Aah, springtime. Crocuses are blooming, squirrels are cavorting, birds are singing ... and the HVc region of the neostriatum, the robust nucleus of the archistriatum and area X of the parolfactory lobe are recrudescing. Those are the bits of a male bird's brain responsible for singing, and they are some of the most interesting nuclei in the history of neuroscience.
One stubborn misunderstanding about the brain is that it's impossible to grow new neurons. Until a few decades ago, it was thought that you get a certain number of neurons at birth and, unlike skin cells or bone cells or pretty much any other part of the body, those original brain cells were what you were stuck with. Aside from a few, mostly overlooked reports in rodents, the research that blew this understanding away came from songbirds.
During breeding season, a rich network of song nuclei takes up a large portion of a male songbird's brain. Some parts are responsible for perceiving song, others for producing song, and some are necessary to learn the correct song when the bird is but a chick. (The system is analogous to the human brain, with Broca's area responsible for producing speech and Wernicke's for understanding speech.) These songbird nuclei shrink in the fall and winter, and then grow back again when the days start getting longer and breeding season approaches. The nuclei don't entirely disappear over the winter, but come springtime, they're brimming with newborn neurons.
Female songbirds learn their species' song as chicks and are wooed by it years later when the season is right. There's some evidence that the female song-perception nuclei grow and shrink seasonally, and the neurons are more responsive to the right song during breeding season.
Why would birds allow these hugely important brain nuclei to atrophy over the winter? Because brains are expensive. It takes a lot of energy to build, maintain and fuel brain tissue. Basically, if there's some part of the brain you can do without, it makes evolutionary sense to just let it go—the body can't afford to keep feeding the brain's version of an appendix.
The human brain, too, is hugely plastic and able to grow new neurons and repurpose old ones. Human birdwatchers, like female songbirds, don't hear birdsong over the winter, and I certainly feel as though the which-bird-sings-that-song nucleus in my own brain shrinks over the winter. And in springtime, when the birds start singing again and I try to match song to species, I can almost feel that nucleus recrudesce.