| The clutch size and sex ratio of offspring of many birds vary with the breeding time,which is usually considered to be related to the change of food resources in habitat and the influence of the birth time on the lifetime reproductive achievement of offspring.Due to the decrease of the food resources in the breeding area with time,the clutch size usually decreases with the time of reproduction.The variation pattern of sex ratio of offspring is different among species,which is related to the difference of disadvantage and lifetime reproductive achievement of individuals of different genders affected by environmental conditions and fledgling time.At present,there are several hypotheses about the physiological mechanism of birds regulating the sex ratio of offspring,some mechanisms may lead to a large amount of energy loss and longer oviposition interval,which means parents need to pay a certain price for regulating the sex of offspring,but also means that sex allocation can provide considerable fitness return for them.Studying the changes of clutch size and sex allocation pattern with breeding time can help us to understand how birds adjust their breeding strategies,weigh the reproductive investment and fitness return,according to environmental and their own conditions.Crested ibis Nipponia nippon is an asynchronous hatching bird.The male nestlings need more energy during their growth and development,and they usually locate in the front hatching order.This phenomenon suggests that crested ibis can regulate the sex ratio of offspring according to the laying sequence,and has the ability of sex allocation.However,the specific mechanism of female regulating the sex ratio of offspring is still unclear.The crested ibis mainly feeds on fish,shrimp,loach,and other aquatic animals.A large number of studies have shown that the food richness of carnivorous birds decreases rapidly with time during the breeding period.However,there is no study on how the crested ibis,which began to lay eggs late,adapt to this situation.In this study,wild crested ibis was taken as the research species,and their breeding season was divided into three periods: early period,middle period,and last period.Using the clutch size data of crested ibis and body traits and gender data of their nestling in 2017 and 2018,we analyzed whether the clutch size and the sex ratio of nestlings changed with the breeding time.In addition,we analyzed the relationship between the oviposition interval and the sex of nestlings,to explored the potential mechanism of sex allocation of crested ibis.The main results are as follows:1.The clutch size declined with the breeding timeBy analyzing the relationship between the first-egg-laying onset and the clutch size in 101 nests of 2017 and 2018,we found an obvious seasonal declined in the clutch size(F=7.443,df=1,P<0.01).There were 18 nests in the last stage,of which 9 nests had only one egg,and the average number of eggs was 1.67 ± 0.84(mean ± se,n=18),which was significantly lower than the previous two periods(2.26 ± 0.70,mean±SE,n=27;2.16±0.78,mean ± SE,n=56).It is suggested that crested ibis can cope with poor environment or its own conditions by reducing the clutch size.2.The sex ratio of nestlings declined with the breeding timeBy analyzing the sex and breeding time of 212 crested ibis nestlings in 101 nests,we found that the sex ratio of crested ibis nestlings declined with the breeding time.there was no bias of sex ratio of nestlings at the early period(55.7%,n=61,P>0.05),while at the last period,it was significantly female biased(30.0%,n=30,P<0.05).The trend of sex ratio declining with time is mainly reflected in the first hatched nestlings,a linear regression model was established based on the sex of the first hatching nestlings and the first-egg-laying onset,The sex of the first egg gradually became female with a significant trend(F=12.61,df=1,P<0.01),while the sex ratio of other hatching orders does not change with the breeding time.The clutch size has a certain influence on the pattern of nestling's sex ratio changing with time.The phenomenon that the sex ratio of first hatched nestlings declined with the breeding time is most obvious in the nests with 2 nestlings.This result is consistent with the prediction of Trivers-Willard hypothesis,that is,when females are at a disadvantage state,they reproduce more female offspring that are less sensitive to external conditions.3.The mechanism of crested ibis is related to the prolongation of oviposition intervalIn order to explore the potential mechanism of crested ibis regulating the primary sex ratio of offspring,this study analyzed the relationship between the sex ratio and the oviposition interval of 109 nestlings whose hatching order of the 2nd,3rd and 4th.The results showed that when the oviposition interval was 2-4 days,the sex ratio of nestlings was significantly more than that of females,but when the oviposition interval was less than 2 or more than 4 days,there was no bias of sex ratio.It is suggested that the primary sex ratio of crested ibis may be regulation by the mechanism which may lead to the prolongation of oviposition interval,such as selective reabsorption,sex-specific fertilisation or atresia the follicle in hierarchical phase.But it should be noted that,there should be other mechanisms to regulate the primary sex ratio of offspring,which needs further study.This study found that the breeding strategy of crested ibis changed with the first-egg-laying onset,and discussed the physiological mechanism of female crested ibis regulating the sex ratio of offspring,which has reference significance for the protection of crested ibis and provides theoretical basis for the related research on sex allocation of birds. |
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