Sierra Nevada birds 2011 to 2019: stable overall, rising in Yosemite, falling in Sequoia

Across Yosemite, Sequoia & Kings Canyon national parks and Devils Postpile National Monument, bird populations were relatively stable from 2011 to 2019, yet many species rose or fell. The picture differed sharply by park: 91 percent of Yosemite’s trends were increases, against only 26 percent in Sequoia & Kings Canyon. The National Park Service’s Sierra Nevada Network, working with The Institute for Bird Populations (IBP), reported these as its first bird trends after nearly a decade of monitoring.

Why count birds

Birds live at every elevation and make up more than 60 percent of vertebrate species in these parks. They are good indicators of change in land ecosystems and sit prominently in most food webs. Warmer temperatures, less snowpack, altered fire regimes and shifting plant communities may change where bird species live, and trend data feed park planning and assessments of how vulnerable birds are to warming, drought and wildfire.

How the counts are made

From May through July, the breeding season, field crews run point counts along mile-long off-trail transects placed at random near the trail systems of Sequoia & Kings Canyon and Yosemite. At the much smaller Devils Postpile a grid of points is monitored each year. At each point a birder records every bird seen or heard within a 7-minute period, plus the habitat. Between 2011 and 2019 technicians detected 166 bird species at 2,048 count stations, 134 species during formal counts and 32 more during other crew work.

Crews must complete three weeks of training in bird identification and safe wilderness practices and pass an identification exam. Each two-person crew has to be at its starting point by sunrise, and each person hikes out to seven transect points to identify birds, much more often by sound than by sight.

The analysis

An IBP research ecologist built models to account for things that affect whether a bird is detected, such as noise from flowing water, vegetation density and the observer, and to test effects on abundance of station location, year, elevation, park and climate (precipitation as snow and mean spring temperature anomalies). Of the 166 species, 62 were detected often enough to estimate detection probability and trend, and 66 were detected more than 100 times.

What the trends show

• Shared increases: Dusky Flycatcher and House Wren rose in all network parks; Red-breasted Nuthatch, Dark-eyed Junco and Lazuli Bunting rose in both Sequoia & Kings Canyon and Yosemite.
• Shared decreases: Western Wood-Pewee and Pine Siskin fell in both.
• Opposite directions: White-breasted Nuthatch, Hermit Thrush and Hermit Warbler declined in Sequoia & Kings Canyon but increased in Yosemite.
• Devils Postpile: MacGillivray’s Warbler fell to less than half its 2011 density estimate by 2019, while Dusky Flycatcher, Steller’s Jay, House Wren, American Robin and Green-tailed Towhee rose.

Taking the two large parks together, 14 species increased in density and 10 decreased. Trends that show at the larger scale are often not visible in single parks. The 14 increasers include Mountain Quail, Hairy Woodpecker, Dusky Flycatcher, Cassin’s Vireo, Red-breasted Nuthatch, House Wren, Townsend’s Solitaire, American Robin, Cassin’s Finch, Dark-eyed Junco, Lincoln’s Sparrow, Green-tailed Towhee, Black-throated Gray Warbler and Lazuli Bunting. The 10 decreasers include Red-breasted Sapsucker, Western Wood-Pewee, Steller’s Jay, Wrentit, Gray-crowned Rosy Finch, Purple Finch, Pine Siskin, Fox Sparrow, Spotted Towhee and Orange-crowned Warbler. Hairy Woodpecker density in Yosemite appeared to double between 2011 and 2019.

What lies behind the pattern

Most populations followed the previous spring’s temperature, the time when the nests that produce the current year’s birds are active: warmer springs went with more birds. Half the populations varied inversely with precipitation as snow, with snowier years bringing fewer birds. So reduced snow cover and milder springs from recent warming and drought may help explain what the network calls the recent, perhaps ephemeral, stability of its birds.

That may not last. If snow cover keeps shrinking in depth and duration, drought could eventually damage vegetation and breeding habitat and lower bird numbers. Sequoia & Kings Canyon, hotter and drier overall than the northernmost parks, had more negative and fewer positive trends. Further research is under way on how warming and drought affect these birds.

Sources

• National Park Service, Sierra Nevada Network, “Bird Population Trends in the Sierra Nevada Network, 2011-2019,” https://www.nps.gov/articles/000/sien-bird-population-trends-2011-2019.htm, which summarizes a 2022 report by Chris Ray and colleagues of The Institute for Bird Populations; rewritten in hubnx’s own words. The page’s figures and photographs, credited to others or to the report’s authors, are left out.