In 1889 a religious book emphasized the spur provided by the possession of a song. This notion matched the quotation under examination:
A bird sings because it has a song and must sing it.
In 1902 a column in “The Brooklyn Daily Eagle” of Brooklyn, New York suggested that duty was not the key motivator: 6
The spring does not pour forth because it is its duty, the little bird does not sing because of duty, but because its little heart would burst if it did not sing.
In 1949 a biographical work called “The Sage of the Hills” highlighted the importance of having a song in the heart: 7
It should be remembered that the Nightingale does not sing just because we listen; it sings because it has a song in its heart. (The above from Quote Investigator.)
I have had conversations with a few naturalists and philosophic minded folks who construe bird song anthropomorphically, stating that birds sing because they are happy and enjoy singing. I always hate to disabuse them of this interpretation, but as a scientist, I know natural selection rarely allows frivolous behavior. It’s nice to think that birds sing because they are happy, and we’ve created lots of songs and greeting cards to reinforce that view. Unfortunately, if there ever was any bird that spent time singing for pure enjoyment, it was removed from the gene pool.
If birds sang because they are happy, then half the birds in the world are depressed, I guess, because they are not songbirds and have no song. Then there are the songbirds like jays and crows and magpies who are apparently unhappy souls, merely squawking rather than singing.
The problem with the idea of a bird singing for pleasure is threefold:
First, it’s usually only the male singing; why not females? Are they unhappy? What about juveniles? Are they unable to express their glee? THere are exceptions. Female Northern Mockingbirds, Northern Cardinals, and Black-headed Grosbeaks have songs as complex as the males’. In the phalaropes, with reversed sexual dimorphism, courtship roles are reversed and the female advertises her presence to males with calls. After the female lays eggs, the male takes over the nest and incubation duties while the female seeks other males to mate with.
Second, singing is usually restricted to breeding season. Does that mean birds are not happy the rest of the year? Some do sing most of the year, so are they happier than birds that become reticent in the winter?
Third, singing advertises a bird’s presence, which is ok if a female notices, but risky if a predator does. So, in the non-breeding season, a singer would be telling predators he’s there for the taking. There are some exceptions such as the European and American Robins which hold winter territories and sing to defend them.
Singing has two major functions: to attract a mate and defend a territory. There has to be a balance between those purposes and making oneself known to predators. So singing “just for fun” is just not adaptive, it’s downright dangerous.