Every year, flocks of red knots criss-cross the globe. In the summer, these shorebirds breed in the Arctic circle, making the most of the exposed vegetation and constant daylight. Then, anticipating the returning ice and continuous night, they fly to the opposite end of the world. Different populations have their own itineraries, but all are epically long: Alaska to Venezuela; Canada to Patagonia; Siberia to Australia.  

These migratory marathons mean that the red knot’s fate in one continent can be decided by conditions half a world away. And that makes it a global indicator, a sentinel for a changing world. It is the proverbial canary in the coalmine, except the mine is the planet.

And the canary is shrinking.

For the last 33 years, European scientists have been measuring a population of red knots that stop at Poland’s Gdansk Bay on their migrations between northern Russia and western Africa. When Jan van Gils from the Royal Netherlands Institute for Sea Research analyzed these measurements, he noticed that the knots have been gradually getting smaller.

They’re not alone. Israeli sparrows, Danish hawks, Alaskan polar bears, North Sea fish, and many other animal populations are getting smaller as the world warms. Some scientists suggest that the trend is adaptive: compact bodies are useful in hotter conditions because smaller individuals have a larger surface area for their size, and so lose heat more quickly. Others say it’s maladaptive: the shrunken species are simply undernourished.

To find out which explanation applied to the knots, van Gils analyzed satellite images of the birds’ Russian breeding grounds. These pictures revealed that the region’s snow has been melting earlier and earlier. Thirty years ago, it was disappearing in mid-July. Now, it’s gone by late June, some two weeks prior. And the earlier the melts happen, the smaller the young knots get.

Why? It’s probably because red knot chicks mainly feed on the insects that emerge from defrosting Arctic soil. If the snow melts too early, the hatchlings miss out on Peak Insect and can’t eat enough to pack on weight. They end up small and stunted.

Van Gils admits that this part of the story is still a little speculative. What’s clearer is that the knots never compensate for their stunted growth. If they leave Russia small, they arrive in Mauritania small. And that’s a big problem.  

Smaller birds have shorter bills and can’t reach the abundant clams buried in the Mauritanian mud. Every missing millimeter translates to a lot of missing meals. For example, van Gils calculated that a bird with a four-centimeter bill can reach two-thirds of all the available clams, but one with a three-centimeter bill can reach just one third. These shorter-billed individuals must resort to eating other species of shellfish (which are rarer) or on seagrass (which is less nutritious).

This inefficient foraging costs them dearly. By tracking individual knots using colored leg rings, Van Gils’ team found that short-billed juveniles were half as likely to show up in Mauritania for a second winter than long-billed peers. It’s possible that they flew off to new pastures, but van Gils doubts it. “There are very few sites where these birds go, and hardly any of our birds turned up at the other places,” he says. “We really think they died.”

This is one of the few studies to clearly investigate the consequences of smaller size, says Celine Teplitsky from the CNRS in France. It shows that climate change can affect the lives of animals in indirect and large-scale ways, “even in places where the change is relatively mild.” Earlier snowmelts in the Arctic mean that knots grow up with smaller bills, can’t eat enough in Africa, and die early. Their bodies are sculpted at the poles but tested in the tropics, thousands of kilometers away.

“This presents a call to arms for the scientific community,” says Meg Crofoot from the University of California, Davis, who notes that many researchers only study migratory species at their breeding sites. “Many long-distance migrants are too small to track using existing technology, so their migratory routes and key stop-over locations are poorly described. In some cases, even the location of their wintering grounds remain a mystery.

Several developments will help to reveal the routes of these unknown migrations. Tracking devices are getting lighter and less obtrusive, allowing researchers to follow the movements of the tiniest of migrants. One such study revealed the 70,000-kilometer round-trip of the Arctic tern. Another showed that three swifts probably flew non-stop for 200 days.  These trackers will allow van Gils and others to track particular birds throughout their lives.

Satellites could help, too. The ICARUS Initiative is developing a system for tracking small animals at global scales, using tags that can be sensed from the International Space Station. Meanwhile, scientists are working to combine and open up their disparate data sets, to get a more comprehensive picture of the world’s animal travelers.

Their work is urgent. In the last thirty years, an estimated 400 million songbirds have disappeared from Europe. Red knot populations have halved. Van Gils has found that the bar-tailed godwit, the world’s longest non-stop flier, is also shrinking in both body and bill. The race is on to understand the lives of these migrants before planes become the only things speeding through intercontinental skies.