Adult hummingbirds are distinguishable from immature ones by their size and their colouring. Young birds are almost naked when they hatch. They have two minuscule rows of feathers on their backs. When they leave the nest, 2 to 3 weeks after they hatch, Ruby-throated Hummingbirds have already grown a lot. In fact, when the nestlings leave the nest, they are adult size. The mother continues to feed them, beak to beak. During this period, the young birds resemble their mother, except that the throats of the young males are usually marked with dark, sometimes red, stripes. Thus, a hummingbird's age cannot be definitely determined simply by observation. At the end of the summer, the bird must be held to determine its age and sex with certainty. In the spring, before migrating north, immature birds moult completely and grow their adult plumage.
Musée de la nature et des sciences de Sherbrooke