broadbill has 9 distinct synonyms spread across 3 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
tropical American heron related to night herons
diving ducks of North America having a bluish-grey bill
freshwater duck of the northern hemisphere having a broad flat bill
small birds of the Old World tropics having bright plumage and short wide bills
Synonyms, antonyms and the broader and narrower relations for broadbill come from Princeton University’s WordNet. Full sources and licences.