In just three lines, this tiny poem sets two sounds against each other. The title, «Give a little whistle», sounds like a cheerful invitation, something light and playful. But the first lines turn it dark: a fired bullet "pretends to sing." As it flies, it whistles through the air, and the poem suggests that this sound is a kind of imitation of birdsong, a deadly thing putting on a harmless disguise.
The last line, "a consternation of birds," answers the bullet. A group of birds is usually given a pretty collective name, but "consternation" means alarm or dismay. Judging by the wording, the birds have heard the shot and scattered in panic, so they aren't fooled by the pretend song at all.
The speaker doesn't state a feeling outright. Instead the poem stays quiet and observant, and that restraint makes the mood uneasy. The playful title makes the violence feel even colder, and the abrupt ending leaves us with a sudden silence after the noise.