[JOAN CARTAN-HANSEN] It's called migration. Scientists who study birds define migration as a repetitive movement from one point to another and back all along the same path.

[CHILD] Huh? What does that mean?

[JOAN] Well it's kind of like - kids catching the school bus. Every morning you go from your house to the same spot to meet the bus. And every afternoon the bus lets you off and you migrate back to your house along the same sidewalk. You do it twice a day. Birds migrate twice a year.

[CHILD] Okay but birds don't have to go to school. Why do they leave home and come back?

[JOAN] That's easy. Food! As winter approaches plants begin to dry up. The berries and seeds that many songbirds feed on disappear. Worms tunnel deep into the soil before it freezes out of the reach of robins and mountain bluebirds kind of have a hard time finding bugs after the first frost. So what do they do? Migrate! It's time to head south to warmer areas where there's food all year-round.

[CHILD] So if there's so much food down south why do they come back home?

[JOAN] The answer's the same - food! Food is back. Birds know that their home territories will be green again. Berries, bugs, seeds and worms - all the yummy foods that birds like to eat will be fresh and plentiful in the spring. And when they fly back to their summer homes they can spread out. They won't have to share with all the other birds that spent the winter in the south.

When animals have to share scientists call it competition. It's kind of like a kid with a candy bar. You might break it in half and share it with your brother but you don't want to share it with the whole neighborhood. There might be nothing left for you and your brother. This is especially important when birds return home because spring is the nesting season and the parent birds want to be sure to have enough food to feed themselves and their young.

[CHILD] Okay. I understand why all those birds who eat bugs and berries have to go down south in the fall but what about hawks and falcons? They don't eat those things.

[JOAN] You're right but birds of prey like falcons and hawks eat songbirds that eat the bugs and berries. It's all part of the web of life. And if your lunch is going south and you're a bird of prey, well, you better follow if you want to eat.

[CHILD] How do you know all this?

[JOAN] Research. A type of scientist called a biologist captures songbirds and birds of prey and marks them with bands to find out where the birds come from and where they go.

[BIOLOGIST] Want to see why it's called a ruby crowned kinglet?

[JOAN] At the Idaho Bird Observatory near Boise, songbirds are captured in these nets then banded, weighed and measured before they're released often with the help of school kids.

[BIOLOGIST] Alright, I'm going to let go now. And give a little bounce. Cool!

[JOAN] Over the years biologists can track the different species of birds that pass through.

[BIOLOGIST] There you are.

[JOAN] In addition, researchers can discover where each species spends the winter.

[BIOLOGIST] Put his leg in the band and make sure it's not going to squish him or anything and you clamp it shut. Whenever you want her to go give her a bounce and blow on her little butt. There you go. Cool!

[JOAN] Of course the birds of prey are nearby because their food is here. The songbirds, red tail hawks, cooper hawks and others are captured just below the ridge top as they search for prey.

[CHILD] What have you got?

[BIOLOGIST] Sharp shinned hawk.

[JOAN] But just like a songbird, this sharp shinned hawk is weighed, measured and banded before it is released.

[BIOLOGIST] Go to mexico. 1, 2, 3.

[CHILD] Fly. There you go.

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