Fig. 4. Circannual reproductive cycles of European and Siberian stonechats. Birds were kept under a constant 12.25 h photoperiod for at least two years. Upper graphs show circannual testicular cycles of individual European males (left) and Siberian (right) males. Lower graph shows median cycles of European (solid black line and black closed circles) and Siberian (grey line and grey triangles) males; years are shown by vertical dotted lines. Data for European stonechats are replotted based on published data (Gwinner, 1991).