Tracking Arctic songbirds as shrubs spread: landbird monitoring in Alaska’s Arctic parks

Alaska's Arctic is getting shrubbier, and the songbirds that live there may be moving north and uphill with it. To find out, National Park Service scientists survey birds in remote country above the treeline. This brief describes the program for the five Arctic Network parks.

A Bluethroat perched in a willow. NPS/Jared Hughey.

Who lives there

About 147 resident and migratory bird species are expected in the five national parks of the Arctic Network: Bering Land Bridge National Preserve, Cape Krusenstern National Monument, Noatak National Preserve, Kobuk Valley National Park, and Gates of the Arctic National Park and Preserve. Among the songbirds breeding above treeline, White-crowned Sparrow, American Tree Sparrow, Savannah Sparrow and Wilson's Warbler are ubiquitous and abundant.

Several species breed in North America mostly in the shrub-tundra regions of Alaska, including the Arctic Warbler, Bluethroat and Eastern Yellow Wagtail. All three are palearctic migrants that winter in Southeast Asia. The Northern Wheatear, which breeds in the same parks, travels from wintering grounds in sub-Saharan Africa.

Northern Wheatear. NPS/Jared Hughey.

Why watch them

Population declines appear to be widespread among North American birds, but the evidence comes largely from roadside surveys at mid-latitudes, such as the Breeding Bird Survey. Little is known about the distribution and trends of landbirds across the vast Arctic. One of the most visible effects of a warming climate in Alaska is the fast spread of erect shrubs into open tundra, and shrub-tundra songbirds appear to be shifting north and to higher elevations, possibly following the shrubs. Tracking birds that breed in the mountains may therefore show how the Arctic and subarctic are responding to climate change, and tracking those populations is needed for conservation.

Landbirds were chosen as "vital signs" for long-term monitoring in the Arctic and Central Alaska Inventory and Monitoring Networks. The objectives are to document trends in landbird density over time and changes in where they live.

The problem with roadside counts

Alaska's parks first monitored landbirds along roads at low and middle elevations: the Park Road in Denali National Park and Preserve since 1995, and the McCarthy and Nabesna Roads in Wrangell-St. Elias National Park and Preserve since 2010. But shrub-tundra and alpine songbirds live higher than those roadside surveys reach. That makes a decline hard to read: has a species moved out of the surveyed elevations, or is its regional population falling? To resolve this, the scientists added off-road surveys above treeline.

They chose six off-road sites, three in each network, in Gates of the Arctic National Park and Preserve, Noatak National Preserve, Denali National Park and Preserve, and Yukon-Charley National Preserve. Each is a similar above-treeline bird community, and each spans elevations from lowland to alpine habitat, with open shrublands, mixed dwarf-shrub tundra and barren slopes. That makes the sites suitable for studying shifts linked to shrub expansion.

How the counting works

• Observers survey fixed routes, recording where each bird is encountered. This kind of spatially explicit distance sampling allows a fine-scale view of how density varies across the landscape and should be more sensitive to shifts in distribution.
• Each route is surveyed two to four times a season, because birds are not always singing.
• Routes lie on a grid of 600 m by 600 m cells centered on an access point, usually the airstrip where the camp is. Routes radiate outward, are designed to be completed in a day and average 5.4 km, and bring observers back near where they began.
• Each site has at least 13 routes covering 120 grid cells on average.

Off-road survey sites in the Arctic Network, with transects. Image from the NPS page.

The data also feed models that predict bird abundance per 30-meter pixel. The page shows predictions for American Tree Sparrow and Savannah Sparrow at the Primus Creek site in Noatak National Preserve in 2016.

Sources

• National Park Service, “Landbirds Resource Brief for the Arctic Network,” https://www.nps.gov/articles/000/landbirds-resource-brief-arctic-network.htm; two prediction maps whose labels do not agree, a photograph credited to a named non-federal photographer and the contact line are left out; rewritten in hubnx's own words.