A Ruby-throated hummingbird’s nest is about the size of half a walnut shell; a quarter will cover the bottom of the structure. In the structure a female usually lay two eggs per clutch.
The tiny eggs are roughly the size of dried black-eye peas. She incubates the eggs for approximately two weeks. Hummingbird chicks are born naked and blind (as is the case with most birds.) Given the size of the eggs, one can visualize how how tiny the chicks are. They are completely dependent on their mother for the first several days of their lives, unable to even regulate their own body temperature until their feathers start growing in. The female spends her time gathering food (tiny insects and sugar water) and brooding her miniature offspring.
However, the chicks grow at an amazing rate, usually are fully grown and leave the nest in about 21 days.
By the time they leave the nest, the fledglings are actually larger than their mother, which is the case with most bird species. The extra mass they carry is an adaptation to help them through the period where they must learn to find food for themselves. They follow their mother around for a few days after fledging. Once they are able to feed themselves, their mother may chase them from her territory while she attempts another nest. Sometimes the female may start another nesting attempt before her first chicks have completely fledged.
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All hummingbird fledglings look like females when they leave the nest; in fact, very recently fledged birds can only be sexed “in hand” by looking at the shape of one particular flight feather, which differs on male birds. The shape of this particular feather is a characteristic males retain throughout life.
Male hummingbirds begin to grow in red throat feathers a few weeks after fledging and continue to do so throughout fall and winter. In late winter, hummingbirds molt prior to their northward migration. Once they’ve achieved adult plumage, male ruby-throateds have made quite a transformation from their juvenile plumage; their tail feathers are shorter and all dark with and obvious forked shape. Their heads are darker, and they have a noticeable “vested” appearance to their breast area. Females retain the same basic appearance they have as juveniles. Male hummingbirds are slightly smaller than females and have shorter bills. Males have a stockier overall appearance, while females appear more slender and elongate.