Meeting Abstract
Index signals convey reliable information about a signaler’s quality because they are constrained by the signaler’s anatomy or physiology. ‘Vocal performance’ in birdsong may be considered an index signal to the extent that certain acoustic features of song are limited by the singer’s vocal capabilities. Many species of birds exhibit a dawn chorus, a period of especially active singing occurring at first light, after which singing becomes relatively less frequent. One hypothesis predicts that birds use dawn choruses to ‘warm up,’ much like human athletes or musicians do, to maximize their vocal performance for the rest of the day. We tested this idea in swamp sparrows (Melospiza georgiana), a species for which the underlying biomechanical constraints on vocal performance are especially well-known and in which the behavioral significance of vocal performance also has been well-studied. We analyzed 1301 songs from a cohort of 11 captive swamp sparrows over the course of 2 days and measured changes in vocal performance across the day. We found that song performance is positively correlated with time of day, suggesting that swamp sparrows do warm up throughout the morning. However, song performance was not correlated with the absolute number of songs that a bird had sung previously in a morning. The performance benefit associated with vocal warm-up may explain why songbirds engage in a dawn chorus to the extent that vocal performance correlates positively with the amount of time a bird has sung. Our finding that the number of previously performed songs does not predict vocal performance suggests that songbirds may experience fatigue of the vocal apparatus, thereby imposing an upper constraint on the optimum number of songs a bird should produce during a single morning.
