Variation in Repertoire Presentation in Northern Mockingbirds

Abstract

Male Northern Mockingbirds (Mimus polyglottos) have exceptionally large vocal repertoires. The manner of presenting this extensive repertoire, as described using five measures, varied with reproductive stage, among situations, and among individuals. All three versatility measures peaked during courtship, declined significantly during incubation, and then slowly increased during nestling and fledgling stages. A fourth measure, bout length, increased as the season progressed, being shortest during courtship and longest during the fledgling stage. A final measure, recurrence interval (number of intervening bouts between two bouts of a particular song type) was shorter during the nestling and fledgling stages than during courtship. Recurrence interval was shortest during patrolling and countersinging with neighboring males. Over 25% of the song types occurred only once in the sampling of singing behavior of four males each over 2 years. Mockingbirds sang these rare song types most commonly during prefemale and courtship stages, thereby increasing the recurrence interval and versatility during these stages. The patterns just described resulted in the greatest number of song types being sung per unit of time during courtship and provide circumstantial support for the hypothesis that song functions intersexually in mockingbirds. The ability to alter the manner of presentation may provide mockingbirds with the flexibility to emphasize particular functions at certain times and other functions at other times. Males with the highest versatility measures and lowest bout length tended to be the first to acquire mates and begin to nest. However, the importance of versatility in attracting females remains speculative and requires further experimental testing because these results were from only four males. Songs sung at night were presented in a manner most similar to the period before a female arrived on a male's territory. Interestingly, under natural lighting conditions, only unmated males sang extensively at night.