This is a hard haiku constraint because AVIAN is more taxonomic than lyrical. It is precise, but not naturally warm, so the risk is that the poem starts sounding like classification rather than perception. Mark’s “A GREAT mystery / CLOAK the paradox in fear / HUMAN? AVIAN?” is a good example of handling that well: the constraint words are absorbed into one compact puzzle, so AVIAN arrives as the meaningful fork in the poem’s logic rather than a visible answer word. Across the set, the stronger entries make the word serve an actual purpose — species question, joke payoff, mental distraction, falconry frame, envy, or bodily choice — which is what keeps the prompt from going dry.