I conducted evaluations of avian use, nest-site selection, and nesting success among birds breeding in mixed aspen (Populus tremuloides) forests of the eastern slope of the Sierra Nevada mountains, California and Nevada, from 2002-2006. Specific questions addressed were: (1) what are the habitat characteristics of aspen stands that maximize total bird abundance and bird species richness, particularly as they relate to conifer encroachment? (2) can chipmunks climb aspen trees; if yes, then why do they not appear to do so, and how might this be affecting avian nest site selection? (3) what are the preferred nest-site characteristics among birds breeding in aspen, and is nest-site selection affected by conifer density?, and (4) what are the important nest-site characteristics affecting nest predation among birds breeding in aspen, and in particular, does conifer density around the nest affect nest success? Indices of total bird abundance and bird species richness were greatest at in pure, mature aspen with a healthy herbaceous understory. Experiments revealed that chipmunks were unable to climb aspen, suggesting that the bark of aspen trees may present a barrier or impediment to small, mammalian nest predators. This may offer partial explanation for our findings of preferential nest-site selection for aspen versus other tree species among arboreal nesters, higher proportional nest success among arboreal nests placed in aspen versus other tree species, and higher rates of nest success among Dusky Flycatchers (Empidonax oberholseri) that placed their nests in aspen trees instead of shrubs. We found little evidence to suggest that birds nesting in aspen select nest placements relative to conifer density. Optimal models of nest success demonstrated that conifer density near nests was negatively correlated with nest success for four of five species. We found relatively high nest success in of four of five species, suggesting that the habitat supported successful breeding of open-cup nesting passerines in general. Promoting larger and purer aspen forests may allow passerines to escape predation related to conifer-associated predators and therefore have the greatest impact on aspen's ability to provide source populations of insectivorous birds in western North American forests.
|Adviser||Dennis D. Murphy|
|School||UNIVERSITY OF NEVADA, RENO|
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