TORONTO — In the shadow of the massive black towers of a bank’s downtown headquarters here was an almost indistinguishable puff of dark gray fluff on the sidewalk.

It was the body of a golden-crowned kinglet, an unlucky one, that had crashed into the iconic Toronto-Dominion Center building somewhere above.

There is no precise ranking of the world’s most deadly cities for migratory birds, but Toronto is considered a top contender for the title. When a British nature documentary crew wanted to film birds killed by crashes into glass, Daniel Klem Jr., an ornithologist at Muhlenberg College in Allentown, Pa., who has been studying the issue for about 40 years, directed them here, where huge numbers of birds streaking through the skies one moment can be plummeting toward the concrete the next.

“They’re getting killed everywhere and anywhere where there’s even the smallest garage window,” Professor Klem said. “In the case of Toronto, perhaps because of the number of buildings and the number of birds, it’s more dramatic.”