Distribution and habitat associations of the northern pygmy-owl in Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/kk91fq76r

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  • I conducted acoustic lure surveys of northern pygmy-owls (Glaucidium gnoma) on transects that were systematically placed in forested areas throughout the state of Oregon. My objectives were to determine if pygmy-owls were associated with particular forest types, ecoregions, or with large-diameter dominant overstory trees. I also examined temporal variation in response rates. All analyses accounted for decreases in counts of owls with increasing day of the survey season (P = 0.01) and with increasing time after sunrise (P = 0.06). Counts were highest in Douglas-fir, ponderosa pine and mixed conifer-hardwood forests, intermediate in mixed conifer, deciduous and high elevation forests, and lowest in western juniper (P = 0.0003). Even after accounting for forest type, numbers of owls detected varied among ecoregions (P = 0.002). Counts of owls increased with the average diameter ranking of dominant overstory trees (P = 0.000 1). Also, diameters of dominant trees were greater at survey points where owls were detected than at survey points where no owls were detected P = 0.008), and greater in the actual stands from which owls responded than around the survey points from which owls were detected (P = 0.0001). My data suggest that widespread decreases in large-diameter trees, or the late-successional forests in which such trees are most abundant, may reduce numbers of northern pygmy-owls. However, this finding needs to be viewed in the context that my surveys also show that the pygmy-owl is a well-distributed and fairly common forest bird in Oregon and is not limited to late-successional forest.
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