The poem is spoken by a house sparrow who has stumbled on a strange human object: a musical greeting card, given to someone who was feeling low. From the bird's point of view, the card is baffling. It sings, the clouds move, and there are stars and crickets, like a tiny garden that only lacks Adam and Eve and their snake. The sparrow, flitting in from "the tree beside the tree beside the tree," is both curious and uneasy at being watched by a giant human eye.
The tone is playful and dry. The sparrow complains that the human is "beginning to creep me out" and wonders about a huge coffee mug with leopard spots. Under the jokes, though, there's a gentle tenderness. The card was meant to cheer someone up, and the sparrow is, in a way, part of that comfort. The final question, about whose "blue egg" this is, is funny and puzzling at once. It may hint at the blue mood of the opening, or at how odd the human world looks from outside it.