The poem starts with an ugly scene: a tree stripped bare by caterpillars and hung with their silk. Then two crows corner a cat, which backs away quietly, pretending it hasn't lost. The crows settle by some bins and talk, and one declares that they "do violence to no-one" and that nobody needs to kill anyone. Yet a sparrow flies past with a caterpillar for its chicks, right in the middle of his speech, and the sentence about harmlessness trips over the evidence. The crow's claim is hollow, and the poem lets us see that.
The final lines shift to "we," the human onlookers, who watch the crows year after year and decide they are tame and "quite likeable." The speaker's tone is dry and gently ironic. The mood is calm on the surface and uneasy underneath. The fable's point seems to be how easily we come to accept bullies who sound reasonable, simply because they've become familiar.