ABSTRACT
It has been shown that the waking-time of the song-thrush (Turdus philomelus) and blackbird (T. merula) are accurately regulated by the time of sunrise (1,3), but that in the case of the starling (Sturnus vulgaris) the correlation is not complete (5). Thus if the time of sunrise throughout the year is plotted graphically, it forms a harmonic curve. The daily times of dispersion of starlings from a roost (i.e. from about July to mid-March) also form a harmonic curve, but its amplitude is less than that of the sunrise curve. In August, for example, the birds leave the roost 40–50 minutes after sunrise, and in January 20–25 minutes before, weather conditions being uniform. There is thus a factor which tends to keep the duration of sleep or of activity constant, which acts as a brake on the changing sunrise factor. This second factor appears to be a physiological rhythm.