Gonad development, moult and seasonal changes in body weight and composition in the Tawny owl Strix aluco were studied by examining the carcasses of 369 owls (mostly road casualties) supplemented by 112 weights of live birds. In breeding females laying was preceded by the accumulation of fat and to a lesser extent protein which meant that they weighed more at this time (February/March) than at any other. Females declined in weight after laying but were still heavy during incubation. In contrast, males and non-breeding females did not increase in weight before the start of the breeding season. Juveniles reached or even exceeded adult weight well before independence due to the deposition of fat. Even after the exclusion of diseased or contaminated individuals, 9·4% of the birds examined were identified as starving; most of these were in the autumn and were probably newly-independent young wandering in search of territories. In both sexes gonad maturation was of brief duration coinciding with the period (mid-March to mid-April) in which eggs are normally laid. Ovarian growth was biphasic. In the three months prior to the breeding season ovarian condition in different birds was positively correlated with body weight and it appeared that the largest ovarian follicles of females in poor condition failed to attain the size from which rapid growth to final ovulation occurs. in males testis size in the breeding season was correlated with pectoral muscle weight (an index to protein condition) but not body weight. The majority of adults commenced wing moult in June. The average duration of primary moult was estimated to be 77 days. Healthy birds replaced the primaries of both wings at the same rate but most diseased birds moulted asymmetrically and/or out of season. First-year birds renewed their body feathers between September and November. In the Tawny owl territory establishment, breeding and moult are temporally separated.