MOTOR STEREOTYPY AND DIVERSITY IN SONGS OF MIMIC THRUSHES

The relationship between the motor and acoustic similarity of song was examined in brown thrashers (Toxostoma rufum) and grey catbirds (Dume tella carolinensis) (family Mimidae), which have very large song reper toires and sometimes mimic other species, Motor similarity was assesse d by cross correlation of syringeal airflows and air sac pressures tha t accompany sound production, Although most syllables were sung only o nce in the song analyzed, some were repeated, either immediately formi ng a couplet, or after a period of intervening song, as a distant repe tition, Both couplets and distant repetitions are produced by distinct ive, stereotyped motor patterns, Their motor similarity does not decre ase as the time interval between repetitions increases, suggesting tha t repeated syllables are stored in memory as fixed motor programs, The acoustic similarity between nonrepeated syllables, as indicated by co rrelation of their spectrograms, has a significant positive correlatio n with their motor similarity, This correlation is weak, however, sugg esting that there is no simple linear relationship between motor actio n and acoustic output and that similar sounds may sometimes be produce d by different motor mechanisms, When compared without regard to the s equence in which they are sung, syllables paired for maximum spectral similarity form a continuum with repeated syllables in terms of their acoustic and motor similarity, The prominence of couplets in the ''syn tax'' of normal song is enhanced by the dissimilarity of successive no nrepeated syllables that make up the remainder of the song. (C) 1996 J ohn Wiley & Sons, Inc.