Song Discrimination Research Articles

Responses in adult pied flycatcher males depend on playback song similarity to local population

Abstract Song divergence driven by social learning has been proposed to be a key factor driving allopatric speciation in oscine birds. Songbirds often respond more to songs deriving from their local population, suggesting the potential for acoustic divergence across populations to shape both intra- and intersexual interactions. However, many of these studies were conducted on species with simple songs and, as a result, we know comparatively little about the emergence of population differences and song discrimination in species with complex songs. We addressed this question in the pied flycatcher (Ficedula hypoleuca) by calculating the dissimilarity of songs from two foreign populations as well as from our study site to the local centroid. We then conducted a paired-design playback experiment where both local and foreign songs were played simultaneously. We found that pied flycatcher males showed significantly stronger responses to those songs that sounded more similar to the local population. This suggests that despite the high complexity of the pied flycatcher song, individuals are still able to discriminate across populations. Our results support the hypothesis that learned song divergence can act as a mechanism for assortative mating and allopatric speciation.

Auditory discrimination learning and acoustic cue weighing in female zebra finches with localized FoxP1 knockdowns.

Rare disruptions of the transcription factor FOXP1 are implicated in a human neurodevelopmental disorder characterized by autism and/or intellectual disability with prominent problems in speech and language abilities. Avian orthologues of this transcription factor are evolutionarily conserved and highly expressed in specific regions of songbird brains, including areas associated with vocal production learning and auditory perception. Here, we investigated possible contributions of FoxP1 to song discrimination and auditory perception in juvenile and adult female zebra finches. They received lentiviral knockdowns of FoxP1 in one of two brain areas involved in auditory stimulus processing, HVC (proper name) or CMM (caudomedial mesopallium). Ninety-six females, distributed over different experimental and control groups were trained to discriminate between two stimulus songs in an operant Go/Nogo paradigm and subsequently tested with an array of stimuli. This made it possible to assess how well they recognized and categorized altered versions of training stimuli and whether localized FoxP1 knockdowns affected the role of different features during discrimination and categorization of song. Although FoxP1 expression was significantly reduced by the knockdowns, neither discrimination of the stimulus songs nor categorization of songs modified in pitch, sequential order of syllables or by reversed playback were affected. Subsequently, we analyzed the full dataset to assess the impact of the different stimulus manipulations for cue weighing in song discrimination. Our findings show that zebra finches rely on multiple parameters for song discrimination, but with relatively more prominent roles for spectral parameters and syllable sequencing as cues for song discrimination.NEW & NOTEWORTHY In humans, mutations of the transcription factor FoxP1 are implicated in speech and language problems. In songbirds, FoxP1 has been linked to male song learning and female preference strength. We found that FoxP1 knockdowns in female HVC and caudomedial mesopallium (CMM) did not alter song discrimination or categorization based on spectral and temporal information. However, this large dataset allowed to validate different cue weights for spectral over temporal information for song recognition.

Early exposure to songs of another subspecies enhances song discrimination in wild sparrow nestlings

Early exposure to songs of another subspecies enhances song discrimination in wild sparrow nestlings

Unmasking hidden genetic, vocal, and size variation in the Masked Flowerpiercer along the Andes supports two species separated by Northern Peruvian Low

Abstract Genetic divergence among isolated populations is not always reflected in phenotypic differentiation. We investigated the genetic and phenotypic differentiation in Diglossa cyanea (Thraupidae; Masked Flowerpiercer), a widely distributed species in the tropical Andes. We found strong evidence for 2 main lineages separated by the Marañón River valley in the Northern Peruvian Low (NPL). These 2 lineages show a deep sequence divergence in mitochondrial DNA (mtDNA; ~6.7% uncorrected p-distance, n = 122), spectral frequency and song structure (with exclusive final whistles in southern populations, n = 88), and wing length (the northern populations are smaller, n = 364). The 2 divergent D. cyanea mitochondrial lineages were not sister to each other, suggesting a possible paraphyly with respect to D. caerulescens (Bluish Flowerpiercer) that remains to be tested with nuclear genomic data. No genetic variation, size difference, or song structure was observed within the extensive range of the southern group (from the NPL to central Bolivia) or within all sampled northern populations (from the NPL to Venezuela). These vocal differences appear to have consequences for song discrimination, and species recognition, according to a previously published playback experiment study. We propose that the southern taxon be elevated to species rank as D. melanopis, a monotypic species (with the proposed name Whistling Masked-Flowerpiercer). In turn, we provide a redefinition of D. cyanea (Warbling Masked-Flowerpiercer), which is now restricted to the northern half of the tropical Andes as a polytypic species with 3 subspecies (tovarensis, obscura, and cyanea). Based on our results, the subspecies dispar should be treated as a junior synonym of cyanea. Our study highlights the need to continue amassing complementary data sets from field observations, experiments, and collection-based assessments to better characterize the evolutionary history, biogeography, bioacoustics, and taxonomy of Neotropical montane birds.

