Chirp, chirrup, cheep, peep, tweet, twitter, chitter can all mean as verbs to make the little sounds characteristic of small animals and as nouns the little sounds so made, and all can be extended to sounds and the making of sounds (as by human beings) that suggest such small animal sounds.
Chirp implies the short, sharp, and thin sound that is made by practically all small birds and some insects; it regularly connotes cheerfulness but often also busyness and immaturity.
Chirrup implies a more sustained effect than chirp, as though a bird is singing or is learning to sing; it also often heightens the connotations of cheerfulness or liveliness.
Cheep implies feebleness yet shrillness of sound such as that made by a very young bird or by a mouse, a bat, or a squirrel.
Peep differs from cheep chiefly in stressing the weakness of the sound and so suggesting its faintness or the animal’s helplessness.
Tweet usually implies the monosyllabic note of a very small bird or the call of small game birds (as the quail or bobwhite).
Twitter implies a succession of notes or sounds, uttered tremulously or excitedly; when applied to persons or their utterances or doings it can suggest feverish or disorderly excitement. The noun, especially, is often used to express a state of agitation.
Chitter, like twitter, implies a succession of sounds, but distinctively it can imply a briskness and sharpness of tone that belong also to chattering and then may carry such connotations as alarm, irritation, or fear.