A bird half wakened in the lunar moon
Sang halfway through its little inborn tune.
Partly because it sang but once all night
And that from no especial bush's height,
Partly because it sang ventriloquist
And had the inspiration to desist
Almost before the prick of hostile ears,
It ventured less in peril than appears.
It could not have come down to us so far,
Through the interstices of things ajar,
On the long bead chain of repeated birth,
To be a bird while we are men on earth,
If singing out of sleep and dream that way
Had made it much more easily a prey.
The first evolutionary concept Frost uses here is "inborn." He sees the traits of the bird as innate, the product of a biological heritage. Then the speaker of the poem wonders why the bird (this species of bird) has survived, why it is fit for survival. Surely a bird singing in the middle of the night like that would be swooped up by an owl or other nocturnal hunter? So he devises an evolutionary explanation of sorts. First of all, he makes a few excuses. It only sang once, and not for very long. Secondly, he makes an almost tautological argument: if the bird did in fact survive, then singing at night once in a while could not have been too maladaptive. After all, the bird is still there! The argument is not strong, because maybe this particular bird was eaten by an owl the minute after the speaker heard the song. Individuals with maladaptive traits exist, even the the species itself has adapted to a particular habitat.