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When a chick pecks at one of a number of alternative stimuli, two questions about mechanisms of decision-making arise. One of these concerns the timing of the peck - this will be discussed later - and the other is about its orientation. In an attempt to answer the second of these questions, a model is proposed, called the Threshold Model. This model then is a possible analogue of the mechanism in a chick which determines at which of the available stimuli each individual peck is directed. The Threshold Model embodies the following assumptions. There is a fluctuating variable of unknown nature called Variable X, which interacts with thresholds corresponding to the external stimuli. A stimulus cannot be pecked unless Variable X exceeds its threshold. Some stimuli have lower thresholds than others and are therefore more likely to be pecked - they are "preferred". If the thresholds of more than one available stimulus are exceeded at any one time, they are equally likely to be pecked. From these assumptions it is possible to deduce a prediction about the precise relationship between the three percentage preferences obtained when any three stimuli are presented to animals in all possible pair combinations. This prediction, called Prediction 1, was tested on domestic chicks, by counting the numbers of pecks given to small hemispherical spots presented in pairs, and differing in colour or some other quality. In general it was well confirmed. The model need not be confined to the pecking response, nor indeed to the domestic chick. Prediction 1 also gave a good fit to the findings in experiments again on domestic chicks involving learned preferences in approach situations. Also, M. Impekoven, J. Kear, W. Muntz, and C. Rees allowed me to test and fairly well confirm Prediction 1 using their data from choice experiments on a number of different species. Continued in thesis ...

Each of 32 neonatal White Leghorn chicks (Gallus gallus domesticus) experienced each of the four environmental situations and motoric and vocal behaviors were observed. The four environments were derived from a 2 X 2 (stimulus availability X temperature) factorial design. The levels of stimulus availability were either present (visible) or absent (not visible) while the two temperature levels were 32.5°C or 18°C. A factor analysis of the observed behaviors yielded six factors; three of which concerned motoric behaviors and three of which concerned vocalizations. A 2 X 2 (stimulus availability X temperature) within subjects analysis of variance for each factor was also performed. These results show that motoric behavior was largely affected by the availability of the imprinted stimulus. Results from the vocalization factors revealed one factor comprised of twitter calls, a second factor comprised of peep calls and a third factor comprised of calls that sonographically resembled an intermediate call. The analysis of variance on these factors revealed that the situational occurrence of the intermediate call factor resembled neither of the other two, more traditional, vocalization categories. Master of Science