Fig. 1.

Relations between changes in performance of the best song type each year (vertical axis, zero means no change in performance) and relative number of song types recorded from each male in the 2 yrs (horizontal axis, zero means same number of song types recorded in both years). Filled and open symbols are mountain and campus males, respectively. Triangles are birds known to be changing from their second to third years of age. In all graphs positive differences in performance mean longitudinal improvements. Consistent longitudinal change in performance would be indicated by an intercept significantly different from the origin (not significant in any case, see text).