Four short lines, and the poem packs in a surprising amount of mischief. The speaker looks at a lark and calls it mad, then pictures it as a young man caught half-dressed, "dishabille," in the middle of a disco. It's a funny, slightly absurd image: a bird's wild, restless song turned into a party guest who has stumbled in without bothering to tidy himself up.
The speaker seems fond of the creature, though. "A fine lad" sounds admiring, almost affectionate, and the final lines suggest he's courting someone while wearing an "ash coat," which could be the lark's dull grey-brown feathers. The plain colour sits oddly against the flashy disco, and that contrast is part of the charm. The title, "Blithe spirit," points to carefree joy, and the poem keeps that tone: playful, quick, a little cheeky. Rather than describe a bird politely, it lets the lark be a bit ridiculous and a lot of fun.