Abstract
The singing behavior of 4 male Northern Mockingbirds (Mimus polyglottos) was studied during 1980 and 1981. Over 10,000 song bouts (repetition of a particular song type) were analyzed spectrographically. Between 102 and 412 distinct song types were identified for each male in a given year. Many of these song types (25.8-57.4%) occurred only once in this sample of singing behavior. Repertoire size was estimated by fitting an exponential curve to a cumulative plot of distinct song types as a function of consecutive bouts sampled. The resulting asymptote was the estimate of repertoire size. Sequences of at least 50 consecutive bouts were used in estimating repertoire size. The technique of using consecutive bouts was required because of the high proportion of rare song types. Several estimates were made for each male and resulted in mean repertoire size estimates of 100-200 song types for the four males. Long sequences (125-150 bouts) of consecutive bouts are preferred to estimate repertoire size accurately. In addition, the social situation in which the singing behavior occurred can alter significantly the estimate of repertoire size. For example, estimates produced from singing during patrolling or countersinging were smaller than estimates generated from singing while associating with a female.
The repertoire size estimate increased between 1980 and 1981 for 3 of 4 males. The one male not showing an increase spent much time patrolling and interacting with neighboring males before dying early in 1981. These results are consistent with the impression that repertoire size increases with age.
