Shape-shifting birds in US skies surprise science

Woodpeckers are one of the species examined by the researchers after the birds had crashed to their deaths. Image: By Robert Woeger on Unsplash

The seasons are changing: American avian migrants are now increasingly shape-shifting birds. Their corpses tell an odd story.

LONDON, 25 June, 2021 − America’s migratory birds are setting off for the breeding grounds ever earlier. That’s not the only change. As global temperatures creep ever higher, the birds’ bodies are getting smaller − but their wings are getting longer. And, a little unexpectedly, the changes producing these shape-shifting birds may not be connected, according to new research.

“We know that bird morphology has a major effect on the efficiency and speed of flight, so we became curious whether the environmental pressure to advance spring migration would lead to natural selection for longer wings,” said Marketa Zimova, of the University of Michigan.

“We found that birds are changing in size and shape independently of changes in their migration timing, which was surprising.”

The study, in the Journal of Animal Ecology, is however a lesson for non-scientists and natural historians in the extraordinary value of museum collections, and a bleak reminder that humankind is casually but relentlessly reducing the numbers and variety of the living things that keep planetary ecosystems − and humans − in good health.

Migrants’ problems

The researchers arrived at their conclusion simply by examining the bodies of birds that had flown into the windows of tall buildings and died on the spot. The scale of this is alarming: between 1978 and 2016 the Field Museum in Chicago assembled 70,716 carcasses of migratory birds, all preserved and recorded with the date of death. Chicago, the researchers write, “is one of the most dangerous cities in the United States for building collisions driven by artificial light at night.”

Within this vast haul of accident victims they counted 11 families, 30 genera and 52 species. All but two of the species − a rail and a woodpecker − were passerines, perching songbirds. To make sure their specimens reliably told a tale of migration timing, the scientists selected only those species of which they had 100 or more individuals and, of those, there had to be at least 10 from each decade in the last 40 years.

Global heating has begun to impose change on the natural world: vulnerable species are at risk, and the sheer numbers of insects, reptiles, amphibians, birds and mammals have fallen dramatically as human numbers and human economies have grown.

Climate change creates special problems for migratory birds because food supplies may not be in step with earlier seasonal shifts, and there has been repeated evidence of change either in bird numbers or bird behaviour as thermometer levels rise. And this is as true for North American birds as for those on any other continent.

“Birds are changing in size and shape independently of changes in their migration timing, which was surprising”

So the mere existence of a huge and growing reservoir of accidentally-killed specimens gave the researchers a chance to examine the links between physical change, higher temperatures and earlier springs in more detail.

On the evidence preserved in the Field Museum the earliest spring migrants are arriving five days earlier than 40 years ago, with the earliest fall migrants heading south 10 days earlier than once they did.

In a warming world, creatures tend to become smaller − because with a bigger surface-to-volume ratio it’s easier to keep cool − but the shift to longer wings is less easy to explain. It’s just possible that with earlier springs, birds flying north don’t need to stop so often.

“And there might be other adjustments that allow birds to migrate faster that we haven’t thought about − maybe some physiological adaptation that might allow faster flight without causing the birds to overheat and lose too much water,” Dr Zimova said. − Climate News Network