Every animal has meat. Humans have meat. But it's not like "meat" shows up on diagrams of the human body. So what part of the body is it then? Meat is muscle. When an animal dies, muscle undergoes a transformation and becomes what we call meat. And the type of muscle it is determines whether that meat is red, dark or white.
First, some terminology: "Red meat" is meat that's a reddish color before cooking, like beef, venison and ostrich. "White meat" is very pale before cooking and includes chicken, turkey and pork; and "dark meat" usually refers to a slightly darker, higher-fat part of an animal that also produces white meat -- like the wing of a chicken. Rabbits are also considered dark meat.
The primary defining factor in whether animals are white meat or red meat is whether their muscles are mostly fast-twitch or mostly slow-twitch. Slow-twitch muscles are used often, for extended activities like constant walking, standing or flying. It has a lot of the protein myoglobin, which stores large amounts of oxygen to support this long-term energy use. Myoglobin is reddish in color, sort of like hemoglobin in blood, which is why red meat can look so bloody. Ostriches, like cows, spend most of their time standing and walking. Even ostrich wings get a lot of exercise, since they play such a central role in steering. Ostrich muscles are mostly the slow-twitch kind. Slow-twitch muscle is red meat.
Chickens and turkeys, on the other hand, don't use their muscles as much. Most of their muscle mass is the fast-twitch kind, used for short bursts of activity, like a quick jump into the air that constitutes most of their flying. Fast-twitch muscles use glycogen for energy -- there's not much myoglobin there. Glycogen is pale in color. Fast-twitch muscle is white meat. (See How Muscles Work to learn more.)
An interesting exception to the rule is the "dark meat" in poultry. Body parts like legs get a lot of activity -- chickens are walking constantly -- so there's more myoglobin in their leg muscles than in, say, their breasts, which are seldom used, as chickens don't fly very much. Because they use their legs for extended periods or time, there's more myoglobin in leg meat, which is why it's darker than breast meat.
For more information on ostriches, ostrich meat and the economics of ostrich farming, hop to the links on the next page.