Multidimensional Variation in an Avian Display: Implications for Social Communication

An analysis ofsong-spread displays performed in social communication by carib grackles, Quiscalus lugubris, revealed that two components of this motor coordination, beak elevation and wing elevation, varied independently. Variation in each depended strongly on the performer's social context, an indication that these components simultaneously reflected different behavioral tendencies of the performer. Independent variation in the elements of a display can permit transmission of more information per display on social context. These two elements of the song-spread display, however, varied independently, regardless of social context. The data reveal that elevation of the beak is more prominent when a displaying male is near another male, while elevation of the wings is more prominent when a displaying male is near a female. Higher elevation of the beak, owing to its similarity in form to the bill-up display, probably reflects stronger agonistic tendencies in males, and thus beak elevation is understandably more pronounced when males are close together. Two-way comparisons of social contexts (Table 2) showed that the distribution of beak' angles was similar when males performed near female's or alone. Hence the higher beak elevations near other males evidently resulted from elicitation of this variant by nearby males, rather than inhibition of beak elevation by nearby females. Similarly, the distribution of wing angles was similar when the male performed near a male or alone. Evidently, higher wing elevation was evoked by proximity to a female, rather than inhibited by proximity to