Forest structure affects resource partitioning between pygmy and white-breasted nuthatches

Abstract

Ecologically similar, sympatric species will theoretically drive one or the other extinct unless those species can divide their resources. Resource partitioning is a common way to avoid competitive exclusion and these differences in resource utilization often represent the “ghost of competition past.” I studied how two closely related species, pygmy (Sitta pygmaea) and white-breasted (Sitta carolinensis) nuthatches, divide tree space. Furthermore, I tested whether pygmy and white-breasted nuthatches foraged in different areas on large- and small-diameter trees and whether pygmy nuthatches foraged more frequently on small-diameter trees. Finally, I tested whether each nuthatch species foraged less frequently in forests where most of the small-diameter trees have been removed. I found that in large trees, pygmy nuthatches foraged higher up in the tree and on the branches than white-breasted nuthatches. White-breasted nuthatches foraged low in the tree and on the trunk. In small-diameter trees, pygmy nuthatches foraged mostly on the lower trunk. White-breasted nuthatches foraged significantly less frequently on small-diameter trees than on large-diameter trees. Overall, pygmy nuthatches foraged more on small-diameter trees. Finally, pygmy nuthatches foraged significantly less frequently in areas of the forest where small-diameter trees have been selectively logged. My results concur with the literature on niche partitioning in forest-dwelling birds, where trees are divided spatially among species. Finally, my data may help explain historical niche relations between these species and how changes in forest structure can alter these niche dynamics.

Keywords:

• pygmy nuthatch
• white-breasted nuthatch
• resource partitioning
• niche
• ponderosa pine

Acknowledgements

I would like to thank Tom Fleischner for mentoring this project and Perri Eason and a reviewer for insightful comments on the manuscript.