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.

Niche partitioning among three tree-climbing bird species in subtropical mountain forest sites with different human disturbance

“…This pattern has even been detected within nuthatch species. For example, in North American ponderosa pine forests, sympatric pygmy and white-breasted nuthatches partition their resources by foraging in different parts of the tree (Stallcup 1968, McEllin 1979, Cloyed 2014. Our results confirm these findings.…”

“…From this data, the author concluded that pygmy nuthatches have a foraging niche in thinned and unthinned forests that is distinct from white-breasted nuthatches, allowing them to coexist without potential population implications. Our data, consistent with the differential tree use in nuthatches reported by Cloyed (2014) and the estimated niche overlaps (including brown creepers), suggest that changes in forest structure could lead to changes in competitive dynamics among them, as discussed below.…”

“…However, changes in tree use by the studied bird species in disturbed forest sites have not been previously reported. In the only study done in this area, Cloyed (2014) examined how pygmy and white-breasted nuthatches divided their resources in two ponderosa pine forest sites, an untreated forest (mostly comprised of trees with trunks higher than 32-cm DBH), and a forest selectively thinned for small-diameter trees (less than 32 cm). He found that the areas used in the trees by the two bird species are different when foraging on large-or small-diameter trees, but partition in tree use remains in both forest conditions.…”

“…Numerous studies of avian communities have elucidated differences between similar species inhabiting the same habitat which lead to coexistence (MacArthur 1958, Hutchinson 1961, Alatalo and Alatalo 1979, Leyequién et al 2007, Lara et al 2011, Cloyed 2014. The general view, in the face of interspecific competition, is that the primary way to achieve this coexistence is for species to show a differential use of resources (Hutchinson 1961, Schoener 1974, Pyke 1982, Young et al 2010.…”

“…Conversely, white-breasted nuthatches (Sitta carolinensis) forage more often on the trunks (Stallcup 1968, McEllin 1979, as do brown creepers (C. americana) (Willson 1970). In this context, differences in the use of hunting tactics or substrates to find prey items should facilitate niche differentiation in sympatric tree-climbing birds (Cloyed 2014).…”