When it comes to habitat, spatial scale is important. You’ll often hear ecologists talk about ecosystem scale, or landscape scale, and sometimes even “microhabitat” scale, but what exactly does that mean and why does it matter? Different habitat characteristics matter at various scales, even when humans are choosing where to live. You look for a place to live downtown because you like to walk to restaurants and museums, and there are lots of apartments. But your passion is gardening, so you need someplace with a yard, and there are few of those downtown. Similarly, an area could have vast expanses of ponds and wetlands swarming with insects, but if there are no trees with cavities or nest boxes in the area, tree swallows will not breed there.

In a new study published this week in the journal Avian Conservation and Ecology, IBP Southwest Avian Ecologist Harry Jones, along with colleagues from IBP and the National Park Service, look at habitat associations of birds breeding in pinyon-juniper woodlands in three Park Service properties: Bandelier National Monument in New Mexico, Grand Canyon National Park in Arizona, and Mesa Verde National Park in Colorado. This study focused on the microhabitat scale, which, for birds during the breeding season, is typically the size of the breeding territory that surrounds a nest site. They examined the breeding season densities of 25 species of birds found in pinyon-juniper woodlands over a period of 11 years from 2008 to 2018.

They used bird and vegetation survey data collected as part of the National Park Service’s Inventory and Monitoring Program, which conducts long-term ecological monitoring within the parks. Since 1998, this program has gathered information that helps park managers make sound, data-backed decisions. This data is also valuable to scientists studying the ecological effects of land management actions and processes outside of the parks, because they can compare their data to ecological data from the parks, where human disturbance and land management are relatively limited.

The Pinyon-Juniper biome is a dwarf woodland that is dominated by pinyon pines and juniper and is found in mid-elevation arid regions of the western US. It is home to several species of birds that specialize on this habitat type: Pinyon Jay, Woodhouse’s Scrub-jay, Juniper Titmouse, and Gray Vireo. Many birds that use this habitat rely on a mix of pinyon pine and juniper, rather than just one species or the other. Many species prefer to forage on pinyon pines, which bear nutritious pine nuts in their cones and tend to host more insects, but prefer the structure of junipers for nesting.

But pinyon-juniper woodlands are changing. A 20-year megadrought and outbreaks of bark beetles in the southwestern US are leading to die-offs of pinyon pine, which leads to juniper-dominated woodlands. Pinyon-juniper woodlands are also being altered by land management actions, primarily mechanical thinning to moderate density-dependent tree die-offs due to drought (fewer trees and shrubs means less competition for water) and to reduce fuel loads in the face of more frequent wildfires.

At the larger landscape scale, there is evidence that pinyon pine die-offs due to drought and bark beetles, and large-scale thinning of woodlands, make the habitat less suitable for pinyon-juniper specialist birds. But looking closer, at the microhabitat scale, to better understand the structure and plant species preferences of different bird species can help scientists and land managers predict how drought will affect pinyon-juniper birds. It also enables them to adapt management techniques, including thinning, to preserve suitable habitat.

Jones and his colleagues found that pinyon-juniper specialist birds were relatively unaffected by localized tree die-offs or thinning, leading to the loss of canopy cover. It was forest generalist species, like Mountain Chickadees, that were most sensitive to canopy cover loss. However, the effect of canopy loss may be different at a larger scale. There were several species that were absent, or very sparse, in Bandelier National Monument, which experienced a landscape-level die-off of pinyon pine that resulted in reduced canopy cover. Those species no longer abundant in the park included pinyon-juniper specialists like Gray Vireo and Pinyon Jays, as well as forest generalists like White-breasted Nuthatch and Plumbeous Vireo.

Despite these woodlands being characterized by their dominant tree species, deciduous shrubs were an important factor for insect-eating birds. Species like Blue-gray Gnatcatcher and Gray Vireo, that forage by gleaning (picking insects off leaves and branches), preferred habitat with greater shrub cover. But species that catch insects in the air, like Gray and Ash-throated Flycatchers, or pick insects off tree trunks, like White-breasted Nuthatch, preferred areas with fewer shrubs in the understory. “Shrubs are an important aspect of local habitat for many insectivorous birds,” says Jones. “Maintaining shrubs on the landscape during treatments of woodlands is important to maintain the full pinyon-juniper bird community.”

Jones and colleagues also found that Pinyon Jays are a bit of an “oddball” in terms of their microhabitat preferences. These jays are often considered “an umbrella species” that represent the suite of pinyon-juniper bird species and their habitat needs. But this study found that they prefer habitats with both older trees, and simplified vertical plant structure (a more typical feature of early successional woodlands), an usual pairing that was unique among the pinyon-juniper bird community. Pinyon Jays likely require both high canopy cover forest for nesting, and relatively open areas to cache the nuts of the pinyon pine (a staple diet item), perhaps one reason why these birds have such large group territories. Pinyon Jay populations are declining, and they are a species of conservation concern, so land managers should consider their habitat needs. However, their needs may not perfectly reflect the needs of other pinyon-juniper specialist birds.

Jones says that though this study found that pinyon-juniper specialist birds were relatively unaffected by localized tree die-offs or mechanical thinning, this should not be used as justification for thinning without considering the wider context. “We only looked at one spatial scale in this study,” he says, “so it is impossible to comment on the effects of large-scale thinning or conifer removal.” He notes that thinning near or at Pinyon Jay nesting colonies has led to nest abandonment. If thinning must occur, he suggests that ensuring that thinning is localized and patchy is important to avoid doing more harm than good. Using thinning techniques that preserve shrubs will also benefit many pinyon-juniper bird species. This research will help land managers both within and outside of the National Parks make more informed and effective pinyon-juniper management decisions.