That is impossible right now for scientists to calculate. One estimate is that about 50% of the migrating population won’t return to their original birthplace.
There has been a lot of talk lately about the whirling blades of wind farms not only creating renewable energy but killing flying birds and bats. It’s already known that the wind turbines should not be built in bird migration routes. But the National Wind Coordinating Committee also came up with the following bird fatality statistics in the United States:
•98 million to 980 million fatal collisions with buildings and windows
•130 million to more than one billion fatal collisions with high-tension lines
•60 million to 80 million deaths caused by automobiles
•4 million to 50 million fatal encounters with communications towers
•72 million birds each year are killed by toxic chemicals, including pesticides
•Domestic cats are estimated to kill hundreds of millions of birds each year
•15 million birds a year in North America are killed in managed annual waterfowl hunt kills
•20,000 to 37,000 fatal collisions with wind turbines
At the East Lansing Wild Birds Unlimited store today we had a Swainson’s Thrush stuck under our awning. It was running back and forth trying to fly up, up, up through the glass ceiling. The House Sparrows that live up there think it’s the best designed building ever because bugs go up there and get stuck too. All day long I watch the sparrows fly up grab a quick meal and fly down.
New birds to the area, however, like the thrush sometimes get stuck. I rescued a hummingbird once with a net because I knew it didn’t have all night to figure out how to get out of the awning. But we watched this thrush for about an hour running back and forth. Finally the sparrows ganged up and pushed the thrush down and out. I’ve seen them do this before with a Goldfinch and a Downy Woodpecker. Whether they were offering a helping hand or just shooing away a stranger from their territory, I was glad he was finally free. I couldn’t help but think of the millions of other birds that find themselves in similar situations during migration and don’t make it.
The good news is I got an up close view of the Swainson’s Thrush Catharus ustulatus. It was named after William Swainson, an English ornithologist, and is also called the Olive-backed Thrush. On its breeding grounds in northern Michigan it is usually seen perched high in a treetop. During migration the bird skulks low on the ground under shrubs and with luck spend its winter in the tropics. A group of thrushes are collectively known as a "hermitage" and a "mutation" of thrushes.