A staggering 600 million birds die every year in the United States after colliding with tall buildings. Birds have been steadily shrinking in size over the last four decades thanks to the increases in temperatures due to climate change, a study at the Field Museum in Chicago has found.
All of the 52 species of birds in the research showed reduced body size and mass over the forty years of records, with an average 2.4 percent decrease in leg bone length. However, the birds’ wingspans increased by an average of 1.3 percent, which the researchers believe allows the birds to continue making long migrations despite having a smaller body to produce the energy needed for flight.
“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 variation in birds,” said Dave Willard, a collections manager at Chicago’s Field Museum.
“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.”
Birds’ size is related to the temperature of their climate. Birds tend to be larger in cooler climates since a larger body mass can help them to stay warm. The researchers say the decrease in size is in keeping with the gradual warming of temperatures in Chicago, where the study was carried out.
The study began by chance in 1978 when Willard heard colleagues that birds were crashing into sides of the huge McCormick Place building, a mile down the road from his job at the Field Museum.
“I was curious, so I went for a walk around the building one morning,” he said. “I found a couple of 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.”
From that day on, scientists and volunteers at the museum collected the birds that collided with the building, taking their measurements and logging them, by hand, in a ledger.
Though the team noticed slight changes in measurements over the years, it wasn’t until the full, four-decade analysis that they realized the extent of the shrinking effect.
Furthermore, this isn’t the only way that climate change has changed bird ecology recently. According to a separate study published in the journal Science in September, an estimated one in four birds have disappeared in the last 50 years in the Western Hemisphere.