LE3 .A278 1987

1987

Smith, Peter

Acadia University

Master of Science

Masters

Biology

Biology

The resident breeding population of Barred Owls Strix varia Barton has increased in Kings County, Nova Scotia since the early 1970s. This increase of nesting pairs was linked with high habitat heterogeneity, reduced interspecific competition with Great Horned Owls and the provisioning of nest boxes. The number of nest boxes increased from 10 to 25 from 1979 to 1986 when the population was studied to assess long-term breeding trends. During this time, the population of potential breeders increased from 3 to 12 pairs. The proportion of pairs which attempted to breed varied among years. This study is the first to, describe Barred Owl population fluctuations incorporating the non-breeding component. Forty-eight nesting attempts which have been documented between 1979 and 1986 are considered. Fifteen of those which occurred in 1985 and 1986 were studied to obtain detailed breeding data. The frequency of breeding, the timing of breeding and clutch size varied within and among years. The most common clutch size of two eggs was less productive than that of three. Chick growth rates examined in 1985 and 1986, generally showed that nestlings which survived to fledge had similar rates of growth. Nestling mortality (starvation) occurred most often among third-hatched chicks in broods of three. The major causes of nest failure were predation of young by Raccoons (n=7) and desertion (n=5) of nests during incubation. Incubation and brooding were conducted soley by the female parents which were fed by males. Intensive brooding stopped approximately two weeks after the oldest chick had hatched; at this time the females began hunting to augment foods provisioned by males. A breeding pair of Barred Owls monitored with a 16 mm cine-camera, showed that hunting was conducted almost exclusively at night. Peaks of hunting activity occurred after sunset and before sunrise. Analyses of adult pellets collected in 1985 (n=ll5) and 1986 (n=83) showed that there were 635 individuals of 36 prey species. Meadow Voles were the major prey and varied in their importance between years. Counts of intact prey found cached in nest boxes when newly hatched chicks were present and analyses of prey remains after chicks fledged from 13 nests in 1985 and 1986, showed that parent owls expanded their dietary width (n=45 prey species) after chicks hatched. Although Barred Owls have traditionally been associated with mature forest habitats, findings of this study suggest that the limiting factors are related more to the abundance of prey resources and nest site availability, rather than the age of the forest. Reproductive rates have previously been interpreted as being affected by individual parental quality, inter-brood competition and brood size. It is proposed that interactions with nest invertebrate communities may increase intraspecific competition within broods during periods of prey shortage. The ability of parent owls to adjust their reproductive effort (i.e. clutch size or brood size) in accordance with short-term changes in prey abundance may be implicated with competition and parasitism by invertebrates found in nest substrates and the nutritional status of young surviving to fledge. An hypothesis is presented to explain the magnitude of nest invertebrate effects.

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