Drought and Landbirds on the Northern Colorado Plateau: Some Species Lose, Others Gain

People have long used birds as early warnings of environmental change — the "canary in the coal mine." In the arid American West, long drought has coincided with declines in many bird species over recent decades, and the region's national parks are projected to warm faster and lose more precipitation than anywhere else in the country.

To help managers plan, scientists from the Northern Colorado Plateau Network and the University of Delaware studied how water deficit affected landbird communities in 11 national parks in Utah and Colorado, using breeding-bird surveys and local climate records for three habitats over 12 years.

At a glance

• Birds of the desert Southwest, a climate-change hotspot, are among the most vulnerable in the United States.
• Responses depended on habitat and species: more drought stress is not simply bad or good for all birds.
• Species that breed in several habitats may adapt more easily.

Soils in the foreground and background hold moisture differently, favoring different plants. Image from the National Park Service

What is water deficit?

Drought used to mean below-average rain. But as temperatures rise, so does evaporation — and hot, dry years together bring extreme water stress. Water deficit captures both: the extra water plants would use if it were there, or, like an overdrawn bank account, the gap between what plants need and what the soil holds.

It is also very local: elevation, slope, aspect and soil — its texture, depth and water-holding capacity — all change how fast ground dries, so places with the same climate can have different water deficits.

Why it matters to birds

Thirsty plants grow and reproduce poorly, which cuts the seeds and insects birds eat. On the Colorado Plateau — in drought for much of the past 20 years — vegetation responded strongly to water deficit during the study (2005–2015 and 2017), differently in different vegetation types, largely because of soils. That tight link let the team test water deficit against bird density — the first attempt to see how water availability might affect aridland breeding birds by vegetation type.

Woodhouse's scrub-jay. NPS / S. Roberts

Winners and losers

Of 30 species–habitat combinations in riparian, pinyon-juniper and sagebrush shrubland habitats, 21 (70%) showed a significant link to water deficit: 8 negative (less water, fewer birds) and 13 positive (less water, more birds).

Gray vireo. NPS / S. Roberts

• Vulnerable: species that decline as water runs short may struggle to adapt — the yellow warbler in riparian areas most of all, along with lazuli bunting, spotted towhee in pinyon-juniper, and dusky flycatcher and green-tailed towhee in sagebrush.
• Less vulnerable: species that increase with drought — such as lark sparrow, ash-throated flycatcher, house finch and gray vireo.
• Flexible: the spotted towhee did worse in riparian and pinyon-juniper habitat but better in sagebrush, so it could shift its breeding there as drought deepens.

As climate change alters the conditions these birds evolved in, some will suffer and others will thrive, driven by a complex mix of factors. Continued monitoring will sharpen the picture and point managers to fine-scale options, depending on which species and habitats they choose to focus on.

Sources

Based on "Localized Drought Impacts on Northern Colorado Plateau Landbirds," Northern Colorado Plateau Network, National Park Service, summarizing S. G. Roberts, D. P. Thoma, D. W. Perkins, E. L. Tymkiw, Z. S. Ladin and W. G. Shriver, "A habitat-based approach to determining the effects of drought on aridland bird communities," Ornithology 138(3), 2021; a work of the United States government in the public domain.