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 Posted: Sun 16:09, 20 Mar 2011 Post subject: mixed-conifer forests rolex submariner |
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Ecological densities, defined as owls per area of suitable habitat, differed significantly (P<0.001) across the subregions and varied from 0.373 to 1.049 owls/km2 [11].[link widoczny dla zalogowanych] Mexican spotted owl densities averaged 0.275 owls/km2 in mixed-conifer forests, 0.08 owls/km2 in pine forests, and 0.022 owls/km2 in pinyon-juniper woodlands of the Sacramento Mountains New Mexico [6].
Spotted owls do not build their own nests. They rely on sites such as trees and snags with cavities or broken tops, and platforms associated with abandoned raptor or squirrel nests, witches' brooms (caused by mistletoe infection) and debris accumulations [12][18][20][21][27][30]. Large, old trees are most often used by spotted owls for nesting. Species used as nest trees vary with region and subspecies.
Several studies indicate that tree cavities are most commonly used for nesting by spotted owls, while the extent of platform use varies. In coniferous forests in Oregon, 60% to 93% of nests were in trees with broken tops. Additionally, broken-topped trees (>21 inches (53.3 cm) DBH with 1 or more secondary crowns) had significantly (P<0.001) higher basal area and density in small plots on and around nest sites than in random plots within spotted owls' home ranges [12]. Platform use may be more common in areas that lack large,[link widoczny dla zalogowanych], old trees and snags and have a greater abundance of witches' brooms. Compared to other habitats within their range, northern spotted owls use platforms more often in mixed-evergreen and mixed-conifer forests [12][30].[link widoczny dla zalogowanych] California spotted owls in southern California use platforms more frequently than those in the Sierra Nevada [4][45]. Platform use also occurred more frequently in oaks than in conifers in the southern Sierra Nevada [20]. The average DBH of California spotted owl platform nest trees was significantly (P<0.01) smaller than that of cavity nest trees in foothill riparian and oak woodlands in the southern Sierra Nevada [21]. In grand fir-dominated stands in eastern Washington, northern spotted owls nested in witches' brooms on trees as small as 12 inches (30 cm) DBH [62]. Mexican spotted owls use cliffs and comparatively open areas as nest sites more frequently than the other subspecies [4][27].[link widoczny dla zalogowanych] Fletcher and Hollis (1994, as cited by [27]) found 9.7% of 248 Mexican spotted owl nests in cliffs, while Steger and others [21] noted only 1 out of 41 California spotted owl nests in a rock cliff in the southern Sierra Nevada.
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