I compared the nesting chronology, body size, food habits, prey size, and nest site characteristics of northern saw-whet owls (Aegolius acadicus) and western screech-owls (Otus kennicottii) nesting in nest-boxes in the Snake River Birds of Prey National Conservation Area (SRBOPNCA) in southwestern Idaho to evaluate the niche overlap and potential interspecific competition between the two owl species. Overlap occurred in the nesting chronology; saw-whet owl median egg laying date was 14 March, median date of egg hatching was 9 April, and the average fledging date was 14 May. The median laying date for screech-owls was 2 April, and median hatching date was 13 April. Body size of the two owl species did not overlap. Overlap was found in the diets of the two owls for prey species, frequency, biomass, and prey size. Dietary overlap was 99% at coarse resolution (class level) and 82% at fine resolution (species level); the lower overlap at species level indicated that the owls ate different mammal species. The prey sizes taken by the two owl species overlapped; saw-whet owls took prey that ranged from 11 – 55 g; screech-owl prey ranged 0. 5 – 400 g.
Nest-site micro-habitat measurements obtained at nest boxes used by saw-whet owls and screech-owls were similar, revealing substantial overlap in nesting habitat for this area. I also examined the relationship of nearest-neighbor distances to nesting success and productivity (clutch size and number of bandable-age young) of western screech-owls and northern saw-whet owls. The nesting success of screech-owls was positively significantly related to the nearest-neighbor distance of screech-owl nest-sites (p = 0.013), but the relationship to saw-whet nest-sites was not significant. Nesting success of saw-whet owls was not related to the nearest-neighbor distances of either owl. Although not significant, there were trends in the relationship of clutch size and number of screech-owl young to the distance to screech-owl and saw-whet owl nest-sites. This indicated that nearest-neighbor distances of screech-owls affected their nesting success, but nesting productivity levels were not determined by the nearest-neighbor distances. Average nearest-neighbor distances for saw-whet owls nest-sites were 4507 m; average distance of screech-owl nest-sites to screech-owl and to saw-whet owl were 3054 m and 13,139 m, respectively. Mean clutch size of saw-whet owls was 5.9 eggs, while that of screech-owls was 4.5 eggs. The average number of young per successful pair was 5.3 for saw-whet owls and 4.1 for screech-owls. The average number of young per nesting attempt was 3.6 for saw-whet owls and 3.0 for screech-owls. Because placement of nest-boxes is sometimes used as a management tool, it is important to understand the possible nest-box requirements of these owl species. I examined the nest-box selection of the two owl species and found that there was no significant difference between nest-boxes selected by saw-whet owls, screech-owls, and unoccupied nest-boxes. However, I found some trends in nest-box selection.