Deconstructing the Morphological Basis of Ecogeographic Variation in the Hand-Wing Index, a Widely Used Proxy for Avian Mobility, in the Yellow Warbler ( <i>Setophaga petechia</i> )

AbstractBird species with high demand for efficient flight (e.g., migrants) tend to have more pointed wing tips than sedentary birds, and indices describing wing tip pointedness, such as the hand-wing index (HWI), are often used as proxies for dispersal propensity in comparative studies. Wing pointedness also varies among closely related populations of the same species that experience different selection pressures on flight, but we know surprisingly little about how variation in bone versus feather lengths contributes to wing pointedness. Here, we compare wing tip shape (HWI) of migratory versus sedentary populations of a widespread songbird, the Yellow Warbler (<i>Setophaga petechia</i>), to deconstruct variation in the individual skeletal and feather components of the hand-wing. Our results reveal that the relatively pointed wing shape of migrants is a consequence of shorter secondary feathers (i.e., a narrower wing) compared with nonmigrants, rather than longer wings. Indeed, despite having more pointed wings, migratory populations have similar wing length (i.e., wing chord) as sedentary continental populations. These populations show similar trunk size, but migrants have significantly shorter limb bones. Our results reveal the morphological underpinnings of a wing shape metric that has been widely used in macroevolutionary and macroecological studies of avian dispersal.