Response of forest Turtur doves to conspecific and congeneric songs in sympatry and allopatry

Birds have a diverse acoustic communication system, and the ability to recognise their own species' song from a distance facilitates complex behaviours related to mate attraction and rival deterrence. However, certain species, including doves, do not learn songs and their vocal repertoires are much simpler than those of better-studied songbirds. In these so-called non-learning birds, relatively little is known about the role that bird song plays in intra- and interspecific interactions, and how such behaviours might be acquired (inherited or learned from experience). To investigate this question, we focused on two species of African wood doves whose long-range songs are used in a territorial context. Specifically, we examined the responses of sympatric and allopatric populations of male blue-headed wood-doves (Turtur brehmeri) and tambourine doves (Turtur tympanistria) to different types of simulated territorial intrusions, i.e. playback of conspecific, congeneric, and control songs. We aimed to assess (i) whether these species, which have similar songs, respond only to their own species' song or exhibit interspecific territoriality, and (ii) if the response pattern is affected by the presence or absence of congeners in the general area. We found that both species responded strongly to playback of their own species in both sympatric and allopatric populations. In allopatry, though, male tambourine doves misdirected their response and also approached the playback of congeneric songs. Our results indicate that, in areas where the studied Turtur doves live in sympatry, they do not exhibit consistent interspecific territoriality. However, we cannot exclude the possibility that the smaller tambourine dove avoids its larger congener during the process of territory establishment. The difference in tambourine doves' response toward the song of present (sympatric) or absent (allopatric) congeners suggests that the ability to discriminate between songs of similarly singing potential competitors is acquired through earlier interactions and learning. This plasticity in response supports the misdirected aggression hypothesis, which argues that interspecific territorialism emerges as a maladaptive by-product of signal similarity. However, on an evolutionary timescale, such an ability could be considered an adaptive cognitive tool useful for resolving competing interests with congeners.

Punctuated evolution in the learned songs of African sunbirds

Learned traits are thought to be subject to different evolutionary dynamics than other phenotypes, but their evolutionary tempo and mode has received little attention. Learned bird song has been thought to be subject to rapid and constant evolution. However, we know little about the evolutionary modes of learned song divergence over long timescales. Here, we provide evidence that aspects of the territorial songs of Eastern Afromontane sky island sunbirds Cinnyris evolve in a punctuated fashion, with periods of stasis of the order of hundreds of thousands of years or more, broken up by evolutionary pulses. Stasis in learned songs is inconsistent with learned traits being subject to constant or frequent change, as would be expected if selection does not constrain song phenotypes over evolutionary timescales. Learned song may instead follow a process resembling peak shifts on adaptive landscapes. While much research has focused on the potential for rapid evolution in bird song, our results suggest that selection can tightly constrain the evolution of learned songs over long timescales. More broadly, these results demonstrate that some aspects of highly variable, plastic traits can exhibit punctuated evolution, with stasis over long time periods.

Interspecific aggression among parapatric and sympatric songbirds on a tropical elevational gradient

Interspecific competition is hypothesized to be a strong force that sets species range limits and drives parapatric distributions of closely related species on tropical mountains. Yet, experimental evidence that competition drives spatial segregation of closely related species on elevational gradients is rare. To test whether competition limits elevational ranges of tropical songbirds, we conducted reciprocal playback experiments on 2 pairs of species with adjacent but nonoverlapping (parapatric) distributions and 1 pair of sympatric species. We found asymmetric interspecific aggression in one parapatric pair (Pycnonotidae) and a complete absence of interspecific aggression in the other (Zosteropidae). We also found asymmetric interspecies aggression in a pair of sympatric flycatchers (Muscicapidae). Our results indicate that interspecific aggression may set range limits in some cases, but it is not a prerequisite for parapatry. Furthermore, the presence of interspecific aggression between co-occurring relatives suggests that while competition may play a role in limiting species distributions, interspecific aggression alone is not sufficient evidence to assert that competition is the primary driver of parapatric distributions.

Social selection parapatry in Afrotropical sunbirds

The extent of range overlap of incipient and recent species depends on the type and magnitude of phenotypic divergence that separates them, and the consequences of phenotypic divergence on their interactions. Signal divergence by social selection likely initiates many speciation events, but may yield niche-conserved lineages predisposed to limit each others' ranges via ecological competition. Here, we examine this neglected aspect of social selection speciation theory in relation to the discovery of a nonecotonal species border between sunbirds. We find that Nectarinia moreaui and Nectarinia fuelleborni meet in a ∼6 km wide contact zone, as estimated by molecular cline analysis. These species exploit similar bioclimatic niches, but sing highly divergent learned songs, consistent with divergence by social selection. Cline analyses suggest that within-species stabilizing social selection on song-learning predispositions maintains species differences in song despite both hybridization and cultural transmission. We conclude that ecological competition between moreaui and fuelleborni contributes to the stabilization of the species border, but that ecological competition acts in conjunction with reproductive interference. The evolutionary maintenance of learned song differences in a hybrid zone recommend this study system for future studies on the mechanisms of learned song divergence and its role in speciation.

