Plastic is a miracle of modern technology, but improperly disposed of, it may harm birds and other wildlife. Each year, hundreds of thousands of birds become poisoned or have their digestive tracts obstructed after eating small pieces of plastic. You can help save the birds in your yard, and even halfway around the world, by making sure that you recycle plastic whenever possible, and otherwise properly dispose of plastic items after you are done with them.
Seabirds--that water bottle cap or other piece of plastic that you might be tempted to throw out your car window can easily be swept into waterways during the next storm and eventually end up floating in the ocean, where dozens of seabird species have been observed to die after eating floating plastic trash. You can easily find dozens of articles about this online, including an early review paper on the subject published in 1987.
California Condors--small pieces of plastic and other trash in the environment are one of the greatest threats to the critically endangered California Condor. For some reason that we don't clearly understand, adult condors feed small rocks and other items to their chicks--perhaps to help their digestion. When the adults find small pieces of trash, they feed them to their young, and the young get sick or die. See this report on microtrash and condors published last year in Bird Conservation International and read the latest report (here) on threats to the California Condor put out by the American Ornithologists Union and Audubon California.
Other Wildlife--while there aren't any studies of birds other than seabirds and condors being killed by eating garbage, trash may also threaten small birds and other wildlife (see this report).
Everyone knows not to litter. Here's just another reason not to--it kills birds and other wildlife. Recycling, putting litter in its place, and organizing a trash pickup not only keeps your neighborhood cleaner, it also protects your local birds as well as those flying over the oceans half a world away.
California Condor photo: Sam Haskins