Seed feeders are the most common type. These can vary in design from tubes to hoppers and trays. Sunflower seed or mixed seed are popular for use in these feeders and will attract many songbirds such as cardinals, finches, and chickadees. Black Oil Sunflower seed is especially popular with bird enthusiasts. The outer shell of Black Oil Sunflower seeds are thinner and easier to crack than other types of Sunflower seeds. In addition, the kernel is larger than striped or white sunflower seeds.[2] Black Oil Sunflower seeds also contain a large amount of fat; therefore they are especially good to use in the winter. Most bird feeders are designed to dispense sunflower-sized foods, but there are specialty "finch feeders" with smaller openings to dispense the tiny Guizotia abyssinica (Niger seed), which is a favorite of smaller finches. Hummingbird feeders, rather than dispensing seed, supply liquid nourishment to hummingbirds in the form of a sugar solution. This mixture is often colored red to attract birds. Likewise, some Hummingbird feeders have red accents or red glass to help attract hummingbirds. Yeasts tend to grow in hummingbird feeders and spoil the solution, so they must be refreshed frequently and kept very clean to avoid harm to the birds. See the article on hummingbirds for more details. Ants and other insects are also attracted to hummingbird nectar. Smearing petroleum jelly on the feeder's may prevent the ants from crawling to the feeder, but the birds cannot clean the petroleum jelly from their feathers, so alternative methods should be found. Oriole feeders, which are traditionally colored orange, also supply such artificial nectar and are designed to serve New World orioles, which have an unusually shaped beak and tongue. These orioles and some other birds also will come to fruit foods, such as grape jelly, or half an orange on a peg.[5] Hummingbirds will also feed from Oriole feeders. Oriole feeders usually have nectar containers made of glass or plastic, which are designed to attract the orioles. Oriole feeders should be cleaned at least once a week and even more often when the temperatures are higher.[6] Oriole feeders also come in top fill, bottom fill and dish-like designs. A suet feeder is typically a metal cage-like construction with a plastic coating that contains a cake or block of suet to feed woodpeckers, flickers, nuthatches, and many other species of insect eaters. Suet logs are also very common. These wooden logs have holes drilled out for suet to be inserted. Suet is high in fat which helps to keep birds warm and nourished during the cold winter.

From: http://en.wikipedia.org/wiki/Bird_feeder