There are around 10000 different species of birds worldwide. Birds have hollow bones which help them fly. Hummingbirds, for example are incredibly lightweight. The average hummingbird is around 4 grams, while the smallest, the bee hummingbird, is closer to 1.6 grams. The largest member of the hummingbird family, the giant hummingbird, can get up to 24 grams—enormous for a hummingbird, but only equivalent to about a handful of loose change.
