Syllable Type Consistency is Related to Age, Social Status, and Reproductive Success in the Tropical Mockingbird

Abstract

Many animals repeat standardized displays multiple times while attracting a mate or deterring a rival. In such contexts it is possible that the ability to perform each display or signal type in a consistent fashion is under direct selection. Studies on sexual selection on song learning in birds have focused on differences in repertoire size with less attention to the potential importance of being able to perform each song/syllable type with high consistency. We present evidence that tropical mockingbirds decrease the variation between renditions of each syllable type as they grow older (i.e., become more consistent) and that more consistent males in this species tend to have higher dominance status and reproductive success. These findings stress the importance of consistency in the performance of sexual displays and suggest that this parameter may be very relevant even in species that are selected for high vocal diversity (i.e., large repertoires). In addition to signalling dominance status and age, we hypothesize that syllable type consistency may also be an indicator of the integrity of brain function in birds analogous to the tests used for neuropsychological assessment in humans.

Figure 1

Selected songs of one of our focal male tropical mockingbirds in Villa de Leyva, Colombia. Lines above the spectrogram indicate syllables and numbers are used to identify syllable types. Grey boxes show how syllable types (in this case, types four and five) are combined in different ways to produce a virtually unlimited number of song types. The songs with the grey boxes were recorded in different years, suggesting that at least part of the syllable type repertoire is conserved throughout an individual’s life. The examples shown were chosen to illustrate the vocal diversity of male tropical mockingbirds and do not reflect the actual types included in our analysis.

Figure 2

Mean within-type consistency (measured as the mean spectral cross-correlation coefficient across renditions of the same syllable type) as a function of social dominance. Boxes represent interquartile ranges, horizontal lines mark the medians, and vertical lines represent the general extent of the data. N = 20 alpha males and 12 beta males.

Figure 3

Distribution of types with low (SPCC ≤ 0.8), intermediate (0.8 < SPCC < 0.9), and high (SPCC ≥ 0.9) average within-type consistency, in alpha and beta male tropical mockingbirds.

Figure 4

Individual changes in within-type consistency (i.e., 2nd sample – 1st sample) as a function of the time elapsed between the two samples.

Figure 5

Change in the distribution of types with low (SPCC ≤ 0.8), intermediate (0.8 < SPCC < 0.9), and high (SPCC ≥ 0.9) average within-type consistency, between breeding attempts.