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 that the greater vulnerability of juveniles to predation by the Merlin may be caused by age-related differences in Dunlin flocking behavior.