Some adorable shorebirds are shrinking.

The red knot, flying long distances and migrating annually from the Arctic to West Africa and back, has been facing great risks as shellfish food is on the decline in the feeding grounds lining its migratory route.

Now climate change is taking its toll on these birds: they are getting smaller in size, and the effects may be speeding up their extinction.

An international team of scientists found that global warming is making red knot chicks miss their insect peak and therefore achieve a smaller size with a shorter bill. The changes are reducing the population’s survival rates in their wintering grounds in the Southern Hemisphere.

Further, the shrinking migrants are paying the price, with the smallest young birds’ survival only half of that of the larger ones. Lead researcher Jan van Gils of the Royal Netherlands Institute for Sea Research said the reason is “quite straightforward” since only larger birds with long bills can reach deeply burrowed shellfish for food.

“Shorter-billed birds were forced to live on seagrass, which is a poor food source for these birds. The poor survival of shrunken first-year birds clearly contributes to the current population decline seen in red knots nowadays,” van Gils explained, warning that this could be an early sign of extinctions that can no longer be prevented.

The North American red knots were listed as “threatened” by the U.S. Fish and Wildlife Service in 2015 due to the dip in horseshoe crab eggs in their key stopover feeding location, Delaware Bay, on their way to their winter home. Since the 1980s, their populations have fallen by up to 75 percent.

Satellite imagery revealed that over the last three decades, snow at the birds’ breeding ground has progressively melted at a half-day rate every year, which brings it to over two weeks at present. This shift has led to a mismatch: plants bloom earlier and insects feeding on the plants emerge earlier, but the chicks eating the insects are too late to feed on them at their peak.

As a result, juvenile red knots encountered along the Baltic coast on their way to Africa demonstrated smaller bodies and shorter bills after warm summers in the Arctic.

Shrinkage in body size is believed to help in climate change adaptation, as a smaller bird is more efficiently dissipating body heat. However, the team saw the opposite: smaller animals are faring worse when it comes to survival.

The dangerous trend will likely continue among other High-Arctic breeding species in the future and bring about serious ecological consequences, warned van Gils.

The findings are now published in the journal Science.

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