Simple categories matching gregarious:

• bird,

Members with property gregarious:

• swan,
• egret,
• rook,
• starling,
• duck,
• pigeon,
• heron,
• goose,
• sparrow,
• chicken,
• common_grackle,
• black_bird,
• grey_currawong,
• swallow,
• penguin,
• pintail,
• oil_bird,