The term 'bird word' can refer to any of the four words found in the crossword puzzle: TWEET, COO, CHEEP, and CHIRP. TWEET is a verb meaning to make a short, high-pitched sound, usually associated with birds. COO is also a verb, meaning to make a low, murmuring sound, often used to describe the sound made by doves and pigeons. CHEEP is a noun meaning a short, high-pitched sound made by a bird, and CHIRP is a verb meaning to make a short, high-pitched sound, usually associated with birds. All four words are commonly used to describe the sounds made by birds.