Pinyon-juniper specialist birds are resilient to local-scale reduction of canopy cover and pinyon pines, but amount of understory shrub cover determines the composition of the insectivore guild
Jones H, Merriell BD, Swan M , Johnson J, Siegel RB. 2025. Pinyon-juniper specialist birds are resilient to local-scale reduction of canopy cover and pinyon pines, but amount of understory shrub cover determines the composition of the insectivore guild. Avian Conservation & Ecology. 20(2)
Abstract: Drought-induced tree die-off and die-back in the southwestern United States is changing the structure and floristic composition of pinyon-juniper woodlands, a widespread middle-elevation community with high bird endemism. Amid increasing prevalence of high-severity wildfires in this habitat, managers have also turned to mechanical thinning to reduce fuel loads. Therefore, in this study, we asked how breeding season bird densities are associated with microhabitat-scale structural features and floristic composition in pinyon-juniper woodlands, features likely to change with die-back and mechanical thinning. We fit Bayesian N-mixture models to eight years of monitoring data at three Colorado Plateau national parks and found supported responses to microhabitat variables across 18 of 25 species, though park-level effects were an order of magnitude larger than microhabitat effects. Pinyon-juniper specialists were resilient to loss of canopy cover at local scales, but generalist forest species showed supported associations with greater canopy cover. We found no threshold values of canopy cover, but nine species showed a supported negative effect of park on density at Bandelier National Monument after a large-scale (> 95%) pinyon pine die-off and associated reduction in canopy cover (< 15% remaining). Species also showed weak or no associations with pinyon-pine basal area, perhaps indicating a selection for both junipers and pinyons. Exceptions included positive associations of cavity-nesting birds with greater pinyon-pine basal area, and negative associations of species associated with canopy gaps. The extent of shrub foliage cover, which may increase in woodlands following thinning or die-off, shaped the composition of the insectivorous bird guild. Foliage-gleaning birds of both canopy and understory showed positive supported associations with increasing shrub cover, while aerial insectivore and bark forager densities were negatively affected. An ordination of species’ responses to all covariates suggests the community segregated along a woodland successional gradient and was generally associated with a simplified vertical vegetation structure.
- Type
- Journal Article
- Authors
- Jones, Harrison; Merriell, Brandon; Swan, Megan; Johnson, James (Matthew); Siegel, Rodney
- Units
- BAND , GRCA , MEVE
- Keywords
- Bayesian hierarchical models, birds, breeding-season bird density, canopy height, foliage height diversity, monitoring, pinyon pine die-off, SCPN, Southern Colorado Plateau