RESPONSES OF MALE TROPICAL MOCKINGBIRDS TO VARIATION IN WITHIN-SONG AND BETWEEN-SONG VERSATILITY

Abstract

Despite their large vocal repertoires and otherwise highly versatile singing style, male mockingbirds sometimes sing in a highly repetitive fashion. We conducted a playback experiment to determine the possible signal value of different syllable presentation patterns during simulated male intrusions in the Tropical Mockingbird (Mimus gilvus) testing the hypothesis that more repetitive singing represents a stronger threat and generates a stronger aggressive response. Responses were measured in terms of approach and singing behavior and were analyzed using McGregor's (1992) multivariate method. We also introduce the use of survival analysis for analyzing response variables for which subjects do not perform the behavior in question in at least one of the replicates (known as 'right-censored variables' in the statistical literature). As predicted by theory, experimental subjects responded more aggressively to songs composed of a single note than to variable ones. However, versatility at the between-song level had an opposite effect as high song switching rates generated stronger responses than low ones. Given the lack of a statistical interaction between within-song versatility and switching rate, we conclude that these two parameters may serve independent purposes and possibly transmit different information. We discuss the possibility that the signal value of variation in vocal versatility lies in the mediation of territorial conflicts, the attraction of female partners and/or the mediation of conflicts over access to reproductive females.

Fig. 1

Variation in the song versatility of the Tropical Mockingbird. The typical song sequences in our population have high within-song versatility (song complexity) and high between-song versatility (switching rate) (a). Occasionally, males become more repetitive at the within-song and between-song level (b). Both sequences were recorded from the same male Tropical Mockingbird within a period of 30 min. New syllable types have been underlined as they are introduced for the first time. A typical courtship solo of a Northern Mockingbird (c) is also shown for comparison.

Fig. 2

Examples of complex songs obtained from recordings of male Tropical Mockingbirds in Bonaire (a, c) and the repetitive songs we derived from them (b, d). Note the similarity between the derived songs and the natural examples of repetitive songs shown in Figure 1b. The bottom panels show a song from a sympatric Icterus icterus (e) and the heterospecific control song derived from it (f).

Fig. 3

Mean proximity response scores during playback trials with low (white bars) and high (gray bars) within-song versatility at low and high between-song switching rates. Error bars reflect one SE. Higher score values correspond to closer approach.

Fig. 4

Kaplan-Meier cumulative plot of the latency to approach during trials. A vertical dotted line marks the end of the playback period. Convention: HSC = High Song Complexity; LSwR = Low Switching Rate.