How sunrise and weather affect timing of rooks' (Corvus frugilegus) morning departure from the winter communal roost

Abstract. The pattern of morning departure of rooks (Corvus frugilegus) from large communal roosts in winter is regular though it is affected by several environmental (weather) variables. A total of 151 records of the morning departure of rooks (and associated jackdaws, Corvus monedula) from two large communal roosts in the Czech Republic during the years 1966 to 1974 were analyzed. On average, the birds departed 36 min before local sunrise and 1.5 min after beginning of local civil twilight. Light intensity was the leading factor that explained 60 % of variability of the departure time. Cloud cover 30 min before sunrise, being interrelated with the light intensity at local sunrise, explained 52 % of the variability. Less important but significant factors were several weather variables such as precipitation, relative humidity of the air, horizontal visibility (fog), snow cover, and air temperature. Two best predictive multivariate regression models for timing of the birds' departure involved factors: light intensity (that accelerated the departure) or inter-related cloud cover before sunrise (it delayed the departure), precipitation (delaying the departure), snow cover (accelerating the departure due to increased food demand), and horizontal visibility (fog delayed the departure). The two predictive multivariate models explained together 75 % of variability of the birds' departure in relation to beginning of local civil twilight.