During migration, birds may store large quantities of fat to fuel long-distance flight. Because mass affects flight performance, fat migrants should behave with greater caution toward aerial predators than lean migrants. We tested a prediction of this hypothesis, namely, that fat birds will remain motionless longer than lean birds after a simulated encounter with a raptor. We captured Gray Catbirds (Dumetella carolinensis) during fall migration, classified their subcutaneous fat, exposed them to a model hawk, and then recorded the time until their first perch change. To determine whether the birds treated the simulated predator as a threat, we also tested them with a hexagon-shaped model and with no model. The birds remained motionless longer with the hawk model than either of the two other treatments. Fat birds remained motionless longer than lean birds when exposed to the hawk model, but we detected no difference with the other two treatments. These results provide evidence that fat affects the predator-avoidance behavior of Gray Catbirds during migratory stopover. That a difference between fat and lean groups was only apparent with the hawk model treatment suggests that the result is attributable to differences in the flight performance of the two groups rather than simply a difference in motivation to begin foraging. We suggest that fat should influence the behavior of a wide variety of migrant birds, particularly those species that typically carry greater fat reserves or that forage in more exposed situations than the Gray Catbirds we tested.
La Grasa Afecta el Comportamiento de Evasión de Depredadores en Dumetella carolinensis durante las Paradas Migratorias