Why Denali's Passerine Birds Are Moving Uphill: Pushed by Conifers, Not Pulled by Shrubs

Climate warming is amplified in the far north, and in boreal-tundra landscapes one result is the spread of conifers and deciduous shrubs. In Denali National Park, Alaska, passerine birds have been shifting uphill fast. The assumption was that shrub-loving birds were simply following shrubs as they spread upslope. Nobody had checked directly.

A northern wheatear (Oenanthe oenanthe) defending its territory in Noatak National Preserve. Photo by Jared Hughey, National Park Service

The study

National Park Service scientists and colleagues looked at the vegetation and the birds together:

What they found

• Shrubs and conifers colonized fastest at middle elevations near treeline.
• Forest birds responded most strongly to spreading woody plants at sites that had been above treeline at the start.
• Shrub birds responded mostly below treeline: conifers filling in hurt them, while more shrubs helped — but above treeline they barely responded to new woody growth.

The conclusion reverses the old assumption. Conifers infilling below treeline appear to be pushing shrub-adapted birds uphill, rather than the birds being pulled upslope by the modest spread of shrubs above treeline.

Why it matters

Tracking where species are found isn't enough to explain why they move. Habitat change can vary a lot over short distances, especially at ecotones such as treeline, where one community gives way to another — and understanding birds' responses means measuring those local changes directly.

Sources

Based on "Bird responses to subarctic tree and shrub expansion," Denali National Park & Preserve, National Park Service, summarizing J. D. Mizel, J. H. Schmidt, C. A. Roland and C. L. McIntyre, "Tree and shrub expansion at treeline drive contrasting responses in a subarctic passerine community," Journal of Animal Ecology (2023); a work of the United States government in the public domain.