| Long-term monitoring of bird population abundance and age ratio is necessary to understand the species population size and trend,as well as the important population parameters such as reproduction rate,mortality and life expectancy,providing important basic data for further study of the impact of environmental changes on population dynamics mechanism.Yangtze River floodplain is essential for wintering waterbird in East Asia.In this study,we summarized population abundance and trend of 10 large-bodied species wintering in Yangtze River floodplain,based on long-term monitoring data and literature investigation,to try to understand their waterbird population dynamics mechanism.We also calculated the average annual mortality and life expectancy based on age ratio monitoring data.Finally,we established the systematic monitoring scheme for a better and long-term age ratio survey.The main results are listed below:During January and February of 2003/2004 to 2019/2020,we conducted wintering waterbird survey in Yangtze River floodplain,together with the Nature Reserve institutes.the age ratio of 10 large-bodied species were checked in autumn and early winter,including waterbird Common Crane Grus grus,Siberian Crane Leucogeranus leucogeranus,Hooded Crane Grus monacha,White-naped Crane Antigone vipio,Swan Goose Anser cygnoides,Lesser White-fronted Goose Anser erythropus,Greater White-fronted Goose Anser albifrons,Greylag Goose Anser Anser Bean Goose Anser fabalis and Bewick's Swan Cygnus columbianus,to study population dynamics.We summarized the population size and trends estimates for three Gruidae species,based on six synchronous wintering waterbird survey including Common Crane,Hooded Crane and White-naped Crane.Based on early published data of annual wintering number and population estimates of seven species(Siberian Crane,Swan Goose,Lesser White-fronted Goose,Greater White-fronted Goose,Greater White-fronted Goose,Greylag Goose,Bean Goose and Bewick's Swan),we got the wintering population trends of the ten species in Yangtze River.The results showed that 7 species of large waterbird(Siberian Crane,Hooded Crane,White-naped Crane,Swan Goose,Lesser White-fronted Goose,Greater White-fronted Goose and Bewick's Swan)are considered to be in decline,while the remainder 3 species are increasing.In order to explain the population trend of wintering waterbird in the Yangtze River,a 4-year investigation on the age ratio of these 10 waterbird species in the Yangtze River was carried out from November to December every year from 2016 to 2019,that is,the ratio of the young birds of each species to the total population was obtained through the identification of adult birds and young birds.The results showed that the age ratio of these 10 species of waterbird changed in the same trend in four years,all of which were V-shaped,that is,the results of the survey in 2017 were the minimum value of the data of the age ratio of all waterbird in four years.The most likely reason was considered that the reproductive success rate of these 10 species were relatively low in Russia(Siberian Crane,Hooded Crane,Lesser White-fronted Goose,Greater White-fronted Goose,Bean Goose and Bewick's Swan)and in Mongolia(White-naped Crane,Common Crane,Swan Goose and Greylag Goose)during the breeding period in 2017,The average age ratio of 7 species of waterbird(Siberian Crane,Hooded Crane,White-naped Crane,Swan Goose,Lesser White-fronted Goose,Greater White-fronted Goose and Bewick's Swan)with declining population trend is 0.168,while the average age ratio of 3 species of waterbird(Common Crane,Greylag Goose and Bean Goose)with increasing population trend is 0.173.The results of student's test showed that the success rate of reproduction is not the dominant factor affecting the population trend(P=0.6013).The average mortality of 10 waterbird species showed that the average mortality of 7 waterbird species with declining population trend was 0.245,and that of 3 waterbird species with increasing population trend was 0.125.The results of student's test show that there are significant differences between them(P=0.006208).Therefore,the high mortality is the leading factor for the decline of wintering population in the Yangtze River.At the same time,the life expectancy of the declining species is 14 years old,while that of the increasing species is 13 years old.Although there is no significant difference between the student's test results(P=0.2822),the declining species still shows a tendency that the life expectancy is slightly higher than that of the increasing population,and correspondingly,its reproductive success rate is slightly lower than that of the increasing population.This phenomenon indicates that the declining population may deepen the "K-strategists" which prolongs the life expectancy of adult birds and reduces the reproductive success rate to regulate the population steady state.The average age ratio of the 10 waterbird species in 4 years is 0.169,and the age ratio of 5 species is lower than the average,including 3 threatened species in the world(Siberian Crane,CR;Swan Goose,VU;Lesser White-fronted Goose,VU),and Greater White-fronted Goose and Bean Goose.The average age ratio of the remaining 5 species of waterbird(Common Crane;Hooded Crane,VU;White-naped Crane,VU;Greylag Goose;Bewick's Swan)is higher than the average.Therefore,it is necessary to focus on the monitoring and protection of the wintering populations of globally threatened species with the declining trend of these three populations.In summary,the population of most species wintering in Yangtze River floodplain are declining,which is thought to be caused by high mortality rather than low reproduction rate.The above work shows the significance of long-term bird monitoring,but the field work is arduous.In order to monitor their age ratio in Yangtze River efficiently,two key monitoring indicators("sites of major flyway importance" and "key points")were introduced,and based on the model results of data from 2016 to 2019,a systematic monitoring scheme was established.The specific contents are as follows:1.Based on satellite tracking data and field investigation results,it was found that most of the 10 species wintering in Yangtze River arrived in November.Therefore,the investigation period was shortened to 15 days(December 1-15)from the original 30 days(November-December),during which the feathers of young birds can still be distinguished from adults;2.Due to the change of waterbird distribution,a total of 17 lakes were investigated in 4 years,with an average of 12 every year.The total points are 501,with an average of 125 points every year.Based on the concept of"investigating lakes with the largest number of birds,increasing investigation points,and avoiding investigating lakes with a small number of birds","sites of major flyway importance"(that is,lakes in which total number of birds in one year or more has reached 5%of the total number of the species surveyed in that year)is retained.There are 6 lakes that meet this criterion(Poyang Lake,East Dongting Lake,Shengjin Lake,Longgan Lake,Caizi Lake and Chen Lake),and the total number of"key points"in these lakes in 4 years is 258;Further delete and optimize the "overlapping sites of survey field of vision",and add new points in the areas not covered by the previous survey,and finally set up 155 "key points" as the survey points of the monitoring scheme for the age ratio of large waterbird in Yangtze River wetlands.Based on the results of "analysis of differences in the age ratio of birds in monitoring period,sites and points",the differences between the results obtained by the monitoring scheme recommended in this paper and the annual survey results were less than 5%,At the same time,the analysis of the complete independence hypothesis test also shows that compared with the Gruidae species(4 species),families with young birds in the Anatidae(6 species)prefer to gather together rather than randomly distributed.Based on this important discovery,the minimum total number of each species is calculated.The total number of 10 species is as follows:Common Crane(1,511),Siberian Crane(1,129),Hooded Crane(1,744),White-naped Crane(1,588),Swan Goose(2,761),Lesser White-fronted Goose(826),Greater White-fronted Goose(6,364),Greylag Goose(1,312),Bean Goose(8,955)and Bewick's Swan(5,844).This study pointed out the direction of follow-up research.While conducting systematic monitoring of wintering waterbird population in Yangtze River,it further investigated the time,place and cause of death,and provided the basis for the precise protection of declining population. |
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