The Audubon Society, protector of all things avian, tackles this question in a blog post.
An illegal fireworks display in Beebe, Ark., a year and a half ago was blamed for the deaths of up to 5,000 red-winged blackbirds. The blasts disturbed and disoriented a winter roost of the birds, sending them flying every which way and crashing into houses, cars, trees, you name it. “Necropsy report shows trauma primarily to the chest,” said Karen Rowe of CSI: Arkansas the Arkansas Game and Fish Commission. “Hemorrhaging in the body cavity, bruised skulls, blood clots in brain. It’s consistent with crashing into something rather than falling.” Yikes.
But before you chain yourself to your local Chamber of Commerce fireworks stash to prevent it from being detonated on the Fourth, it appears that the Arkansas carnage was unusual.
The problem in Beebe, Arkansas was timing. Kevin McGowan from the Cornell Lab of Ornithology points out that the red-winged blackbirds were grouped together in one location, typical behavior for the winter season. … During the summer months, birds are usually more spread out, which means fewer bird casualties. “You’re going to scare a few robins here and there, but that’s not going to affect a large number of birds,” [says McGowan].
Still, it’s better if you don’t set off your own fireworks:
If you want to see your fireworks and protect birds, too, the best thing to do is attend a commercial display, rather than setting off your own pyrotechnic devices. Commercial fireworks are concentrated in one location, rather than in several locations at once, which is what often happens in neighborhoods. This allows birds to take off and land again in a “safer” location rather than continuing to flee noises coming at them from all directions. In addition, professional displays often take into account the natural environment and any impacts they might have.
So put down the pyrotechnics, sit your ass down with an organic beer, and watch the professionals at work. Hyper-local isn’t always best.
Also check out what Grist advice columnist Umbra Fisk has to say about the eco-impact of fireworks.
Do Fireworks Cause Bird Deaths? What to Do on the Fourth of July, Audubon blog.