Effect of weather conditions on the communal roosting behaviour of common ravens Corvus corax with unlimited food resources

Avian time-activity budgets are known to be affected by many environmental factors. The aim of this study was to examine whether roosting times of common ravens Corvus corax are affected by inclement weather conditions in response to increased energy demands. The study was conducted at a communal roost of ravens in Thuringia, Germany, from August 2003 to January 2005. The number of roosting ravens fluctuated substantially, ranging from zero individuals in summer to 574 in winter. During the entire study period, a composting facility served as the main feeding ground for roosting ravens. There, the feeding-to-resting ratio did not depend on flock size, indicating the ravens were not food limited. Ravens arrived at the preroost and the roost later relative to sunset on cloudless evenings and on days with shorter length of daylight. Weather conditions measured as ambient temperature, wind speed, and cumulative daily precipitation had no effect on arrival times at the preroost and the roost. Likewise, moonlight did not affect roosting times. Apparently, flexible time-activity budgets resulting from a superabundant food supply may have enabled ravens to moderate the effects of environmental conditions on arrival times at the preroost and the roost.