Physiologic constraints at the syrinx determine song frequency variation in the dialects of the Rufous-collared Sparrow and its association with habitat

LIJTMAER, DARÍO A.; ALLIENDE, JORGE A.; LAJE, RODRIGO; MINDLIN, GABRIEL B.; TUBARO, PABLO L.

Ithaca, Estados Unidos

Congreso; 12th International Behavioral Ecology Congress; 2008

International Society for Behavioural Ecology

The patterns of association between the temporal structure of song dialects and habitat in the Rufous-collared Sparrow (Zonotrichia capensis) are consistent with signals being adapted to reduce their degradation during transmission. However, this interpretation cannot explain song frequency variation or its association with habitat, because the patterns are variable and because lower frequencies reduce song attenuation independently of the environment. We studied frequency variation in this species focusing on the limitations imposed by the syrinx during song production, using a recently developed model that considers sound pressure in the air sac, which determines amplitude, and tension in the syringeal labia, which determines frequency (Laje et al. 2002, Physical Review E 65:051921). We first studied the respiratory pattern in singing birds by simultaneously recording them and measuring their thoracic air sac pressure. This showed that Rufous-collared Sparrows can produce vocalizations with two different respiratory patterns, depending on the interval between notes. Then, we analyzed the correlation between temporal and frequency variables of the trill, and found that shorter intervals between notes correlate with lower bandwidths (achieved by reducing maximum frequency or increasing minimum frequency). The model easily explains this association, because small intervals between notes limit tension differences in the labia between the end of a note and the beginning of the following one. This novel approach explains the patterns of frequency variation in this species and shows that the association between frequency and habitat is an indirect consequence of the direct effect of habitat on the temporal structure of song.