Hundreds of dead migrating songbirds covered the ground like a carpet on Thursday morning.

Nearly 1,000 songbirds died throughout the night after slamming into the glass of the McCormick Place Lakeside Center, the consequence of a lethal confluence of excellent migratory circumstances, rain, and the low-slung exhibition hall's lighting and window-lined walls, according to avian specialists.

Window strikes are a problem in practically every major city in the United States. Birds cannot see transparent or reflecting glass and are unaware that it is a dangerous barrier. They rush for plants or bushes they see through windows or reflected in them, killing themselves in the process.

Night-migrating birds, such as sparrows and warblers, rely on the stars for navigation. Building lights both attract and confuse them, resulting in window hits or birds fluttering around the lights until they die of exhaustion, a phenomena known as fatal light attraction.

According to Stan Temple, a retired wildlife ecology professor and avian expert from University of Wisconsin-Madison, conditions were ideal for a big wave of songbird southern migration over Chicago on Wednesday evening.

"You had all these birds that were just raring to go, but they've been held up with this weird September and October with temperatures way above normal," Temple said.

To avoid air turbulence and predators, little songbirds graze during the day and travel at night. They've been waiting for northerly breezes to push them south, but September's exceptionally warm southern winds left the birds in a holding position here, according to Temple. A front moved south on Wednesday evening, generating a tailwind, and millions of birds took to the air. 

Pre-dawn rain forced the birds to drop to lower altitudes, where they found McCormick Place's lights on, said David Willard, a retired bird division collections manager at the Chicago Field Museum.

According to Anna Pidgeon, an avian biologist at the University of Wisconsin-Madison, window hits and fatal light attraction are simply avoidable. Building managers can simply decrease their lights, and architects can create windows with bird-friendly marks in the glass. People can also add screens, paint their windows, or put stickers on the glass.

(With input from AP)