the proportion of sons decreased with brood size and increased with the number of core offspring.It seems that females may control the quality of sons and the quantity of daughters by adjustment of primary offspring sex ratio.The growth of nestling body mass showed that sons had larger body mass than daughters,but those smaller daughters can also fledge successfully.This result implies that parents ensure the quality of sons by providing more food towards sons than daughters,and simultaneously they ensure the survivorship of daughters by increasing the total amount of food via the assistance of helpers.Last,dominant females and males both engaged in high frequency of extra-pair copulations(EPCs).Although there were opposite-sexed helpers within the cooperative group,neither dominant female nor male tended to mate with helpers;they,on the contrary,engaged EPCs with out-group individuals.Dominant females allocated their reproductive rights to three types of males,including social mates,male helpers and out-group males.Dominant females in EPCs preferred to select distant-related males rather than their social mates,although the difference did not reach a significant level owing to the small sample sizes.The reproductive sharing of male helpers within the cooperative group was positively related to their relatedness with the dominant female,whereas the reproductive sharing of either dominant male or out-group male was unrelated to their relatedness with the dominant female.In conclusion,this study reveals that,when azure-winged magpies intrude the high-altitude habitats,the occurrence of cooperative breeding changes with the number of vacant niches.Dominant females adjust both primary and secondary offspring sex ratios that seems unrelated to the identity of helpers.When dominant females allocate their reproductive rights,they prefer out-group males rather than male helpers.The reproductive sharing that dominant female allocate to male helpers is based on their relatedness to male helpers.