Food Caching in the Screech Owl

Abstract

Instances of food storing or caching in raptors have been reported (Pierce, Condor 39:140, 1937; Tordoff, Wilson Bull. 67:138, 1955; Stendell and Waian, Condor 70:187, 1968). Food storage may be appropriate when a food resource is periodic and/or unpredictable. It has been proposed that this behavior is adaptive for avian predators such as shrikes (Lanius) (Cade, Living Bird 6:43-86, 1967) and Snowy Owls (Nyctea scandiaca) (Pitelka et al., Ecol. Monogr. 25:85, 1955). In April 1974, on Amherst Island, near Kingston, Ontario, Canada, I found a Screech Owl (Otus asio) storing meadow voles (Microtus pennsylvanicus) on a rafter inside an old shack. Three visits were made to the site. At each visit, a Screech Owl was flushed from the shack. Numerous regurgitated pellets were found in the shack, indicating that the owl had been using it for a long period. On April 12, seven voles were found on the rafters. Each vole was weighed and eartagged for subsequent identification. Two days later, there were only five voles, all with eartags. A week later, I found six voles, four of them eartagged. The size of the voles in the cache was between 13 and 26 g (mean 20.1 g; N = 9). I could find no apparent signs of external injury on the voles in the cache. The owl appeared to be storing vole carcasses it had picked up nearby. At the time of my observations, the vole population on the island was declining in density, from approximately 165 voles/ha to about 33/ha. Vole carcasses were abundant in the vicinity of the shack. My findings show turnover in the cache. However, some stored items remained longer than those observed by Stendell and Waian (1968). During forty days of observation of a kestrel (Falco sparverius) cache, prey items were stored for seven days at most. The storage of prey by raptors must be brief because decay may often be rapid. Decay is probably the chief factor limiting the usefulness of a cache. When decay is slow, as in winter, or the turnover rate of the cache is high, as while feeding young, decay is less limiting. Food caching is probably adaptive in many situations, providing a readily accessible food supply. In shrikes, this behavior is necessitated by the similarity in size of the predator and the prey. In the Snowy Owl, the accumulation of prey items is probably in response to the great demand for prey by the female and young during the nesting period. It is possible that the caching behavior in this instance was also related to breeding as Screech Owls in this locality begin nesting in mid-April. However, there was no evidence that the owl in question was nesting. As a result of caching behavior, foraging time may be reduced, permitting more time for other activities (e.g. breeding). It seems possible that caching could occur before actual breeding activity commences if physiological changes themselves are adequate to .elicit this behavior at the onset of the breeding season. Thus, caching could provide a margin of safety in the provision of food, while substantially reducing the time and energy costs normally associated with foraging. These observations were made while conducting field work supported by Canadian Wildlife Service grant CWS 7475/043.