Effects of exogenous melatonin on sun-compass orientation of homing pigeons.

Experiments were performed to test whether melatonin plays a role in sun-compass orientation of homing pigeons. Birds were kept for a period of time in dim continuous light (LL) or in artificial light-dark (LD) cycles and then released under the sun from unfamiliar sites. Control birds in dim LL were oriented homeward in all cases. Birds with melatonin implants in LD were capable of a correct use of the sun compass at release. Birds with melatonin implants in dim LL, on the contrary, performed very poorly in orientation. The present results demonstrate for the first time that melatonin is involved in the control of the circadian rhythms underlying sun-compass orientation in birds.