AGE AND SEX DIFFERENCES IN WING LOADING AND OTHER AERODYNAMIC CHARACTERISTICS OF MERLINS

Abstract

We examined age and sex differences in wing loading, aspect ratio, and wing span in a sample of 208 Merlins (Falco columbarius) captured at Cedar Grove, Wisconsin, during fall migration, 1978–1993. We also examined differences in tail loading of 166 of these Merlins. Adult males had significantly greater mass and wing loading than juvenile males. Adult females differed significantly from juvenile females only in mass. Females were significantly greater than males in every measure except aspect ratio. There were no apparent age differences in tail area or flight surface loading, but females had greater values in both. Merlins show fewer age differences in aerodynamic characteristics than Sharp-shinned Hawks (Accipiter striatus), probably because of differences in how the two species pursue and capture avian prey. Merlins usually capture prey in the air, sometimes after multiple stoops and at high flight speeds. In contrast, Sharp-shinned Hawks take birds from their perch or after a brief chase, often in dense vegetation, at relatively low flight speeds. Slower flight speeds require larger control surfaces and can explain the increased age differences in wing and tail areas in Sharp-shinned Hawks.