The importance of body reserves for successful reproduction in the Tawny owl (Strix aluco)

Abstract

With 3 figures in the text)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.

Effects of Variable Food Supply on the Body Condition of Breeding Tawny OwlsStrix alucoin Southern Finland

“…In Europe, the southern populations of vole-eaters, for example, common kestrels Falco tinnunculus [22,23] and tawny owls Strix aluco [7], show, in general, relatively minor annual variations in body mass, characterising the relatively stable prey base of the region [24] and generalist foraging habits of the species [25]. Between seasons of year, however, variations may be considerable [17,23].…”

“…[7,21]) but the variations should be dampened according to availability of alternative prey [29,31,34,35].…”

“…[18,45,46]). Breeding is probably a comprehensively strenuous duty for the body condition of males [7,13,17], while in females condition first increases before the onset of egg laying and then declines during breeding [4,14,20] at a rate determined by the local availability of food and foraging capacity of the male.…”

“…Birds of prey are capital breeders that acquire their resources in advance and store them until they are needed to supply aspects of offspring production [5,6]. The sufficiency of their body condition for breeding seems to be largely determined by the foraging conditions during the months immediately preceding the onset of egg laying [7,8]. The main strategies to overcome the negative effects of variable food supply are (1) to follow the abundance of the staple prey spatially (nomadic specialists), (2) when necessary, to rely on some alternative prey (stationary generalists), or (3) to abandon breeding altogether [9][10][11][12].…”

Microhabitat Features at Mexican Spotted Owl Nest and Roost Sites in the Guadalupe Mountains

“…For example, Hirons (1985) has shown than large body reserves of far and protein are essential during incubation by female tawny owls for successful reproduction. Abundant prey populations during winter and early spring periods increase the likelihood of egg laying and decrease the rate of nest abandonment (Hirons 1985).…”