Avian studies have shown that food quality (amino acid load) can act as a limiting factor in reproduction and possibly breeding density. In owls, the role of food quality may have been largely overlooked due to the strong effect of food quantity. I examined evidence of food quality limitation in owl reproduction from four sets of data: (1) population data in Tawny Owls and diurnal birds of prey in a degenerate forest, (2) post-laying breast muscle conditions of Tawny Owls Strix aluco and Sparrowhawks Accipiter nisus which strongly relied on internal amino acid stores during egg formation, (3) amino acid deficiency-related embryo anomaly data in failed eggs of Barn Owls Tyto alba, Little Owls Athene noctua, and two reference species, and (4) growth of Tawny Owl chicks experimentally fed with day-old chickens or mice. The Tawny Owl population was found to be stable whereas sympatric bird of prey species greatly declined; breeding female Tawny Owls (n = 5) did not show a decline of breast muscle tissue as was found in Sparrowhawks. Embryo mortality from amino acid deficiency was estimated at 63% and 12% in Barn Owls and Little Owls, respectively. Growth rate of Tawny Owl chicks did not relate to food type. I conclude that egg amino acid investments can affect embryo viability in owls, but mainly in relation to reproductive strategy instead of environment. I hypothesize that owls can obtain additional amino acids from their caecal bacteria, which are lacking in birds of prey, and thereby are less dependent on amino acid availability in the food chain to acquire the amino acids needed for egg formation and chick growth.
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