Additional file 1: Figure S1. Exemplary picture from a temperature-sensitive radio transmitter attached to a wild Great Tit in the field. Photo by Aurelia F. T. Strauß. Figure S2. Skin temperature (10-min binned and unfiltered) against date and time for 18 Great Tits (female (F) and male (M)) kept in outdoor aviaries during winter. Colour indicates if days are rhythmic (black) or arrhythmic (red) according to the 3-day sliding window filtering. Two individuals were additionally...