Processes underlying complex patterns of song trait evolution in a Setophaga hybrid zone

Abstract

During secondary contact between two species when hybrids are less fit than parents, mating signals are expected to diverge, while aggressive signals are expected to converge. If a single signal trait is used in both mating and aggression, then the dynamics between these two forces could influence the evolutionary trajectory of that trait. We studied such a situation in an avian hybrid zone between two Setophaga species, where birdsong is used in both mate attraction and territory defense. We hypothesized that song modules of the two species will show separate and distinct geographic patterns due to the influence of selective pressures for effective territorial aggression and for effective mate attraction. We conducted geographic cline analyses and playback experiments across this hybrid zone. We found an unexpected geographic pattern of asymmetric introgression of song rhythm, which may be explained by results of the playback experiments that suggest that differences in song rhythm serve a greater role in mate attraction than in territory defense. In contrast, differences in syllable morphology show little evidence of importance in mate attraction or territorial defense. Song features converge in the hybrid zone, yet patterns of trait change suggest that the song production modules may vary in their modes of development and inheritance. Syringeal motor gesturing, which gives rise to syllable morphology, shows a nonclinal mosaic pattern, suggesting that this trait may be predominantly learned. In contrast, respiratory patterning, which forms song rhythm, shows a clinal geographic transition, suggesting that this trait could be more innate. The results indicate that opposing forces act independently on song via distinct modules of the song production mechanism, driving complex patterns of song trait evolution.

Keywords: aggression; birdsong; mating signals; reinforcement; vocal learning.

FIGURE 1

Map of study area. Crosses show sample locations. Colors indicate plumage‐based hybrid index. Red = Hermit Warbler, purple = hybrid, blue – Townsend's Warbler

FIGURE 2

Playback songs of sympatric Hermit and Townsend's warblers varied by number of notes in the introductory syllables. Upper four (a–d) songs were sung by sympatric Hermit warblers, and lower four (e–h) songs were sung by sympatric Townsend's warblers. Allopatric Hermit warblers use multinote introductory syllables (red box, as in a, c, e, g), while allopatric Townsend's warblers use single‐note introductory syllables (blue box, as in b, d, f, h). In sympatry, both parent species use both multi‐ and single‐note intro syllables. Graphics are representative, showing divergent plumage

FIGURE 3

Asymmetrical convergence of song features in the hybrid zone. (a) Density distributions plots of Hermit (red), Townsend's (blue), and hybrid (purple) song features. Left panels: Largely distinct allopatric song rhythm converges on Townsend's‐like values (positive rhythm LD1) in sympatry. Right panels: Allopatric syllable morphology overlaps broadly, but means are significantly different. In sympatry, syllable morphology converges to intermediate values. (b) Hermit (red) and Townsend's (blue) song traits in allopatry and sympatry. Rhythm converges to Townsend's‐like values in sympatry, while syllable morphology converges to intermediate values in sympatry. Bold dots and bars show mean +95% CI

FIGURE 4

Geographic cline analysis. Samples were averaged every 1 km. Black line represents the maximum‐likelihood cline. Shaded region is 95% credible cline region. Red and blue lines indicate trait cline‐based center and edges of hybrid zone. Plots produced with HZAR package (Derryberry et al., 2014)

FIGURE 5

Variation in responses to different song rhythm (left) and average syllable morphology (right) LD1 scores by females (orange) and males (green). Circles are Hermits, squares are Townsend's, and triangles are hybrids. Lines show sex‐specific linear regressions. Female response is greater with positive (Townsend's‐like) rhythm LD1 scores (p = .017). Males and females show little difference in response to songs with different average syllable morphology. Points are jittered to increase visibility. Includes data from sympatric and allopatric populations