Asymmetric song recognition does not influence gene flow in an emergent songbird hybrid zone

Abstract Hybrid zones can be used to examine the mechanisms, like song recognition, that affect reproductive isolation and speciation. Song has mixed support as a driver of speciation; we did not find song to be associated with reproductive isolation in White-crowned Sparrow subspecies (Zonotrichia leucophrys pugetensis and Z. l. gambelii). We examined an emerging secondary contact zone in these subspecies by measuring song variation, song recognition, plumage, morphology, and mitochondrial DNA (mtDNA). Plumage and morphological characters provided evidence of hybridization in the contact zone, with some birds possessing intermediate phenotype and song characteristics, and some possessing discordant phenotype and mitochondrial haplotype. Playback experiments revealed asymmetric song recognition: male pugetensis displayed greater response to their own song than gambelii song, whereas gambelii did not discriminate significantly. If female choice operates similarly to male song discrimination, we predicted asymmetric gene flow, resulting in a greater number of hybrids with gambelii mtDNA. Contrary to our prediction, more gambelii and putative hybrids in the contact zone possessed pugetensis mtDNA haplotypes, possibly due to greater pugetensis abundance and female-biased dispersal.

Species-specific song responses emerge as a by-product of tuning to the local dialect

Oscine birds preferentially respond to certain sounds over others from an early age, which focuses subsequent learning onto sexually relevant songs.1,2,3 Songs vary both across species and, due to cultural evolution, among populations of the same species. As a result, early song responses are expected to be shaped by selection both to avoid the fitness costs of cross-species learning4 and to promote learning of population-typical songs.5 These sources of selection are not mutually exclusive but can result in distinct geographic patterns of song responses in juvenile birds: if the risks of interspecific mating are the main driver of early song discrimination, then discrimination should be strongest where closely related species co-occur.4 In contrast, if early discrimination primarily facilitates learning local songs, then it should be tuned to songs typical of the local dialect.5,6,7 Here, we experimentally assess the drivers of song discrimination in nestling pied flycatchers (Ficedula hypoleuca). We first demonstrate that early discrimination against the songs of the closely related collared flycatcher (F.albicollis) is not strongly affected by co-occurrence. Second, across six European populations, we show that nestlings' early song responses are tuned to their local song dialect and that responses to the songs of collared flycatchers are similarly weak as to those of other conspecific dialects. Taken together, these findings provide clear experimental support for the hypothesis that cultural evolution, in conjunction with associated learning predispositions, drives the emergence of pre-mating reproductive barriers.

Faster evolution of a premating reproductive barrier is not associated with faster speciation rates in New World passerine birds.

Why are speciation rates so variable across the tree of life? One hypothesis is that this variation is explained by how rapidly reproductive barriers evolve. We tested this hypothesis by conducting a comparative study of the evolution of bird song, a premating barrier to reproduction. Speciation in birds is typically initiated when geographically isolated (allopatric) populations evolve reproductive barriers. We measured the strength of song as a premating barrier between closely related allopatric populations by conducting 2339 field experiments to measure song discrimination for 175 taxon pairs of allopatric or parapatric New World passerine birds, and estimated recent speciation rates from molecular phylogenies. We found evidence that song discrimination is indeed an important reproductive barrier: taxon pairs with high song discrimination in allopatry did not regularly interbreed in parapatry. However, evolutionary rates of song discrimination were not associated with recent speciation rates. Evolutionary rates of song discrimination were also unrelated to latitude or elevation, but species with innate song (suboscines) evolved song discrimination much faster than species with learned song (oscines). We conclude that song is a key premating reproductive barrier in birds, but faster evolution of this reproductive barrier between populations does not consistently result in faster diversification between species.

