The red knot shorebird is a tough, long-distance flier that migrates yearly from the Arctic into the Southern Hemisphere and back. For decades, the bird has been at risk because its food sources, such as crab eggs, have declined in the feeding grounds along its migratory route.
Now an alarming discovery by an international team of scientists, published this week in Science, shows that the effects of climate change on the robin-size shorebird may hasten its extinction.
“It’s even worse than we thought,” says Michael Reed, a conservation biologist and bird expert who teaches at Tufts University outside Boston. “This is going to increase their extinction risk, and the red knots are already in the highest risk category.”
The study found that warming is contributing to malnutrition of red knot chicks in their breeding grounds in the high Arctic, leading to smaller body sizes with shorter bills. Those changes, in turn, are reducing the survival rates of the birds in their winter grounds farther south.
“This could be a first early warning signal for extinctions that are awaiting us that we cannot prevent,” he says.
The North American red knot, was listed by the U.S. Fish and Wildlife Service as “threatened” under the Endangered Species Act in 2015 because of the decline in horseshoe crab eggs in Delaware Bay, a key stopover feeding site en route to its winter home in Tierra del Fuego in South America. Red knot populations in some areas have declined by as much as 75 percent since 1980.
It’s Not Easy Being Small
For the new study, the team tracked the subspecies of red knot that nests in northern Russia and winters on the tropical coast of Mauritania in West Africa. The scientists analyzed more than three decades of satellite imagery to look for a correlation between snow melt and development of red knot chicks.
The team found that the red knot’s Arctic breeding grounds have been progressively melting earlier, at a rate of a half day a year. The snow now melts two weeks earlier than it did 33 years ago. This shift has created a trophic mismatch, meaning plants bloom earlier, and insects that eat the plants emerge earlier, while the red knot chicks that eat the insects are hatching too late to feed on them at their peak.
Consequently, the body size of juvenile red knots is smaller and the birds’ bills are shorter. After they fly to their wintering grounds, it is harder for them to reach deep into the sand and retrieve clams and mussels—proteins essential to their diet. Instead, the birds are eating more sea grass.
The survival rates among the shrunken red knots are half that of larger juvenile red knots, van Gils says, because birds with larger bills are still able to retrieve bivalves burrowed in the sand.
Until recently, body size shrinkage was regarded as a possible adaption to climate change, because a smaller bird is better able to dissipate body heat, van Gils says.
“The prediction was that smaller animals do better,” he says. “Our study found the opposite. Smaller animals are having a harder time surviving.”
Fly Me to the Moon
All birds making long migrations are constantly vulnerable—in summer, in winter, and along the route, says Stuart Pimm, a bird scientist at Duke University in Durham, North Carolina. The red knot makes one of the longest migrations in the animal kingdom.
In recent years, the red knot has become known as the moon bird, named for an individual labeled B95 whose route was tracked in the Americas for more than 18 years. In that time, B95 flew an estimated 320,000 miles (515,000 kilometers)—a distance equal to flying to the moon and halfway back.
The bird is also known for the distinctive way in which it feeds. It flies virtually nonstop, often traveling at least half the route before stopping to refuel. Then the red knot takes on board so much food that the body size swells, before it departs for the next leg.
It would seem that the evolutionary answer to the red knots’ threat is for the birds to fly to the Arctic earlier to catch the insects at their peak and preserve body size. Some red knots are heading north earlier, van Gils says, but not early enough to make a difference.
“The ones leaving the tropics earlier do the best,” he says. “But they winter so far away, it’s impossible for them to anticipate a very early summer in the Arctic.”
And for the species as a whole, such an evolutionary change may not be possible. The birds’ annual migration to northern nesting grounds is triggered by changes in daylight, not warming global temperatures, and the seasonal daylight cycles remain constant.
Reed, the Tufts bird expert, points to racehorse breeding as a clear example of the complications in evolutionary change.
“We spend millions of dollars breeding the best and the fastest, but if you look at the winning times for the Kentucky Derby over the last 40 years, the horses are not going faster. They can’t,” he says. “For the red knots, no matter how hard evolution says to leave earlier, if there is no variation in the genes to leave earlier, they can’t.”