There is another group of birds that nest in tree holes, but cannot excavate the holes themselves. These include bluebirds, nuthatches, many wrens, some swallows and some owls. They are dependent upon the excavators for their nest sites, an this is why it is a good conservation measure to leave dead trees standing if they have nest holes in them. They will be useful to the birds. Feathers - Hummingbirds have no downy feathers at all.
Hummers have the least number of feathers of all birds.
Brain - The brain of the hummer is larger in proportion to body size than that of any other bird. Flight - Hummingbirds have the fastest wing beat. Muscles - The breast muscles of a hummingbird are the largest of any bird relative to body size. The most rapid heartbeat belongs to the hummer. Metabolism - Hummingbirds have the greatest energy output relative to body size of all warmblooded animals.