A team of international researchers from the Netherlands, France, Australia, Poland and Russia found that climate change is putting too much pressure to robin-sized migratory birds forcing them to alter their migratory timeframe and affecting their size.

The study, published in the journal Science, suggests that climate change is shrinking the size of red knots, a medium-sized migratory shorebird, making it more difficult for them to continue their long distance travels and can potential push them to extinction.

Body shrinkage is considered to the universal response of animals to climate change because smaller body size can dissipate body heat better due to the larger surface to volume ratio, but researchers discovered that this is not the case for the red knots.

According to the study, climate change causes the arctic ice to melt earlier during summer, about two weeks earlier. Due to this, insects living in the arctic rescheduled their hatching season timing it to the ice melt. Insects are the primary source of nutrition of red knots before they leave the arctic for their lengthy travels. Unlike the insects, red knot did not alter their breeding season. As an effect, when new red knots were hatched, it is already passed the insect peak, making them smaller due to malnutrition.

Researchers also discovered that smaller red knots also possess shorter bills. This can also contribute to the shrinking body size of the bird. When the red knots reached the tropics in July, their primary source of diet is clams and other shellfish food burrowed deeply in the ground. The shorter bill of the undersized birds is unable to reach for the clams. As a result they are forced to consume more shallowly buried sea grass, which is far less nutritious than the bivalves.

"Since smaller birds do worse than larger ones, we reject the hypothesis that body shrinkage is evolutionarily beneficial. Instead, we suggest that a so-called 'trophic mismatch' during chick stage underlies the smaller knot body: due to the rapidly advancing Arctic summer, juvenile knots are now simply born after the rapidly advancing insect peak. We therefore propose that changes in body size and shape, and the negative population dynamical consequences, will be widespread among other High-Arctic breeding species in the future. This is a very serious ecological effect that requires our immediate attention," said Jan van Gils from NIOZ Royal Netherlands Institute for Sea Research and first author of the study, in a statement.

According to the report from National Geographic, the North American red knot, was listed by the U.S. Fish and Wildlife Service as "threatened" under the Endangered Species Act in 2015.