Sunrise, Skylight Polarization, and the Early Morning Orientation of Night-migrating Warblers

Field observations reveal that night-migrating passerines sometimes fly during early morning hours. Functional consideration of daytime flights have overshadowed proximate questions regarding the sensory basis of daytime orientation. I conducted a series of early morning cage-orientation experiments in spring with four species of migrating warblers (Parulinae) on the north coast of the Gulf of Mexico. Whereas birds were poorly oriented on overcast mornings, the headings for three species were oriented in a seasonally appropriate direction when individuals were tested on clear mornings with the horizon glow from the rising sun visible. Moreover, the direction of activity was influenced in a predictable way by manipulating the pattern of skylight polarization. Skylight at sunrise appears to function as a source of directional information for morning flights of migrating warblers. This interpretation is consistent with field observations of the orientation behavior of free-flying migrants, and reinforces speculation that solar-related information is involved in the orientation of ongoing migratory flight, or redetermined orientation following nighttime displacements. In any case, the responsiveness of several species in a cage situation may allow controlled experiments on the question of course correction and redetermined migratory flight.