Squirrels often make a "barking" sound as an alarm call. This sound is called a "kuk." It is sharp and short, with a broad frequency similar to a dog's bark.
Squirrels typically vocalize to each other as a warning of danger, but alarm calls also serve to let an intruder know the squirrel is aware of its presence. Alarm calls like the "kuk" are often successful in causing predators to cease stalking, since the hunters know they can no longer surprise their prey. Once a predator leaves a squirrel's sight line, the "kuk" usually lulls to a softer "quaa" sound.