Vocalization in chicks, and the concept of “stimulus contrast”

Abstract 1. 1. The calls of the chick form a single system of responses, in which increasing intensity is shown by increasing length, pitch and number of trilling cycles. 2. 2. The twitter and related calls are elicited by any stimulus which contrasts with background stimulation. Such stimuli are said here to have “intrinsic contrast”. 3. 3. Naive chicks twitter when pecking at any type of small particle. In experienced chicks, twitters are given during pecks at food when hungry. Chicks trained to peck a lever for a food reward gave most of their twitters at the time when they perceived cues announcing the arrival of food. After long deprivation of food the number of calls given remained high throughout a session of fifty rewards, but after brief deprivation the number of calls were high for only the first few rewards and then became low. Twitters are also given to cues which announce the automatic presentation of food or, in thirsty birds, water. An explanation in terms of conditioning of the twitter is discussed and shown to be improbable. It is likely that the cues announcing food are given contrast because of the chick's set to perceive them, and that this set is affected by hunger. Contrast given to stimuli in this way is here termed “signal contrast”. 4. 4. It is argued that all the calls of the chick form a single system of responses evoked by stimulus contrast of differing persistence and intensity. Persistent and relatively intense contrast, such as results from cold or absence of the imprinting object, is needed to evoke calls towards the peep end of the twitter-peep series, whilst very intense contrast such as that provided by a painful electric shock is needed to elicit a shriek. 5. 5. The association between twitters and approach to the stimulus source, and between peeps and avoidance, can be explained by assuming that approach may be elicited by moderate stimulus contrast, and avoidance by intense contrast. 6. 6. Other groups of responses, which may be best regarded as elicited by stimulus contrast, include alert responses, protective responses and certain autonomic responses associated with conflict. The latter two groups have probably both given rise to display components.