The relationship of body weight, fat deposit and moult to the reproductive cycles in wild Tawny Owls and Barn Owls.
Annual variation in body weight and physiological condition has been described for very few predatory birds largely because of the difficulties in obtaining adequate sample sizes. Limited information is now available for four Falconiformes, the European sparrow-hawk (Accipiter nisus) (Newton, 1979) and kestrel (Falco tinnunculus) (Village, 1979), the Swedish goshawk (Accipiter gentilis) (Marcstrom and Kenward, in press) and the African species, Ruppell's griffon vulture (Gyps ruppellii) (Houston, 1976). However, published data are available for only one strigiform - the little owl (Athene noctua) (Marples, 1942) following an examination of 250 shot birds in New Zealand.
A large sample of dead tawny owls (Strix aluco) and barn owls (Tyto alba) collected as part of a pollution monitoring programme (Stanley and Elliott, 1976) afforded the opportunity of examining in detail annual cycles of body weight, condition, gonad development and moult. Data for the tawny owl are being published in greater detail elsewhere (Hirons et al, in prep.) but in this paper we compare variations in weight, fat content and moult between tawny and barn owls.