Hummingbird feeders usually have red accents or red glass to help attract hummingbirds. The sugar mixture is sometimes colored with red food coloring to attract birds, though this is not necessary if the feeder itself is red, and may actually be harmful to the birds. 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 stem or cap of the feeder (away from the perch or flower part where the bird may come into contact with it) may prevent the ants from crawling to the feeder. When placing a hummingbird feeder, the feeder is best suited 15 to 20 feet from windows; 10 to 15 feet from the nearest cover, like shrubs or bushes; and in an open area that receives partial sun, so that hummingbirds can move from nectar source to nectar source.