For red knot shorebirds that nest in the Arctic and fly across the globe every year, timing is everything — a matter of life and death.

They nest in the coldest place in the world, feeding bugs to chicks to nourish them for a marathon flight to the tropics when winter approaches. Historically, their departure to tropical beaches and their arrival back to the Arctic after the cold relents were perfectly timed, when plenty of food was available in both areas.

But a new study published Thursday in Science says global warming has changed that. Warming has caused Arctic snow to retreat earlier, causing insect populations that peak as the snow melts to rise and fall before chicks can eat as many as they need to grow and power the grueling flights to come.

As a result, red knots are physically shrinking. And because the smaller birds are weaker, they’re dying off and causing the population to shrink as well.

“Juvenile red knots that we caught along the Baltic Coast while on their way to West Africa were smaller and had shorter bills after warm Arctic summers,” said Jan van Gils, a researcher at the NIOZ Royal Netherlands Institute for Sea Research, who was the study’s lead author.

According to estimates calculated at the turn of the century, red knot numbers have fallen by nearly 60,000, and “the threat of extinction is more than real” as they continue to drop, said Eldar Rahimberdiev, a researcher who helped author the study.

This is why it’s a life and death matter for the red knots that nest in the northern Taimyr Peninsula of Russia, where the study took place. After the birds fly a marathon to the west African tropic of Banc d’Arguin National Park in Mauritania for the winter, they usually gulp down bivalves such as oysters to recharge and also power up for the next leg of their journey.

But many juvenile red knots, already under nourished and ill-prepared for the flight they undertook, have beaks too small to reach bivalves buried in the wet sand. As bigger birds with normal beaks eat meals rich in protein, undersized birds settle for sea grasses, a vegan meal that can’t help them fully recover. Many never make it out of Africa.

The shrinkage is a fairly rapid evolution that happened over the past three decades. “Analysis of satellite images has shown that over the past 33 years, snow at the red knot’s breeding grounds has progressively melted earlier, at a rate of half a day per year, so that’s now more than two weeks,” van Gils said.

This non-genetic change in size due to a lack of nutrition could lead to genetic effects later, van Gils said. “For example, the smaller birds will lay smaller eggs themselves. Imagine that the chicks hatching from those eggs grow up under ideal circumstances. They [could] become bigger than their parents, but they will still be smaller than they should be because they started small. So there is some generation-to-generation effect in there as well.”

Animals shrinking as a result of warming is not a new observation, but it was attributed largely to “a universal response to climate change,” as survival mechanism that allowed them to more easily dissipate heat. But the hypothesis that animals are shrinking, along with developing other changes in morphology, because climate change keeps them from getting “enough of the right food at the right time, leading to malnutrition during the juvenile life stage,” is new.

The outlook for red knots with smaller bodies and beaks is grim, a statement that announced the study said. “The poor survival of shrunken first-year birds clearly contributes to the current population decline seen in red knots nowadays.”

Global warming hurts red knots that nest in Nunavut, Canada in another way. That group winters on the southern tip of Brazil and mistimes its 9,000-mile trip back to the Arctic at its breeding ground because of faulty climatic cues. They take off too late to catch the peak of a horseshoe crab spawn, where they gorge on eggs and double their weight during a layover in Delaware.

Biologists knew from past events that the crab feast is crucial. In the 1990s, horseshoe crabs were overfished for shellfish bait. As the crabs went, so went the red knot. By 2000, a population of about 100,000 had fallen to about 44,000, a stunning decrease, said wildlife biologists for the Delaware Bay Estuary Project.

The researchers for the current study said their work is to the first to examine the entire annual migration to piece together the puzzle to why red knots were declining.

As a student, Rahimberdiev said, he actually fought with other researchers over the reason for the red knot decline. Some attributed it to mysterious causes during migration. But researchers in Africa said there was no problem there. “We thought that the problem was at the breeding grounds,” he said. “Now it becomes clear that all these parts are interconnected.”

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