Age-related Mortality in a Wintering Population of Dunlin

--Despite considerable evidence that juvenile shorebirds experience significantly higher annual mortality rates than adults, identification and quantification of the sources of mortality have received little attention. We found that the proportion of juvenile Dunlins (Calidris alpina) in the kills of a Merlin (Falco columbarius) one winter at Bolinas Lagoon, California was greater than the proportion of juveniles in the lagoon's winter population. This is evidence that raptor predation may be one of the factors contributing to the age differences in annual mortality rates of shorebirds. We suggest hat the greater vulnerability of juveniles to predation by the Merlin may be caused by age-related differences in Dunlin flocking behavior. Received 23 March 1983, accepted 6 September 1983. STUDIES of the population dynamics of shorebirds (Charadrii) indicate that, in general, juveniles experience higher annual mortality rates than adults (Goss-Custard 1980). This differential mortality has been reported for a wide range of European (Boyd 1962 and references therein) and North American species (e.g. Holmes 1966, Myers 1980, Page et al. 1983). Although a difference in survivorship between age-classes appears to be widespread among shorebird populations, little attention has been directed toward identifying and quantifying specific sources of mortality and evaluating their contribution to this pattern. Heavy mortality among first-year birds is generally thought to be the result of their relative inexperience in dealing with such selective pressures as predation, feeding efficiency, and extremes in environmental conditions (see references above); these hypotheses have not been tested empirically in shorebirds, how-