An Analysis of the Temporal Patterning of Pecking in Chicks
Authors: Machlis Lee 
Published: April 2008

In this paper aspects of the temporal and sequential patterning of pecks in the domestic chick are examined. To this end experiments were done in which young chicks were allowed to peck at pairs of coloured stimuli - the time of each peck being recorded automatically by a small computer. A particularly striking feature of the chicks' behaviour was the tendency of the pecks to occur in rapid bursts. Such bouts have been described (and quantified) for behaviours in a number of species, but the bout itself has proven to be a particularly difficult unit to define empirically. Accordingly a model is proposed which describes the order independent features of the intervals between pecks, and also objectively defines a bout criterion interval. This model assumes that not only do pecks tend to cluster into bouts but that the bouts themselves occur in clusters. Consequently two types of between bout intervals will be generated: "not pecking" intervals, whose mean length is long, and which separate bout clusters; and intervals of medium average length, separating bouts within a cluster. It is assumed that these between bout intervals are generated by two random (Poisson) processes of sharply differing rate constants. To test this model an Algol procedure was written which describes the survivorship curve of intervals between pecks in terms of components that can be adequately characterized as negative exponential. It is found that with large sample sizes three distributions are consistently delineated; the "not pecking" intervals are characterized by an exponential with a rate constant of order 10-3, the intervals within a cluster by one of 10-2 and the within bout intervals by one of 10-1 (if the latter generating process is assumed to have a dead time). In addition, a stable bout criterion ranging from 1.9 to 2.3 secs is defined. It is also argued that degeneracies in this model can occur when sample sizes are small - in particular that the intervals of medium length are either not distinguished from those belonging to the outer state of long intervals, or fail to be completely delineated from the two other components. Part II of this paper examines additional features of the pecking behaviour within the framework of this model. The question of how a colour is preferred is explored and it is found that preferred colours in these experiments receive from 1.6 to 3.2 times as many bouts, which are on average 1.1 to 1.3 times as long as those directed at the less preferred colour. Some information on sequencing of bouts is presented in which it is shown that there is a probability of about .60 of continuing to "bout" at the same position when the two stimuli are the same colour. This finding is combined with a measure of overall colour preference to allow fairly consistent predictions of the transition probabilities between bouts when the stimuli differ in colour. Upon examining the intervals within a bout it is found that although they peak strongly at .3 secs they can range between .1 and 2.3 secs. Through indirect evidence it is argued that the number of pecks within a bout also varies substantially. However, it is found that the distribution of intervals within a bout remains nearly invariant regardless of what colours the chicks are pecking. It is suggested that this invariance provides some objective verification for the concept of a "bout unit". However, upon closer examination, differences in the distribution of within bout intervals are found. These differences are interpreted as resulting from the rather surprising occurrence of mixed bouts, that is bouts during which pecks were elicited by both stimuli. Intervals within such mixed bouts were on average longer than those occurring in bouts where all pecks were directed at the same stimulus and consequently effected shifts in the mean and increased the spread in some of the within bout interval distributions. It is tentatively suggested that such mixed bouts are elicited when the chicks attend to cues upon which the stimuli cannot be distinguished. An important assumption of the model is that the interval distributions arising from the three generating processes overlap and that while a stable and obj ective bout criterion can be defined it does not provide a neat means of classifying intervals into either within or between bout intervals. Some intervals less than the bout criterion are assumed to belong to the processes generating between bout intervals. For this reason the total number of bouts defined by the criterion interval will underestimate the true number of bouts occurring - some bouts will be merged together when one of these very short between bout intervals occurs. The occurrence of mixed bouts, along with the substantial proportion of undetectable between bout intervals is shown to hinder extensive examination of the properties of the bouts defined by the criterion interval; it is argued that considerable ambiguity exists in the bouts so defined. The origin of this ambiguity is discussed within the framework of current studies in which bouts of behaviour, defined by a criterion interval, are used as a basic analytical unit.