Nest site selection of Dusky and Hammond’s flycatchers over an elevational gradient

Abstract

Release from interspecific competition can lead to niche shifts and have positive fitness consequences. I studied two closely related and ecologically similar Empidonaxspecies that breed across an elevational gradient in the southern Sierra Nevada, California: Dusky Flycatcher (Empidonax oberholseri) and Hammond’s flycatcher (E. hammondii). The two species coexisted at low and moderate elevations but only Dusky Flycatchers occurred in high elevation forests. Where they co-occurred, Dusky Flycatchers nested primarily in shrubs but at high elevations they nested primarily in trees and snags. My objective was to examine whether ecological release from interspecific competition was responsible for the apparent niche shift in Dusky Flycatcher nests sites at high elevations where Hammond’s Flycatchers do not occur. Both species nested higher and had higher nest survival at higher elevations. Dusky Flycatcher nest survival was higher when nesting in trees or snags compared to shrubs and Hammond’s Flycatcher nest survival increased with increasing nest height. Upon closer inspection, Dusky Flycatchers did not occupy the nest site niche of Hammond’s Flycatchers at high elevations. When nesting in trees, Dusky Flycatcher nests were much lower (mean = 4.3 m) than Hammond’s Flycatcher nests (mean = 18.0 m). The observed distributions of these two species over the elevational gradient appeared to correspond to the habitat preferences of the two species, rather than interactions between the species and their preferred nesting locations. Should warming due to climate change trigger upslope range expansion, Hammond’s Flycatchers’ preferred nesting locations would not be available and they would likely incur fitness consequences. Increased vertical overlap in habitat use could lead to increased competitive interactions with Dusky Flycatchers. These results provide important findings related to the habitat requirements, species’ ranges, and ecological niches of these two species and help us to better predict species’ responses to climate change and inform conservation strategies.

Citation

Purcell, Kathryn. 2024. Nest site selection of Dusky and Hammond’s flycatchers over an elevational gradient. Journal of Field Ornithology. 95(4). https://doi.org/10.5751/JFO-00574-950407.