Song post and foraging location characteristics of breeding varied thrushes in coastal redwood forests of Northwestern California

Maurie Beck

Graduation Date

1997

Document Type

Thesis

Program

Other

Program

Thesis (M.S.)--Humboldt State University, Wildlife Management, 1997

Committee Chair Name

T. Luke George

Committee Chair Affiliation

HSU Faculty or Staff

Keywords

Thrushes, Habitat selection, Humboldt State University -- Theses -- Wildlife Management, Habitat, Northern California

Abstract

In this study I demonstrated the effectiveness of a hierarchical approach in describing habitat characteristics of song posts and foraging sites used by Varied Thrushes (Ixoreus naevius). I evaluated characteristics of song posts and foraging locations at four study sites in coastal redwood (Sequoia sempervirens) forests of northwestern California from March - August of 1994 and 1995. I measured a suite of mesohabitat (0.04 ha circular plots) and microhabitat (0.5 m radius) scale attributes centered on occupied and random song post and foraging locations within Varied Thrush territories. Varied Thrushes appeared to select song post and foraging locations at multiple scales in a hierarchical process. Ninety-five percent (n = 57) of song posts were in trees or snags. There was no difference in proportion of tree species used as song posts between bird-centered and random locations. The most parsimonious matched-pairs logistic regression (MPLR) model included distance to water, tree DBH (diameter at breast height), tree type, slope, and tree density. Male thrushes used song posts with low foliage density near the top of large DBH conifers, located on steeper slopes, surrounded by a high density of trees, and centered in drainages closer to water compared to random locations. Seventy-eight percent (n = 43) of foraging locations were on the ground or on downed logs and 16% (n = 9) were in berry bushes. Varied Thrushes foraged almost exclusively on the ground early in the breeding season. Subsequently, thrushes included fruit (predominantly red huckleberry Vaccinium parvifolium) in their diet after young fledged. Most ground foraging locations were on trails or other disturbed habitat types with little ground foliage density. Though many variables were correlated with ground foraging locations, microhabitat foliage density had the greatest correlation to foraging location and was the only variable included in the final MPLR model. This highlights the importance of measuring habitat characteristics at the correct spatial scale. Also, within foraging locations, thrushes used microsites with much less shrub cover compared to what was available in bird-centered plots. It appears that Varied Thrushes selected foraging sites primarily at the microhabitat, rather than the mesohabitat scale. Based on my results, a multi-scale, hierarchical methodology should be considered when investigating habitat associations of animals.

https://scholarworks.calstate.edu/concern/theses/kd17cw21h

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