Ledgers record body size measurements, measuring tools, and a Tennessee Warbler from Field Museum scientist Dave Willard who measured 70,716 specimens of migratory birds that collided with buildings in Chicago over a period of about four decades. photo published December 4, 2019. Field Museum / Kate Golembiewski / Press Release via REUTERS
WASHINGTON (Reuters) – Since 1978, researchers have been collecting and measuring tens of thousands of birds that died after colliding with buildings in Chicago during spring and autumn migrations. Their work has documented what might be called the incredible shrinking bird.
A study published Wednesday involving 70,716 birds killed from 1978 to 2016 in such collisions in the third largest city in the US found that the average size of their bodies declined steadily over time, although their wings increased.
The findings suggest that a warm climate is shrinking the size of certain bird species in North America and perhaps worldwide, the researchers said. They cited a phenomenon called the Bergmann Rule, where individuals within a species tend to be smaller in warmer regions and larger in colder regions, as a reason to believe that species can become smaller over time as temperature increases.
The study focused on 52 species – mainly songbirds dominated by various sparrows, warblers and thrushes – that breed in cold North American regions and winter in locations south of Chicago. The researchers measured and weighed a parade of birds that collided with the building's windows and fell to the floor.
Over the four decades, body size has decreased in all 52 species. Average body mass fell by 2.6%. The length of the leg bone fell 2.4%. The wingspan increased by 1.3%, possibly to allow species to continue to make long migrations, even with smaller bodies.
"In other words, climate change seems to be changing the size and shape of these species," said biologist Brian Weeks of the University of Michigan School of Environment and Sustainability, lead author of the study published in the journal Ecology Letters.
"Virtually everyone agrees that the weather is warming, but examples of how it is affecting the natural world are now emerging," added Dave Willard, emeritus manager of collections at the Chicago Field Museum, which measured all birds.
The study provides new evidence of worrying trends for US birds. A study published in September documented a 29 percent drop in the US and Canadian bird population since 1970 and a net loss of about 2.9 billion birds.
"I think this is the message," said Weeks. "As humans change the world at an unprecedented rate and scale, there are probably widespread and consistent biotic responses to environmental changes."
Will Dunham report; Editing by Lisa Shumaker
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