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.