A study has found out that many birds are shrinking in size while their wingspans are increasing as the climate warms. Researchers from the University of Michigan in the US analysed a dataset of 70,000 North American migratory birds from 52 species that died when they collided with buildings in Chicago, US. They found that, from 1978 through 2016, body size decreased in all 52 species, with statistically significant declines in 49 species. The findings, published in the journal Ecology Letters, also show that wing length increased significantly in 40 species over the same period.
Brian Weeks, an assistant professor at the University of Michigan said, the new study is the largest specimen-based analysis of body-size responses to recent warming. He added, the study shows the most consistent large-scale responses for a diverse group of birds. According to the researchers, several lines of evidence suggest a causal relationship between warming temperatures and the observed declines in avian body size. The researchers said, the strongest evidence is that embedded within the long-term trends of declining body size and increasing temperature.
There are numerous short-term fluctuations in body size and temperature that appear to be synchronised. The researchers added, within animal species, individuals tend to be smaller in warmer parts of their range, a pattern known as Bergmann’s rule. For each bird, the researchers measured the length of a lower leg bone called the tarsus, bill length, wing length, and body mass. In birds, tarsus length is considered the most precise single measure of within-species variation in body size.