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Use of riparian woodlands by cavity-nesting birds in the Sacramento Valley, California
My study aimed to quantify nesting habitat availability for cavity-nesting birds in restored and remnant patches of riparian forest on the Sacramento River. I explored three hypotheses: 1) that older restored forests will attract more cavity-nesting birds in comparison to younger restored forests because more mature habitat features are available; 2) that the vegetation structure between remnant forests, younger restored forests, and older restored forests are distinct, owing to temporal developmental processes that differ in each subset of forest; and, 3) that the vegetation structure within each forest type affects the presence of secondary cavity- nesting birds and primary cavity-nesting birds differently because of their distinct nesting requirements. I performed vegetation surveys, cavity surveys, and bird point-count surveys to describe habitat structure and estimate bird density, cavity availability, and suitable cavity- building substrate (potential nesting substrate) in the form of snags and broken tree limbs. Surveys were conducted in each of six different aged riparian forests (remnant and 13- to 21- year old restored) in Glenn and Tehama Counties in Northern California. In restored forests, I determined that restoration sites between the ages of 13- and 21- years old (time since initial planting) do not have significantly different habitat availability for cavity-nesting birds. Features included in this analysis were potential nesting substrate and cavity density (p=0.176 and p=0.121 respectively). When comparing remnant forests with restored forests, the potential nesting substrate and cavity density were significantly different (p=0.005 and 0.003, respectively). A multivariate analysis of the vegetation structure comparing restoration forests and remnant forests showed there are trends in vegetation features that can differentiate remnant forests and 13-year old restored forests. Through a mixed model linear regression, I determined that primary cavity nesters were more sensitive to specific habitat features than secondary cavity nesters. Tree density, snag density, and forest age were important for both groups, but secondary cavity nesters were also sensitive to the number of different tree species present and the amount of potential nesting substrate available. It may be inferred that secondary cavity nesters are more likely to be observed in remnant forests, since these forests have a wider variety of tree species and far more potential nesting substrate available than restored forests.