Rising temperatures are making birds smaller

Birds are shrinking as the climate warms

Birds are shrinking as the climate warms

"Despite the diversity of so many species, they are all changing in the same way", said Brian Weeks, an evolutionary ecologist at the University of MI who was the lead author of the study.

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

Co-author Dr David Willard, an ornithologist at the city's Field Museum, measured each specimen - producing a remarkably detailed dataset.

Along with a volunteer group, since 1978 he has collected more than 100,000 dead birds, carefully measuring them with a caliber and scale and cataloging the results by hand in a ledger.

The study provides fresh evidence of worrisome trends for North American birds. 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, Dr Willard headed down there one morning in 1978 to see what he could find.

The birds had died after colliding with buildings in Chicago, Illinois. "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".

"Periods of rapid warming are followed really closely by periods of decline in body size, and vice versa", Weeks said.

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.

Within animal species individuals tend to be smaller in warmer parts of their range because they conserve less heat. In particular, the length of the tarsus or lower leg bone, shrank by 2.4%.

This dropped 2.4 percent across species - while wing length showed an average increase of 1.3 percent.

Meanwhile, wing length showed a mean increase of 1.3%, with the species showing the fastest declines in tarsus length also showing the most rapid gains in wing length.

Body size decreased significantly as the mercury rose warmed. The changes appear to be a response to a warming climate.

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.

"In other words, climate change seems to be changing both the size and shape of these species", said biologist Brian Weeks of the University of Michigan's School for Environment and Sustainability, lead author of the study published in the journal Ecology Letters.

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

The extreme energetic demands of flying thousands of miles have shaped the morphology of migrating birds - their form and structure - for efficient flight. The number of birds we get each day is highly variable depending on whether it's a big day of migration.

The changes were too small to be apparent to the naked eye, the scientists said, amounting to a gram or so in weight per bird and a few millimeters change in individual wing length. "The phrase "climate change" as a modern phenomenon was barely on the horizon". "The results of this study highlight how essential long-term data sets are for identifying and analyzing trends caused by changes in our environment", says Willard, a collections manager emeritus at the Field Museum and one of the authors of a new study in Ecology Letters presenting these findings, along with lead author Brian Weeks and senior author Ben Winger from the University of MI.