Dave Willard, Field Museum scientist, took body size measurements of 70,716 migratory birds specimens. There are more than a thousand birds that die after crashing into building windows during their migrations in Chicago. A study found that though the wingspans of birds were increasing, their body sizes on an average were getting smaller gradually over the period. Wingpans increased possibly to enable longer duration travel of birds compared to their smaller body size. According to the researchers, climate warming is the root cause of a gradual reduction in the sizes of birds in North America.
Bergmann’s rule says that individuals within the same species become larger in colder regions and smaller in warmer regions. This phenomenon may be the reason for a gradual reduction in body sizes in warmer climatic conditions. The study included most of the songbirds like thrushes, warblers, sparrows with almost 52 species over the four decades. The birds breed in North America colder regions and spend winters in south Chicago. Body size reduced in all the 52 species with a reduction in bone length by 2.4% and body mass by 2.6%. These results suggest that change in climatic condition seems to change both the shape and body size of these species.
Dave Willard, who is collection manager emeritus at the Field Museum in Chicago, said that everyone agrees to change in climatic conditions, and this reduction in body sizes is just a supporting example of how the natural world is getting affected. This study proves to be an alarming situation for North American birds. US and Canada had lost about 2.9 billion birds. And there has been a 29% reduction in the avian population since 1970, according to the study published in September 2019. This study is a takeaway that the changing world of humans had led to environmental change at an unexpected scale and rate, and now this is likely to widen.