What's happening to red knots is a reminder of why youngsters should eat well to grow big and strong. For these shorebirds that fly global marathons every year, malnutrition has become a matter of life and death.
Red knot chicks often lack proper nutrition because they don't get enough food. Historically, they have hatched in the Arctic when the snow has begun its spring thaw and insect populations that are a staple of their diet have been at their peak. But as part of a problem linked to global warming, their parents linger in tropical areas where the birds winter and arrive in the Arctic to nest after the insect peak.
As a result, red knots are shrinking physically. And because the smaller birds are weaker, they're dying off and causing the population to shrink as well.
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 published Thursday in the journal Science.
Here's why global warming is a life-and-death matter, the study said. When winter approaches at their nesting grounds in the northern Taimyr Peninsula of Russia, adults lead the juveniles on a journey to the west African tropic of Banc d'Arguin National Park in Mauritania, where they winter. But juvenile red knots are unprepared for the rigors of such an epic migrations.
"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.
They can't reach a smorgasbord of bivalves buried in the wet sand of beaches at Banc d'Arguin. As bigger birds gulp down meals rich in protein to replenish the energy spent in long flights and to prepare for even more flying, the smaller birds can't reach deep enough because of their shrunken beaks. They 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 also hurts red knots in other ways. Another group that winters on the southern tip of Brazil mistimes its 9,000-mile trip back to the Arctic at its Canadian breeding ground in Nunavut, thrown off by faulty climatic cues. Timing is key, because the birds can miss 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."