The male Ruby-throated Hummingbird weighs about 3 grams and is about 9 cm long. Its plumage is a shiny green with a metallic cast on its back, head and wings. The greyish white of its breast and underwings contrasts with its throat, often red but sometimes orange or black, depending on the angle of the light. Its eyes are edged in black. Its black, forked tail sports feathers that are tapered towards the end. It has a long, straight, slender, dark beak.
Females weigh about 3.3 g and are about 9 cm in length. Their eyes and beaks resemble those of the males. Their plumage is also similar, except for their throats, which are greyish white. Their tails are not forked, but rounded and some of the outer feathers have white spots. Immature hummingbirds resemble the female. Young males have dark stripes on their throats. At times, a touch of iridescent red may be seen there.
The size of nests varies from 1.5 to 4.5 cm in diameter and 2.5 to 5 cm in height. They are built from bud scales, lichen and blades of grass. This is all held together with spider webs or insect silk and lined with plant down for the nestlings' comfort. Nests are usually built in a tree 3 to 6 metres from the ground, but they may also be constructed on the end of a branch of a conifer or a broad-leaved tree. The eggs, 1 or 2 in number, are the size of a pea. The female usually has one clutch per year, but at times will have two. She will lay her eggs with a one-day interval and incubate them for about 16 days.
The Ruby-throated Hummingbird is adapted to a wide variety of habitats: gardens, clearings, orchards and the edges of mixed and hardwood forests. It appears to prefer open spaces. Its diet varies according to month and location. It feeds on nectar and on tiny insects. It is particularly attracted to tubular flowers such as the honeysuckle, impatiens and lilies and has a weakness for red and orange flowers. Tree holes pecked by Yellow-bellied Sapsuckers providing access to insects, and feeders filled with sweetened water are other food sources.