What a bird feeder must have?

There are different types of bird feeders. Some are fly through, which means it is open and the birds fly into it and eat. Then there are perch type bird feeders where the birds sit on perches and eat. There are also weight sensitive bird feeders where you can set the weight you want. If a bird is big, the openings in the bird feeder closes, keeping them out of the food. There are also squirrel-proof bird feeders which keep the squirrels out of your wild bird feed.