Effect of sympatry on discrimination of heterospecific song by varied tits

Many animals recognize their own species through acoustic signals, and erroneous recognition incurs costs. Birds discriminate between conspecific and heterospecific songs; for example, male territory owners respond aggressively to songs of conspecific males within their territories, but not to heterospecific songs. This discrimination is affected by the costs associated with territory defence behaviours. In populations where birds live with a related species that sings similar songs, males may waste time and energy on useless territory defence behaviours if they mistakenly recognize heterospecific songs as conspecific ones. On the other hand, if males in allopatric populations recognize conspecific songs as heterospecific songs, they may risk losing territories because the songs are sung by conspecific rivals in the absence of heterospecifics. For this reason, the potential costs of erroneous species recognition are expected to differ between sympatric and allopatric populations. Thus, it can be hypothesized that, in species without interspecific territoriality, males in sympatry respond less strongly or not at all to heterospecific songs similar to their own, and males in allopatry respond equally to heterospecific and conspecific songs. In this study, I examined this hypothesis using varied tits, Poecile varius , in the Ryukyu Archipelago, Japan. Their songs are similar in structure to those of confamilial Japanese tits, Parus minor . I used song playbacks and evaluated the aggressive responses of 355 territorial males in seven populations of varied tits. Overall, varied tit males responded strongly to conspecific songs but only weakly to Japanese tit songs. Furthermore, responses to the heterospecific songs were much weaker in the sympatric populations than in the allopatric ones, which is consistent with the hypothesis tested. This study suggests that when secondary contact occurs, song discrimination may influence the effectiveness of territorial behaviours and the reproductive success of individual males from sympatric and allopatric populations. • I studied the effect of closely related species on song discrimination in birds. • I used playbacks of conspecific and heterospecific songs and compared the responses. • Male responses were weaker to heterospecific songs than to conspecific songs. • This effect was greater in sympatric populations than in allopatric populations. • Coexistence of related species may influence discrimination of conspecific song.

The role of introductory alarm calls for song discrimination in Ficedula flycatchers

Assortative mating depends on species distinctiveness in mating traits and preferences, which can be challenging to maintain when traits and/or preferences are learned. This is because learning may cause individuals to copy heterospecific signals. Juvenile songbirds possess innate sensory biases favouring them to learn and to prefer conspecific songs, but the effectiveness of these biases relies on consistent and sufficient differences between the songs produced by different species. However, mating signals, including learned songs, sometimes converge in sympatry, and the species-specific cues that individuals use to shape their preferences are often unknown. In Ficedula flycatchers, a stereotyped and highly species-specific alarm call is often incorporated as the first syllable of their songs. However, where the two species co-occur, pied flycatchers, Ficedula hypoleuca , learn to incorporate the introductory calls of the closely related collared flycatcher, Ficedula albicollis , into their songs. In this study, we investigated the role of introductory alarm calls for song discrimination in collared flycatchers, using playback experiments of both manipulated and unmanipulated songs on adults and nestlings within the hybrid zone of Öland, Sweden. We predicted that the introductory alarm call would be sufficient to trigger song responses, such that adults and nestlings would respond similarly to song phrases including the call, whether it is followed by conspecific or heterospecific notes. Our results provide evidence that the introductory alarm call is sufficient to trigger song discrimination in nestlings, but not in adult males, potentially due to their greater experience with songs and, therefore, subtler discrimination. Altogether, this study highlights the often-overlooked importance of calls within or associated with songs. • Collared flycatchers often start their songs with an alarm call. • The alarm call seems sufficient to drive song responses in nestlings. • We argue that the call could guide learning onto appropriate songs. • However, the call alone is unlikely to be sufficient for discrimination in adults. • We discuss the broader implication of incorporation of calls into songs.

Behavioral discrimination and time-series phenotyping of birdsong performance.

Variation in the acoustic structure of vocal signals is important to communicate social information. However, relatively little is known about the features that receivers extract to decipher relevant social information. Here, we took an expansive, bottom-up approach to delineate the feature space that could be important for processing social information in zebra finch song. Using operant techniques, we discovered that female zebra finches can consistently discriminate brief song phrases (“motifs”) from different social contexts. We then applied machine learning algorithms to classify motifs based on thousands of time-series features and to uncover acoustic features for motif discrimination. In addition to highlighting classic acoustic features, the resulting algorithm revealed novel features for song discrimination, for example, measures of time irreversibility (i.e., the degree to which the statistical properties of the actual and time-reversed signal differ). Moreover, the algorithm accurately predicted female performance on individual motif exemplars. These data underscore and expand the promise of broad time-series phenotyping to acoustic analyses and social decision-making.