Sharp acoustic boundaries across an altitudinal avian hybrid zone despite asymmetric introgression

Birdsong is a sexually selected trait that could play an important evolutionary role when related taxa come into secondary contact. Many songbird species, however, learn their songs through copying one or more tutors, which complicates the evolutionary outcome of such contact. Two subspecies of a presumed vocal learner, the grey-breasted wood-wren (Henicorhina leucophrys), replace each other altitudinally across the western slope of the Ecuadorian Andes. These subspecies are morphologically very similar, but show striking differences in their song. We examined variation in acoustic traits and genetic composition across the altitudinal range covered by both subspecies and between two allopatric populations. The acoustic boundary between the subspecies was found to be highly abrupt across a narrow elevational range with virtually no evidence of song convergence. Mixed singing and use of hetero-subspecific song occurred in the contact zone and was biased towards the use of leucophrys song types. Hetero-subspecific song copying by hilaris and not by leucophrys reflected a previously found asymmetric pattern of response to song playback. Using amplified fragment length polymorphisms (AFLP) markers, we detected hybridization in the contact zone and asymmetric introgression in parapatric populations, with more leucophrys alleles present in hilaris populations than vice versa. This pattern may be a trail of introgression due to upslope displacement of leucophrys by hilaris. Our data suggest that song learning may impact speciation and hybridization in contrasting ways at different spatial scales: although learning may speed up population divergence in songs, thereby enhancing assortative mating and reducing gene flow, it may at a local level also lead to the copying of heterospecific songs, therefore allowing some level of hybridization and introgression.

Asymmetric Response of Costa Rican White-Breasted Wood-Wrens (Henicorhina leucosticta) to Vocalizations from Allopatric Populations

Divergence in song between allopatric populations can contribute to premating reproductive isolation in territorial birds. Song divergence is typically measured by quantifying divergence in vocal traits using audio recordings, but field playback experiments provide a more direct way to behaviorally measure song divergence between allopatric populations. The White-breasted Wood-Wren (Henicorhina leucosticta; hereafter “WBWW”) is an abundant Neotropical species with four mitochondrial clades (in Central America, the Darién, the Chocó and the Amazon) that are deeply divergent (~5–16% sequence divergence). We assessed the possibility that the WBWW as currently defined may represent multiple biological species by conducting both statistical analysis of vocal characters and field playback experiments within three clades (Central America, Chocó and Amazon). Our analysis of vocal traits revealed that Central American songs overlapped in acoustic space with Chocó songs, indicating vocal similarity between these two populations, but that Central American songs were largely divergent from Amazonian songs. Playback experiments in the Caribbean lowlands of Costa Rica revealed that Central American WBWWs typically responded aggressively to songs from the Chocó population but did not respond to playback of songs from the Amazonian population, echoing the results of the vocal trait analysis. This marked difference in behavioral response demonstrates that the songs of Central American and Amazonian WBWWs (but not Central American and Chocó WBWWs) have diverged sufficiently that Central American WBWWs no longer recognize song from Amazonian WBWWs as a signal to elicit territorial defense. This suggests that significant premating reproductive isolation has evolved between these two populations, at least from the perspective of the Central American population, and is consistent with the possibility that Central American and Amazonian populations represent distinct biological species. We conclude by advocating for the further use of field playback experiments to assess premating reproductive isolation (and species limits) between allopatric songbird populations, a situation where behavioral systematics can answer questions that phylogenetic systematics cannot.

Differential rates of phenotypic introgression are associated with male behavioral responses to multiple signals

Sexual selection on multiple signals may lead to differential rates of signal introgression across hybrid zones if some signals contribute to reproductive isolation but others facilitate gene flow. Competition among males is one powerful form of sexual selection, but male behavioral responses to multiple traits have not been considered in a system where traits have introgressed differentially. Using playbacks, mounts, and a reciprocal experimental design, we tested the hypothesis that male responses to song and plumage in two subspecies of red-backed fairy-wren (Malurus melanocephalus) explain patterns of differential signal introgression (song has not introgressed, whereas plumage color has introgressed asymmetrically). We found that males of both subspecies discriminated symmetrically between subspecies' songs at a long range, but at a close range, we found that aggression was equal for both subspecies' plumage and songs. Taken together, our results suggest that male behavioral responses hinder the introgression of song, but allow for the observed asymmetrical introgression of plumage. Our results highlight how behavioral responses are a key component of signal evolution when recently divergent taxa come together, and how differential responses to multiple signals may lead to differential signal introgression and novel trait combinations.

Playbacks revisited: asymmetry in behavioural response across an acoustic boundary between two parapatric bird species

Behavioural barriers to gene flow can play a key role in speciation and hybridisation. Birdsong is well-known for its potential contribution to such behavioural barriers as it may affect gene flow through an effect on territorial and mating success across population boundaries. Conspecific recognition and heterospecific discrimination of acoustic variation can prevent or limit hybridization in areas where closely related species meet. Here we tested the impact of song differences on territorial response levels between two adjacent Henicorhina wood-wren species along an elevational gradient in Colombia. In an earlier study, playback results had revealed an asymmetric response pattern, with low-elevation H. leucophrys bangsi responding strongly to any conspecific or heterospecific song variant, whereas high-elevation H. anachoreta birds discriminated, responding more strongly to their own songs than to those of bangsi. However, in that study we could not exclude a role for relative familiarity to the song stimuli. In the current study we confirm the asymmetric response pattern with song stimuli recorded close to and on both sides of the distinct acoustic boundary. Furthermore, we also show a previously unnoticed divergence in singing style between these two wood-wren species, which may contribute to an acoustically guided barrier to hybridization in this secondary contact zone.