Birds are getting smaller, study finds

James Marshall
December 7, 2019

The scientists aren't exactly sure why warmer temperatures cause birds to shrink.

At the same time their wings have got longer - to offset the loss in weight, say scientists.

"When we began collecting the data analyzed in this study, we were addressing a few simple questions about year-to-year and season-to-season variations in birds", said Willard.

The evidence suggests warming temperatures caused the decrease in body size, which in turn caused the increase in wing length.

"We had good reason to expect that increasing temperatures would lead to reductions in body size, based on previous studies", said Brian Weeks, an assistant professor at the University of Michigan School for Environment and Sustainability and lead author of the study that published Wednesday in the journal Ecology Letters.

"The species were pretty diverse, but responding in a similar way", he said.

Mr Weeks said the body of specimens was the result of a "herculean effort" by Dave Willard, co-author of the study and an ornithologist at the Field Museum in Chicago. It isn't the most cheerful task, but the measurements have offered scientists novel insights into the physiological changes triggered by global warming.

The researchers measured and weighed a parade of birds that crashed into building windows and went splat onto the ground.

Numerous birds hit the hulking glass McCormick Place, North America's largest convention centre. And since McCormick Place is just over a mile away from the Field, Willard headed down there one morning in 1978 to see what he could find.

"That's one major implication", he said.

"I found a couple dead birds and I brought them back to the museum - I've always wondered if there had been no birds that morning whether I would have ever bothered to go back". But this study shows the morphology of the body is the third important aspect.

Senior author Dr Ben Winger, also at MI, described it as a "herculean effort" to get such valuable information from birds that would otherwise have been discarded.

Animals' body sizes are often tied to the climate they live in-within a species, individuals that live in cold climates tend to be bigger than their counterparts in warmer areas.

Researchers are now examining the same data set to better understand why the wing length of so many species increased over the last several decades.

"There is nothing particularly hard about making the actual measurements - there is a special ruler for measuring wings and we use a digital caliper for bill length and tarsus length and a digital scale for weight", Williard said. Brief respites of cooling amidst the broader warming trends were reflected in small increases in bird body size.

As the climate warms, many birds are shrinking in size while their wingspans are increasing, according to a study. The identical 12 months, one other examine discovered salamanders had shrunk quickly in response to local weather change.

The consistency of the body-size declines reported suggests such changes should be added to the list of challenges facing wildlife in a rapidly warming world.

He says the birds most likely to survive migration were the ones with longer wingspans that compensated for their smaller bodies.

The birds analysed are small-bodied songbirds that breed north of Chicago in the summer and migrate through the region in high numbers.

Several species of sparrow, warbler and thrush make up the majority of the dataset - with thousands of individuals of each species documented as lethal collisions.

Three measures of body size - tarsus length, body mass and PC1, a common measure of overall body size that combines several key body-part measurements - showed statistically significant declines.

"The phrase "climate trade" as a most modern phenomenon used to be barely on the horizon. The outcomes from this gaze highlight how predominant lengthy-time period recordsdata sets are for figuring out and analyzing traits precipitated by changes in our atmosphere".

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