How many juvenile Teal Anas crecca reach the wintering grounds? Flyway-scale survival rate inferred from wing age-ratios

摘要

Autumn postnuptial migration is critical in the dabbling duck annual cycle, when first-year birds in particular suffer high losses to natural and hunting mortality. Mortality rates in this age-class are generally unknown in Europe where winter ringing predominates. We used data from large-scale wing collections from hunters in Finland, Denmark and France to test the prediction that juvenile proportions among killed Teal (Anas crecca) would decline with distance along the flyway. As expected, this proportion decreased from 89% in Northern Finland to 58% in Western France. Potential biases linked with age determination from the wings, differential migration of age-classes, relative susceptibility to different forms hunting and gradual improvement of juvenile survival as they learn to avoid hunters could not explain the observed decline of juveniles in the shot population. This pattern was therefore considered to be genuine, the result of the cumulative depletion of first-years along the flyway, likely through hunting. On this assumption, combined with known adult monthly survival rates during August-November (94.2%), monthly juvenile survival rate was estimated at 52.8%, i.e. 14.7% (range 13.9-15.4% based on extreme values of adult survival) amongst Scandinavian juveniles reaching wintering quarters in Western France. Despite lack of precision in such estimates based on relative proportions, there is little doubt about the magnitude of autumn juvenile mortality and its consequences for the population dynamics of Teal. Lack of correlations between annual proportions of juveniles in the hunting bag and an index of Teal breeding success in Finland may result from such high and variable inter-annual mortality.