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In the fall there are a lot of birds flying south.

A recent study by the Cornell Lab of Ornithology estimated that 4 billion birds fly from Canada into the United States and 4.7 billion leave the southern U.S. and fly to Central or South America. 

The researchers made their estimates of how many birds migrate by using information from 143 weather radar stations across America and then did a lot of math.

In the springtime, birds fly north from wherever they spent the winter.

The researchers found that many birds don’t survive the winter, though. Based on their estimates, 2.6 billion birds fly from the U.S. back into Canada, and 3.5 billion fly from the tropics back to the U.S. That’s a loss of about 2.6 billion birds!

"Contrary to popular thought, birds wintering in the tropics survive the winter better than birds wintering in the U.S.," said Andrew Farnsworth, co-author of the study. "That's despite the fact that tropical wintering birds migrate three to four times farther than the birds staying in the U.S."

Some of the reasons more wintering birds die in the United States might be that there are more buildings with windows to crash into. Americans also own a lot of house cats. These cuddly pets may eat as many as 1.4 billion birds a year or more. That's a good reason to not let your cat out of the house.

The researchers also note that birds wintering in the U.S. lay lots of eggs in an attempt to balance out how many die. Birds that spend their winters in the tropics have fewer chicks, but more adults survive even though they fly longer distances during migration.

How birds figure out where to fly and how to lay a certain amount of eggs is pretty fascinating. Since birds are the modern versions of dinosaurs that lived millions of years ago, they've had a while to figure things out.

— Brett French, french@billingsgazette.com

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