The Influence of Moonlight on the Timing of Roosting Flights in Common Cranes Grus grus

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Abstract
The timing of roosting flights of wintering Common Cranes Grus grus was correlated with the time of sunset for the whole population as well as for the last flock. Birds usually flew to the roost with a certain delay with respect to sunset. The magnitude of this delay was clearly associated with the amount of additional illumination from the moon. The delay in the roosting time of the last flock was almost esclusively determined by the amount of moonlight, while the average delay of the population increased linearly throughout the entire lunar cycle. This suggests that endogenous components might be involved in regulating the roosting time of the majority of the birds, while certain individuals adjust their roosting time as late as light conditions allow. Reduced food availability was most probably the factor determining the delay, as shown by the significant inverse correlations between both delay variables studied and food abundance in the study area.

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