Early Auditory Experience Modifies Neuronal Firing Properties in the Zebra Finch Auditory Cortex.

Songbirds learn to sing much as humans learn to speak. In zebra finches, one of the premier songbird models, males learn to sing for later courtship through a multistep learning process during the developmental period. They first listen to and memorize the song of a tutor (normally their father) during the sensory learning period. Then, in the subsequent sensory-motor learning phase (with large overlap), they match their vocalizations to the memorized tutor song via auditory feedback and develop their own unique songs, which they maintain throughout their lives. Previous studies have suggested that memories of tutor songs are shaped in the caudomedial nidopallium (NCM) of the brain, which is analogous to the mammalian higher auditory cortex. Isolation during development, which extends the sensory learning period in males, alters song preference in adult females, and NCM inactivation decreases song preference. However, the development of neurophysiological properties of neurons in this area and the effect of isolation on these neurons have not yet been explained. Here, we performed whole-cell patch-clamp recording on NCM neurons from juvenile zebra finches during the sensory learning period, 20, 40, or 60 days post-hatching (DPH) and examined their neurophysiological properties. In contrast to previous reports in adult NCM neurons, the majority of NCM neurons of juvenile zebra finches showed spontaneous firing with or without burst firing patterns, and the percentage of neurons that fired increased in the middle of the sensory learning period (40 DPH) and then decreased at the end (60 DPH) in both males and females. We further found that auditory isolation from tutor songs alters developmental changes in the proportions of firing neurons both in males and females, and also changes those of burst neurons differently between males that sing and females that do not. Taken together, these findings suggest that NCM neurons develop their neurophysiological properties depending on auditory experiences during the sensory song learning period, which underlies memory formation for song learning in males and song discrimination in females.

Variation in auditory neural activation in response to strain-specific songs in wild and domesticated female Bengalese finches

Female songbirds identify and prefer conspecific male songs. Songs are an important cue for species discrimination. Bengalese finches are domesticated species and their male songs seem to have evolved as they comprise more complex sequences and tonal sounds than the songs of their wild ancestors, white-rumped munias. Previous research suggested that the degeneration of song functionality for species identification may have been one of the factors that promoted the evolution of song complexity in domestic strains. We hypothesized that female responses to conspecific songs have changed between the two strains: white-rumped munias could distinguish songs of their own species more readily than Bengalese finches. Because the song discrimination is affected by developmental experiences, we used adult female Bengalese finches and white-rumped munias reared with or without exposure to songs of their own strains (i.e., socially-reared or untutored). To evaluate their song discrimination, we quantified zenk-labeled cells in the auditory areas after exposure to song stimuli, either with songs of own strains or those of other strains. Socially-reared white-rumped munias exposed to songs of Bengalese finches showed lesser zenk expression than munias exposed to songs of their own strain. However, there were no significant differences among the groups in Bengalese finches. The result suggests that white-rumped munias could distinguish songs of their own species more strictly than Bengalese finches.

The Role of Nestling Acoustic Experience in Song Discrimination in a Sparrow

Oscine songbirds are an ideal system for investigating how early experience affects vocal behavior. Young songbirds face a challenging task: how to recognize and selectively learn only their own species’ song, often during a time-limited window. Because birds are capable of hearing birdsong very early in life, early exposure to song could plausibly affect recognition of appropriate models; however, this idea conflicts with the traditional view that song learning occurs only after a bird leaves the nest. Thus, it remains unknown whether natural variation in acoustic exposure prior to song learning affects the template for recognition. In a population where sister species, golden-crowned and white-crowned sparrows, breed syntopically, we found that nestlings discriminate between heterospecific and conspecific song playbacks prior to the onset of song memorization. We then asked whether natural exposure to more frequent or louder heterospecific song explained any variation in golden-crowned nestling response to heterospecific song playbacks. We characterized the amount of each species’ song audible in golden-crowned sparrow nests and showed that even in a relatively small area, the ratio of heterospecific to conspecific song exposure varies from 0 to 20%. However, although many songbirds hear and respond to acoustic signals before fledging, golden-crowned sparrow nestlings that heard different amounts of heterospecific song did not behave differently in response to heterospecific playbacks. This study provides the first evidence that song discrimination at the onset of song learning is robust to the presence of closely related heterospecifics in nature, which may be an important adaptation in sympatry between potentially interbreeding taxa.

