I want to add some words of my own here, but, really, I'm not being lazy. The Audubon Society has said it way better than I could.
"Every year, billions of birds migrate north in the spring and south in the fall. Most fly at night. As they pass over big cities on their way, they can become disoriented by bright lights, especially those on buildings and other structures that reach high into the sky directly in their path. Millions die. [italics are mine]
"Some are casualties of nighttime collisions with windows. Others circle in confusion until they become exhausted; when they land they fall prey to other urban threats. Dozens of species are affected, including such priority species—those we’ve identified as most in need of and most likely to benefit from our help—as the Allen’s Hummingbird, Varied Thrush, Golden-winged Warbler, and Seaside Sparrow.
"Lights Out is a national effort to reduce this problem. The strategy is simple: By convincing building owners and managers to turn off excess lighting during the months migrating birds are flying overhead, we help to provide them safe passage between their nesting and wintering grounds.
"For a list of existing Lights Out programs around the country, click here."