Abstract:We present experimental evidence for endogenous control of circannual rhythms in a long-distance migratory wader, the Knot Calidris canutus. Six individuals of the subspecies canutus were caught during northward migration whilst staging in France, and were held together under constant temperature and photoperiodic conditions (15°C, LD 12:12) for 20 months. Rhythms of contour feather and flight feather moult and of body mass change persisted throughout the experiment in most birds, and showed two cycles over this period. At the beginning of the experiment the six birds were synchronous, but in the course of the experiment inter-individual differences in their circannual periods developed. Desynchronisation between the rhythms of moult and mass change in the same individual occurred in some birds. The first cycle was delayed with respect to the natural schedule, while the second cycle had a duration of about a year on average. Since the second cycle was less well expressed than the first cycle in most birds, the endogenous rhythm is likely to be damped rather than self-sustained. The complexity of the annual changes in photoperiod experienced by transequatorial migrants like Knots calls for more extensive analysis to unravel the nature of the synchronisation of the endogenous circannual rhythm.