Testosterone synthesis in the female songbird brain

Decades of work have established the brain as a source of steroid hormones, termed ‘neurosteroids’. The neurosteroid neuroestradiol is produced in discrete brain areas and influences cognition, sensory processing, reproduction, neurotransmission, and disease. A prevailing research focus on neuroestradiol has essentially ignored whether its immediate synthesis precursor - the androgen testosterone - is also dynamically regulated within the brain. Testosterone itself can rapidly influence neurophysiology and behavior, and there is indirect evidence that the female brain may synthesize significant quantities of testosterone to regulate cognition, reproduction, and behavior. In songbirds, acoustic communication is regulated by neuroestrogens. Neuroestrogens are rapidly synthetized in the caudomedial nidopallium (NCM) of the auditory cortex of zebra finches in response to song and can influence auditory processing and song discrimination. Here, we examined the in vivo dynamics of NCM levels of the neuroestrogen synthesis precursor, testosterone. Unlike estradiol, testosterone did not appear to fluctuate in the female NCM during song exposure. However, a substantial song-induced elevation of testosterone was revealed in the left hemisphere NCM of females when local aromatization (i.e., conversion to estrogens) was locally blocked. This elevation was eliminated when local androgen synthesis was concomitantly blocked. Further, no parallel elevation was observed in the circulation in response to song playback, consistent with a local, neural origin of testosterone synthesis. To our knowledge, this study provides the first direct demonstration that testosterone fluctuates rapidly in the brain in response to socially-relevant environmental stimuli. Our findings suggest therefore that locally-derived ‘neuroandrogens’ can dynamically influence brain function and behavior. Significance statementThis study demonstrates that androgen synthesis occurs rapidly in vivo in the brain in response to social cues, in a lateralized manner. Specifically, testosterone synthesis occurs within the left secondary auditory cortex when female zebra finches hear male song. Therefore, testosterone could act as a neuromodulator to rapidly shape sensory processing. Androgens have been linked to functions such as the control of female libido, and many steroidal drugs used for contraception, anti-cancer treatments, and sexual dysfunction likely influence the brain synthesis and action of testosterone. The current findings therefore establish a clear role for androgen synthesis in the female brain with implications for understanding neural circuit function and behavior in animals, including humans.

Diversification in trophic morphology and a mating signal are coupled in the early stages of sympatric divergence in crossbills

AbstractUnderstanding the mechanisms generating diversity in mating signals is critical to understanding the process of speciation. One mechanism of mating signal diversification occurs when phenotypes that experience divergent ecological selection also affect the production of mating signals, resulting in a coupling between ecological diversification and mating signal diversification. Here, we present evidence that rapid diversification in bill size has resulted in the diversification of some components of song structure in a young adaptive radiation of seed-eating finches (red crossbill, Loxia curvirostra complex). Specifically, we find that larger-billed ecotypes sing songs with lower minimum frequencies, lower syllable repetition rates and greater vocal deviation (i.e. lower performance) than smaller-billed ecotypes for pure tonal syllables. In contrast, bill size was not correlated with maximum frequency or frequency bandwidth, and we found no relationship between bill size and any song parameters in buzzy syllables. Furthermore, we found no evidence for a relationship between the degree of bill size divergence and the potential for song discrimination between sympatric ecotypes. Because bill size is correlated with some features of pure tonal syllables (which appear to be most important for courtship in crossbills) in crossbill song, our results suggest that there was an early-evolving link between ecological and mating signal diversification that may have influenced the rapid evolution of reproductive isolation between sympatric ecotypes.

Song playbacks demonstrate slower evolution of song discrimination in birds from Amazonia than from temperate North America.

Genetic data indicate differences in speciation rate across latitudes, but underlying causes have been difficult to assess because a critical phase of the speciation process is initiated in allopatry, in which, by definition, individuals from different taxa do not interact. We conducted song playback experiments between 109 related pairs of mostly allopatric bird species or subspecies in Amazonia and North America to compare the rate of evolution of male discrimination of songs. Relative to local controls, the number of flyovers and approach to the speaker were higher in Amazonia. We estimate that responses to songs of relatives are being lost about 6 times more slowly in Amazonia than in North America. The slow loss of response holds even after accounting for differences in song frequency and song length. Amazonian species with year-round territories are losing aggressive responses especially slowly. We suggest the presence of many species and extensive interspecific territoriality favors recognition of songs sung by sympatric heterospecifics, which results in a broader window of recognition and hence an ongoing response to novel similar songs. These aggressive responses should slow the establishment of sympatry between recently diverged forms. If male responses to novel allopatric taxa reflect female responses, then premating reproductive isolation is also evolving more slowly in Amazonia. The findings are consistent with previously demonstrated slower recent rates of expansion of sister taxa into sympatry, slower rates of evolution of traits important for premating isolation, and slower rates of speciation in general in Amazonia than in temperate North America.

