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One-quarter of Tawny owl nests fail to hatch young, mainly because the eggs are chilled and/or deserted. In 1973–74 automatic photography was employed at four nests near Oxford to relate the incubation behaviour of females to the ration of prey supplied to them by their mates. The eggs did not hatch in two nests and young fledged from only one of the others. Females were less attentive at the nests which failed during incubation and on average received less prey, but even at successful nests there were some nights when the female was supplied with less than her estimated daily food requirement. Female Tawny owls accumulate large reserves of fat and protein before laying. These buffer against any temporary inability of the male to provide sufficient food during incubation, and enable the female to stay on the nest rather than hunt for herself and risk the eggs becoming chilled. When prey are scarce, many females do not lay at all, and the ultimate factor determining whether breeding takes place may be the female's ability to acquire body reserves sufficient to provide a chance of breeding successfully.