Courtesy of Jan van Gils, NIOZ
The red knot as it is now (l.) and an (exaggerated) projection of how the future red knot might look like (r.): smaller, but having maintained its relatively long bill.
Courtesy of Jan van de Kam
Only long-billed red knots are able to access the deeply burrowed bivalves at their tropical wintering grounds. Shorter-billed birds are forced to make a living from shallowly burrowed seagrass rhizomes.

Climate change could be shrinking these Arctic birds

Warming in the high Arctic may be fueling rapid evolution of red knots, according to new research. 

Honey, we shrunk the shorebirds.

The red knot, a species of bird that breeds in the Arctic, is rapidly changing. These birds have become noticeably smaller with passing generations, a change researchers attribute to warming conditions in the region. Their findings were published Friday in a paper in the journal Science.

Red knots incubate their eggs in Arctic snow so that their offspring will hatch just as the insect population peaks. Chicks normally grow up on a steady diet of insects before migrating to their wintering grounds in West Africa. Once there, first-year birds use their bills to dig out burrowed shellfish, which is their main food source.

But due to warming conditions in the Arctic, red knot hatchlings are missing the insect peak. As a result of limited food availability, the average chick simply doesn’t grow as large as was typical in previous decades. Smaller birds mean smaller beaks, so migrating juveniles struggle to reach their usual food and often settle for less-nutritious seagrass. Researchers call this effect “trophic mismatch.”

“I had never [known] body shrinkage was a becoming a universal response to climate change until we found that our study species was doing so,” Jan van Gils, who co-authored the study, tells The Christian Science Monitor in an interview. “Then I started digging in the literature and found that many species were shrinking.”

Dr. van Gils, who is a senior scientist with NIOZ Royal Netherlands Institute for Sea Research, was part of an international effort to investigate this trend. Until now, researchers have not found overwhelmingly negative consequences of body shrinkage. In fact, some scientists have argued that smaller-bodied animals might dissipate heat more effectively due to a larger surface to volume ratio.

But by linking several different data sets – 33 years of archived data on red knot body size, satellite images to assess snowmelt dates, and survival rates collected annually in Mauritania – researchers were able to show that shrinkage is actually hurting red knot populations.

According to van Gils and colleagues, the smallest knots were the poorest survivors. Suddenly unable to access their main food source, shrunken birds had an increased mortality rate. Researchers suggest that a dwindling red knot population, which has dropped 50 percent since the 1980s, may be a result of the “negative survival effects” of body shrinkage.

As the Arctic continues to warm, van Gils and colleagues expect to see even more changes in red knot anatomy. The change in body size, they suggest, may be a kind of “rapid evolution,” which is a plastic response to an environmental stimulus, rather than a product of generations of genetic mutation. Meanwhile, even as the birds continue to shrink, the individuals with longer beaks will be more biologically successful. In other words, there’s an evolutionary imperative towards small birds with large bills.

This trend, researchers say, requires immediate attention. According to van Gils, it could have serious ecological implications – not just for red knots, but for other animals as well.

“We think that many high-Arctic breeding animals are facing this problem,” van Gils says, “as they often overwinter in southern regions where there is no chance they monitor, let alone anticipate, the onset of an advanced spring in the Arctic. In fact, in the other high-Arctic breeding shorebird that we study at NIOZ, the bar-tailed godwit, we just discovered the same trend: a decline in body size and bill length.”

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