As Arctic temperatures rise and shrubs spread northward into the tundra, many branch-perching birds are benefiting. But that benefit has a limit because, at some point, woody shrubs become too tall for the birds.
That is the conclusion of a U.S. Geological Survey study of birds in Western Alaska's Seward Peninsula. The study, published in the scientific journal PLOS ONE, uses counts of birds and shrub characteristics recorded from 2012 to 2014.
Thousands of birds were counted at 247 sites on the Seward Peninsula. The surveys tracked 68 bird species; the study zeroes in on the 17 most prevalent.
Among those 17 species, the study found, shrub height, density and cover had significant effects on bird abundance — both positive and negative, depending on species.
But even for some birds that thrive among shrubs, there's a Goldilocks effect: They like plants that are not too small but not too tall. That means shrub proliferation, which has been dramatic on the Seward Peninsula and other parts of northern Alaska, has diminishing benefits for many birds, and ultimately can hurt some species.
"We can expect of lot of birds to move in as shrubs come in, but that benefit is not likely to continue," said lead author Sarah Thompson, a USGS wildlife biologist.
The point is made in the study's title: "When Winners Become Losers."
Of the 17 species in the study, only one — the gray-cheeked thrush — had abundance that continued to rise as shrub heights did. Other shrub-using species had numbers that topped out at certain heights. Arctic warblers, the study found, decreased in abundance when shrubs were higher than about a foot. And shorebirds stayed away from shrubs entirely.
As shrubs become more plentiful and dense on the Seward Peninsula, several species — like fox sparrows, Savannah sparrows and yellow warblers — will benefit, at least for a while, while a few others, like arctic warblers and bristle-thighed curlews, will be harmed.
In the long term, however, a "considerable number" of bird species will probably find the region's habitat to be "unsuitable," the study concludes.
With its boreal-Arctic transition ecosystem, the Seward Peninsula has a wide mix of habitats, making it a good place to study many different types of birds, Thompson said.
"The interesting thing about birds is you have so much diversity in what kind of habitats they prefer and what kind of habitats they can tolerate," she said.
Just why too much shrubbery is bad for some birds is the subject of follow-up study already underway.
There are some theories, Thompson said: Taller and more plentiful shrubs might change supplies of insects or seeds that birds eat. They might be ill-suited for some of the birds' nests. And they might be drawing in more predators, like high-perching ravens.
Steve Matsuoka, a USGS wildlife biologist leading that phase of the study, said that field work started last summer will continue this summer, with findings to be published later.
His team has been examining different types of habitat, with varying types of vegetation, and recording the nesting behavior, food supplies and other conditions.
"We're tying to kind of link together how the habitat types are changing and how birds are either capitalizing on the changes or being forced out," he said.
The bird studies are only part of the research being conducted on the Seward Peninsula, he said. Other scientists are studying changes to such features as permafrost and ponds, he said.
"It's right at the border of the boreal and the Arctic, so I think a lot of people are looking at that transition," he said.