The Physiology and Geophysics of Bird Navigation

Radar observations often show nocturnal migrants well oriented under opaque layers of cloud, and perhaps even inside or between cloud layers. Some homing pigeons can orient towards home from unfamiliar territory under overcast skies. In searching for non-visual sensory capabilities of birds it is important to consider not only their sensory physiology but also the atmospheric ecology of migration when neither sky nor ground is visible. Either inertial naviation or orientation to the earth's magnetic field requires considerably greater sensitivity than any yet demostrated in birds. Natural or artificial sounds from the surface, or even ground echoes of flight calls, might facilitate orientation under some conditions. Birds communicating by flight calls might detect the differential wind drift experienced by upper and lower members of a flock, and thus determine the direction of wind shear. Under many meteorological conditions such patterns of air movement as roll vortices or internal gravity waves produce readily detectable updrafts capable of providing both lift and information about the direction of the wind or wind shear.