A low-threshold potassium current enhances sparseness and reliability in a model of avian auditory cortex.

Birdsong is a complex vocal communication signal, and like humans, birds need to discriminate between similar sequences of sound with different meanings. The caudal mesopallium (CM) is a cortical-level auditory area implicated in song discrimination. CM neurons respond sparsely to conspecific song and are tolerant of production variability. Intracellular recordings in CM have identified a diversity of intrinsic membrane dynamics, which could contribute to the emergence of these higher-order functional properties. We investigated this hypothesis using a novel linear-dynamical cascade model that incorporated detailed biophysical dynamics to simulate auditory responses to birdsong. Neuron models that included a low-threshold potassium current present in a subset of CM neurons showed increased selectivity and coding efficiency relative to models without this current. These results demonstrate the impact of intrinsic dynamics on sensory coding and the importance of including the biophysical characteristics of neural populations in simulation studies.

Genomic divergence in allopatric Northern Cardinals of the North American warm deserts is linked to behavioral differentiation.

Biogeographic barriers are considered important in initiating speciation through geographic isolation, but they rarely indiscriminately and completely reduce gene flow across entire communities. Explicitly demonstrating which factors are associated with gene‐flow levels across barriers would help elucidate how speciation is initiated and isolation maintained. Here, we investigated the association of behavioral isolation on population differentiation in Northern Cardinals (Cardinalis cardinalis) distributed across the Cochise Filter Barrier, a region of transitional habitat which separates the Sonoran and Chihuahuan deserts of North America. Using genomewide markers, we modeled demographic history by fitting the data to isolation and isolation‐with‐migration models. The best‐fit model indicated that desert populations diverged in the Pleistocene with low, historic, and asymmetric gene flow across the barrier. We then tested behavioral isolation using reciprocal call‐broadcast experiments to compare song recognition between deserts, controlling for song dialect changes within deserts. We found that male Northern Cardinals in both deserts were most aggressive to local songs and failed to recognize across‐barrier songs. A correlation of genomic differentiation and strong song discrimination is consistent with a model where speciation is initiated across a barrier and maintained by behavioral isolation.

Auditory experience controls the maturation of song discrimination and sexual response in Drosophila.

In birds and higher mammals, auditory experience during development is critical to discriminate sound patterns in adulthood. However, the neural and molecular nature of this acquired ability remains elusive. In fruit flies, acoustic perception has been thought to be innate. Here we report, surprisingly, that auditory experience of a species-specific courtship song in developing Drosophila shapes adult song perception and resultant sexual behavior. Preferences in the song-response behaviors of both males and females were tuned by social acoustic exposure during development. We examined the molecular and cellular determinants of this social acoustic learning and found that GABA signaling acting on the GABAA receptor Rdl in the pC1 neurons, the integration node for courtship stimuli, regulated auditory tuning and sexual behavior. These findings demonstrate that maturation of auditory perception in flies is unexpectedly plastic and is acquired socially, providing a model to investigate how song learning regulates mating preference in insects.

Evolution and plasticity: Divergence of song discrimination is faster in birds with innate song than in song learners in Neotropical passerine birds.

Plasticity is often thought to accelerate trait evolution and speciation. For example, plasticity in birdsong may partially explain why clades of song learners are more diverse than related clades with innate song. This "song learning" hypothesis predicts that (1) differences in song traits evolve faster in song learners, and (2) behavioral discrimination against allopatric song (a proxy for premating reproductive isolation) evolves faster in song learners. We tested these predictions by analyzing acoustic traits and conducting playback experiments in allopatric Central American sister pairs of song learning oscines (N = 42) and nonlearning suboscines (N = 27). We found that nonlearners evolved mean acoustic differences slightly faster than did leaners, and that the mean evolutionary rate of song discrimination was 4.3 times faster in nonlearners than in learners. These unexpected results may be a consequence of significantly greater variability in song traits in song learners (by 54-79%) that requires song-learning oscines to evolve greater absolute differences in song before achieving the same level of behavioral song discrimination as nonlearning suboscines. This points to "a downside of learning" for the evolution of species discrimination, and represents an important example of plasticity reducing the rate of evolution and diversification by increasing variability.