Title: FLIGHT BEHAVIOR OF MIGRATORY RAPTORS IN RELATION TO MORPHOLOGY, WEATHER, AND AGE

Program: Raptor Biology MS

Committee Chair: Julie A. Heath

Committee: Julie A. Heath, Jen Cruz, Laurie Goodrich, Kimberly Thompson

Abstract: Migrating raptors use a mix of soaring, gliding, and flapping flight to reduce the energetic cost of long-distance movements, but how species- and age- specific flight behavior changes in response to weather remains unknown. We examined the flight behavior of three morphologically distinct Neotropical migrants: Broad-winged Hawks (Buteo platypterus), Swainson's Hawks (Buteo swainsoni), and Turkey Vultures (Cathartes aura) during Fall migration across the Isthmus of Panama, a migratory bottleneck with variable weather conditions. We hypothesized that wing loading, weather, or the interaction between wing loading and weather would influence flight behavior. We predicted that lighter species would use flapping flight more often than heavier species. During 30-second observations (N = 1584) we tallied full wingbeats (flaps) of individual birds. Broad-winged Hawks (lowest wing loading) had the highest flap rates, while Swainson’s Hawks and Turkey Vultures flapped less. Both hawk species flapped during periods of higher wind speeds and weak thermals. In contrast, Turkey Vultures flapped more at lower wind speeds and less at high wind speeds, regardless of thermal presence. This pattern may reflect vulture’s strategy of briefly flapping to intercept rising air, then relying on efficient, high-aspect ratio, dihedral wings to remain airborne – even in turbulent conditions. Juvenile hawks flapped more than adults, perhaps because of inexperience in locating or exploiting lift. These findings are consistent with previous studies of wing loading and flight behavior, and highlight that other morphological traits, such as wing posture and angle (dihedral), modulate flight behavior responses to weather. These results underscore how weather conditions shape flight behavior in migrating birds and produce species-specific shifts in behavior that could influence flight costs.