The exact opposite is true of people, of course; we are hypersocial and use language in highly social situations.
Fitch uses bird song as a comparative case, rightly noting that we can learn a lot from analogy. One thing we can’t learn is why language evolved. Bird song is intimately connected with holding territory. The most highly intelligent and sociable birds—crows and ravens—have lost the ability. Several other highly social birds that descend from songbirds have also lost it, or reduced it to a low level. The exact opposite is true of people, of course; we are hypersocial and use language in highly social situations. It is grossly overengineered for small-family communication. Bird song, as Fitch notes, is also very much about mating, whereas language is about everything (and then some). He points out that language developed in large part for the purpose of teaching children; evidence is the striking speed with which children learn their languages.