英语时差：自然直升机
Yael: Hey Don. What's up?
D: I'm making a top ten list of the best animals of all time.
Y: What's on the list?
D: Dingo, aardvark, python, tapir, rhesus monkey, black widow, bullfrog, owl, osprey.
Y: That's only nine. What's the tenth?
D: I'm not sure yet . . .
Y: What about the hummingbird?
D: Hmm...tell me more.
Y: Well, hummingbirds are really amazing. For one thing, they can hover in mid air.
D: That is impressive. How do they do it?
Y: Most birds flap their wings up and down, but hummingbirds move their wings in a figure-eight pattern. With each flap they twist their wings one-hundred-eighty degrees so that they create a downward thrust on both the forward and backward strokes. If each loop of the wings is of the same strength, they hover. They can go forward, backwards, or sideways by tilting their wings in the direction they want to go.
D: Kind of like a helicopter.
Y: Exactly. In order to hover hummingbirds have to be really strong. Their wing muscles are thirty percent of their total weight. When a small hummingbird hovers it flaps its wings about eighty times per second.
D: That sounds tiring.
Y: It is. Hummingbirds have to eat close to fifty percent of their weight in food everyday to keep up their strength. They eat things that are packed with calories and produce lots of reusable energy, like seeds, nuts, nectar, and fruit.
D: So that's why in those nature programs you always see hummingbirds hovering near a flower and sticking its beak inside.
Y: That's right. It's drinking nectar.
D: All right, you've convinced me. Number ten, hummingbird.
Y: Number ten?
D: All right. Hummingbird is number one.
