Immature dunlins Calidris alpina migrate towards wintering grounds later than adults in years of low breeding success

Abstract

AbstractDunlin (Calidris alpina) females migrate ahead of males towards their wintering grounds, but the factors influencing the timing of this migration is incompletely understood. In our long-term (1991–2000) study of the autumn migration phenology of male and female dunlins, as adult and immature birds separately, we noted that the difference in mean migration dates between sexes was the greatest in years of high breeding success, reaching 5 days in immature birds and 7 days in adult birds. In other, relatively less successful years, this difference could reach 2 days in immature birds and 3 days in adult birds. As Dunlin females are not fully involved in parental care, the timing of their migration appears to be determined mainly by the timing of the start of the breeding season. In males, the timing of the autumn migration appears to depend primarily on the time spent on breeding and parental care. Hence, in years of high breeding success, with a high number of offspring, the males stayed longer at the breeding grounds. In the years with a high and moderate production of offspring, immature birds departed ahead of adults, probably due to their very low breeding success. A novel finding of our study is that in the years of overall low breeding success the opposite was true: immature birds migrated later than adults and much later than in other years. The plausible explanation is that lack of experience may result in greater tendency of immature birds to renest after failure of the first clutch, especially when high breeding failures of earlier arriving adult Dunlins may lead to decreasing competition at the breeding ground. However, in years of low breeding success, there was no difference in mean migration date between immature males and females, indicating that although immature birds stayed longer at the breeding grounds, they had a very low breeding success.