How many letters does the top answer for “relating to or characteristic of the passeriform birds” have?

The most common answer is passerine (9 letters).

Crossword clue: relating to or characteristic of the passeriform birds

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

passerine 9 letters wn-def

perching birds mostly small and living near the ground with feet having 4 toes arranged to allow for gripping the perch;