The influence of riparian-canopy structure and coverage on the breeding distribution of the southwestern willow flycatcher
Brodhead, Katherine May.
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There is a long history of relating bird species diversity and distributions to heterogeneity in foliage structure as seen from within the habitat and measured from the ground up. There is also an overwhelming contribution in the literature promoting and justifying a broad-scale approach to characterizing spatial patterns, especially for the purpose of relating to, and predicting, species distributions. This study draws from the relationship between birds and habitat structure but assesses heterogeneity in structure from a broader perspective. For this analysis, I compared the spatial distribution of a breeding population of the southwestern willow flycatcher (Empidonax extimus traillii) to the structure of a riparian zone with an emphasis on capturing patterns that are measured horizontally across the zone. Riparian zones are dynamic by nature and are structurally diverse in a natural, healthy system, and support a high density of breeding birds. The flycatcher prefers dense riparian habitat in close proximity to lentic water. The structural characteristics of the riparian zone where dense vegetation and water are present were hypothesized to be more structurally heterogeneous and to support a broader riparian zone. Riparian zone structure was mapped and stand characteristics were extracted, with the intent of relating spatial patterns in stand heterogeneity and riparian extent to the spatial distribution of the southwestern willow flycatcher. A moving window function in a GIS assessed the stand characteristics, which were imported as attributes to presence/absence data points. The extent of the analysis window was varied to determine the scale at which the habitat characteristics were most highly correlated with flycatcher presence. Correlation between the structural characteristics of the stand and the presence/absence of the flycatcher was modeled with logistic regression. Results show the flycatcher is more likely to occupy habitat that is structurally heterogeneous and has more riparian vegetation. The results also show that selection is most sensitive to habitat characteristics within a relatively close proximity.