Whirlybird is a colloquial term that refers to a helicopter or a machine that rotates like a helicopter. Some synonyms for the word 'whirlybird' include chopper, heli, whirlygig, gyro, copter, and rotary-wing aircraft. These terms are often used interchangeably to refer to a motorized flying machine that uses rotary blades to lift off the ground and hover in the air. The term 'chopper' is widely used when referring to military helicopters, while the term 'heli' is popular in the aviation industry. 'Whirlygig' is a playful term that is often used to refer to toy helicopters or small-scale models. In summary, the synonym you choose will depend on the context in which it is used.