The New York Times, May 12, 2016
Animal migrations combine staggering endurance and exquisite timing.
Consider the odyssey of a bird known as the red knot. Each spring, flocks of the intrepid shorebirds fly up to 9,300 miles from the tropics to the Arctic. As the snow melts, they mate and produce a new generation of chicks. The chicks gorge themselves on insects, and then all the red knots head back south.
“They are there less than two months,” said Jan A. van Gils, an ecologist at the Royal Netherlands Institute for Sea Research. “It’s a very tight schedule.”
It is also a vulnerable one. The precipitous decline of the red knots that winter in West Africa may provide a small but telling parable of the perils of climate change.
In a study published Thursday in the journal Science, Dr. van Gils and his colleagues present evidence that indicates climate change is putting pressure on the birds along their entire journey, possibly helping to drive down the birds’ population and making them yet another of many species around the world being affected by climate change.
The new study shows how climate change can create ecological ripples that can threaten a species in unexpected ways.
“I think it’s a fascinating study,” said David S. Wilcove, an expert on animal migrations at Princeton University. “It illustrates the odd and almost unpredictable ways in which climate change is affecting biodiversity.”
And if Dr. van Gils and his colleagues are right, further warming will make the plight of the red knot even more dire.
“I foresee some sort of crash,” Dr. van Gils said.
The researchers got their first clue that something was amiss when they studied a subspecies of red knots that winters on the coast of Mauritania in West Africa. Typically, red knots dig into the sand to harvest clams with their long bills. The scientists were intrigued to discover that some of the birds were digging up sea grass roots and eating them instead.
“That puzzled me,” Dr. van Gils said. “So I wondered, ‘Why is this?’”
It turns out that the clams are buried deeper in the sand than the sea grass roots are. Small red knots with short bills could not reach the clams, so they had to make do with the less nutritious sea grass roots. “These smaller birds go for something they don’t like so much,” Dr. van Gils said.
To understand why the birds grew to different sizes, the team began a study of the red knots’ full migratory path. In June, the birds leave Mauritania and fly to the Arctic coast of Russia. At the end of July, they make a return trip. The adults stop off to refuel in the Netherlands, while the juvenile red knots travel through Poland.
The scientists found a disturbing trend: Over the past 30 years, the juveniles arriving in Poland have been shrinking in size. On average, they are about 15 percent smaller today than in 1985.
Looking at satellite images of their Arctic summer habitat, the scientists found a clue to this trend. The Arctic has been warming up earlier because of climate change. Today, the snow is melting two weeks earlier than in 1985.
The insects in the Arctic are responding to the shift by hatching earlier. But the red knots are not adjusting their schedule. By the time their chicks hatch, the insects are far past their peak, and the birds can’t find as much food as they could 30 years ago.
When the birds arrive in Mauritania in July, the smaller juveniles with shorter beaks cannot dig deep enough to eat their regular diet of clams. Instead, they are eating more sea grass. This new diet appears to be taking a toll: Dr. van Gils and his colleagues have found that juvenile red knots with short bills are more likely to die than birds with long bills.
(How the shorebirds, which turn a rusty color in spring, got their name is a bit of a mystery. One theory suggests that the pattern of the birds feeding along the tide line evoked the image of Canute, a Norse king whose failed efforts to hold back the tide became legend. Another possible notion could be that the birds’ grunting call sounds like the word knot.)
Martin Wikelski, a zoologist at the Max Planck Institute for Ornithology who was not involved in the research, cautioned that some questions about the birds were still unanswered. For example, the team kept track of the birds in Mauritania by banding them with colored rings. When the birds disappeared, the scientists couldn’t know for sure if they were dead or had just gone elsewhere.
“It’s the first of its kind,” Dr. Wikelski said of the new study, “but it might not be true.”
Scientists will be able to get better data about migratory birds, Dr. Wikelski said, as miniature tracking devices become more powerful and affordable. Experiments will also allow scientists to test hypotheses about the animals. This summer, for example, Dr. van Gils is going to study a population of red knots that summer in Alaska. He will move eggs to places where insects emerge at different times in the summer. He predicts the timing will affect the chicks’ growth.
Dr. van Gils said that climate change might be a factor behind the decline of the red knots that winter in Mauritania. They have decreased from about half a million birds to a quarter of a million.
“If that continues, they’re going to go extinct,” he said.
Dr. Wikelski still wants to see more evidence to judge the future of migratory birds in a warmer world. But he has seen enough to be worried.
“Maybe they’re sentinels,” he said about the red knots. “We should take these early warning signs seriously.”
Copyright 2016 The New York Times Company. Reprinted with permission.