Ruby-throated hummingbirds are about 7.5 to 9 cm long and weigh 3.5 grams. Males have an emerald green back, an iridescent ruby red throat, gray flanks, and an entirely dark, forked tail. The female differs from the male as she is larger with a longer bill, has a white breast and throat, and has a rounded tail with white tips. Immature birds resemble the female. These birds feed exclusively on nectar and insects, but they will eat tree sap when nectar is scarce. Ruby-throated hummingbirds are the only breeding hummingbird in the eastern United States.
Ruby-throated hummingbirds winter in tropical deciduous or dry forests, scrubland, citrus groves, and second growth forests. They breed in mixed woodlands, eastern deciduous and pine forests, gardens, and orchards. The population is stable and common in its range.