The natural control of a population of Tawny owls (Strix aluco)

Abstract

A long‐term study was made (1947‐59) of the numbers and breeding success of the Tawny owl (Strix aluco L.) in a woodland habitat near Oxford. Parallel studies were made of the numbers and distribution of the owl's two main prey species, the Wood mouse (Apodemus sylvaticus (L.)) and the Bank vole (Clethrionomys glareolus (Schr.)). The life‐cycles of all these three interrelated species were worked out and special attention was paid to the habit of strict territoriality of the owl (on the evidence of vocalizations and of the recovery, from the regurgitated pellets of the owl, of marking rings placed on the rodents) and to the sequence of losses which occurred during the breeding of the owl. The owls' vocal defence of their territories enabled an accurate census to be made of the adult population each spring and the fact that the fledged young remained for a long time in their parents' territories made it possible to count the number of young produced each year. Investigation of the breeding habits of the owls showed that the number of fledged young produced fell far short of what was possible: some pairs of owls refrained from breeding at all, others laid eggs but failed to hatch them, yet others hatched young but failed to rear them to fledging. By and large this degree of reproductive “failure” was associated with the numbers and availability of rodent prey. At an exceptionally low density of prey no owls even attempted to breed; as densities increased, correspondingly greater numbers of fledged young were produced up to a ceiling where no more young were produced however much higher the level of abundance of the rodents rose. In spite of these wide variations in the numbers of prey and in the numbers of fledged young produced, the population of adult owls remained notably stable, increasing from a low level of 17 pairs in 1947, owing to the emergency mortality of an exceptionally hard winter, to about 30 pairs in 1955, after which the population remained stationary until the end of the study in 1959. It is clear that this stability is maintained by those young which fail to find a territory either starving or moving outside the study area, which in most cases amounts to the same thing. Thus regulation of the population studied was due to territorial intolerance acting to produce subsequent mortality, though it is not certain that this applies over a larger area.

Factors Affecting Timing of Breeding in the Tawny Owl

“…8,10,20]. In the North, the availability of food for breeding tawny owls seems to be particularly affected by the pronounced and more or less cyclic fluctuations of vole populations [21][22][23][24] and it is modified by habitat [25,26] and various climatic factors [8,20,22,27,28].…”

“…Birds were sexed on the basis of their size, wing length, body mass and the prominent incubation patch of the females [9,20,31]. I determined the age of birds by the plumage patterns [32].…”

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

“…In owls, reproductive success is often correlated with prey abundance (Craighead and Craighead 1956, Southern 1970, Lundberg 1976, Wendland 1984, Korpimiiki and Norrdahl 1991. The postulated mechanism behind this relationship is energetically based.…”

Seasonal changes in Tawny Owl (Strix aluco) diet in an oak forest in Eastern Ukraine

“…This results in a substantial number of winter pellets found under winter roosting sites, while summer pellets are widely distributed between numerous hollows and perches used for roosting in summer time. As a result, the summer diet of the Tawny Owl is less known than the winter diet (Southern, 1970). Furthermore, very few pellets are usually found under their breeding hollows during the breeding season (Yatsiuk, 2009).…”

“…These rodent species show noncyclic fluctuations in their numbers (Jedrzejewsky et al, 1996;Petty, 1999) but they are rather abundant in temperate forests. This allows the Tawny Owl to reach maximal densities in temperate broadleaved forests (Southern, 1970;Galeotti, 2001) and in broadleaved forests of the Mediterranean region (Salvati and Ranazzi, 2002).…”