Science.org – Some 3.5 billion birds fly through the southern border area of the contiguous United States each spring, then head north through the center of the country, Cornell ecologist Adriaan Dokter found in a 2018 study using weather radar. Come fall, after breeding season, roughly 4.7 billion birds travel south, this time along routes that cluster more toward the eastern United States. Collisions with buildings take out anywhere from 365 million to 1 billion of them each year, according to one landmark meta-analysis of papers and bird collision data sets published in 2014. That study put buildings behind only domestic cats on the list of top anthropogenic bird killers.
