Hidden Dialects in the Cloud Forests: Hooded Grosbeak Flight Calls Reveal Cryptic Populations

Deep in the mountain forests stretching from northern Mexico to Central America lives a bird so poorly studied that fewer than 200 audio recordings of its voice existed worldwide until now. Yet those scarce recordings have just yielded a discovery that could reshape how scientists view the species: the Hooded Grosbeak (Hesperiphona abeillei) appears to harbor a complex system of learned flight call dialects, with at least three distinct vocal types, one of which is confined to a geographically separate population running from southern Mexico into Guatemala. The finding, published in Frontiers in Bird Science, suggests that this tropical finch may conceal cryptic subpopulations that differ in voice but not in plumage, echoing patterns previously documented only in a handful of temperate finches.

The research team, led by Caleb T. Centanni of Oregon State University and the K. Lisa Yang Center for Conservation Bioacoustics at the Cornell Lab of Ornithology, together with William M. Kirsch, Matthew A. Young of the Finch Research Network, and W. Douglas Robinson, set out to accomplish something that had never been formally done: describe the vocal repertoire of the Hooded Grosbeak. The species belongs to the cardueline finches (Fringillidae), a family famous among bioacousticians for intraspecific diversity in social calls. Its close relative, the Evening Grosbeak (Hesperiphona vespertinus), is already known to produce quantitatively distinct flight call types that roughly align with subspecies groups, and the Red Crossbill (Loxia curvirostra) has become a textbook example of vocal types accompanying differences in foraging behavior, breeding ranges, bill morphology, and mating preferences. But the majority of fringillid species, especially those in the tropics, have never had their vocal variation formally described at all.

To fill that gap, the researchers downloaded every one of the 134 Hooded Grosbeak recordings archived in Xeno-Canto and the Macaulay Library, the two most comprehensive digital sound archives for neotropical birds, which together likely represent most or all of the accessible audio data for the species. Each file was analyzed in Raven Pro 1.6 with spectrographic measurements of duration, beginning and ending frequency, maximum frequency, and the timing of frequency peaks. The team employed a deliberately two-pronged strategy. First, they manually sorted the vocalizations into three putative categories: songs, flight calls, and trills. Sequences with at least three temporally distinguished elements, or plastic sequences that appeared as modifications of the same phrase throughout a recording, were flagged as putative songs. Stereotyped single elements were treated as putative flight calls, while rapid sequences of sharply modulated sub-elements producing a uniformly harsh or buzzy tone were classified as trills.

Crucially, the qualitative classification was performed blind to location and date. Two reviewers independently attempted to sort the flight calls and trills into discrete categories showing less internal than external variation, then compared results and retained only the categories on which they agreed. This manual step partially replicated the methods used to identify Evening Grosbeak call types, providing a biological baseline against which to check the more objective statistical analyses, which can over- or under-classify depending on input parameters. Calls that appeared interspersed among plastic song sequences were labeled Type 0, or possible song elements, while apparent flight calls that did not match at least two other recorded calls were labeled Type 4, or unknown flight calls.

The quantitative half of the analysis used a hierarchical cluster analysis with Ward’s method, applied to all putative flight calls using six spectrographic features: duration, initial, maximum, and terminal frequency, total change in frequency, and change from maximum to terminal frequency. The algorithm operated without any reference to the qualitative assignments, allowing calls to form natural groupings in multivariate acoustic space rather than imposing a predefined classification. The researchers visually inspected discordant cases, revisiting original spectrograms to correct only clear and unambiguous measurement errors, which amounted to just four corrections, less than 3 percent of the dataset. A canonical discriminant function analysis then tested whether the same structure emerged when all samples retained their cluster-derived labels. Variables were left untransformed because none deviated significantly from normal distributions, and all analyses were performed in JMP Pro 17.

The convergence of all three independent approaches was striking. Across the 134 recordings, the team identified 25 songs, 44 flight calls, and 13 trills, and both the expert opinion and the statistical methods concurrently identified at least three distinct subgroups of flight calls. The short rising flight call, tentatively Type 1 (n = 24), is a brief, clear-toned ascending element reminiscent of the whit call of Empidonax flycatchers. The long rising flight call, tentatively Type 2 (n = 10), begins with a rapid ascent like Type 1 but quickly levels out into a whistled quality similar to the flight call of a Swainson’s Thrush. Putative Type 3 (n = 15), the peaked descent call, is much higher in pitch, consisting of a nearly vertical ascent to a peak followed by a long descent, a quality reminiscent of the poorly known Evening Grosbeak Type 5, which is hypothesized to overlap in range with Hooded Grosbeaks. Some Type 3 recordings were accompanied by a distinctive peaked polyphonic element above the highest point of the call. The entropy R-squared of the discriminant analysis was exceptionally high at 0.94, indicating that the acoustic variables explained nearly all of the discrimination among types.

The geographic mapping of these call types may prove to be the study’s most consequential result. All Type 1 recordings came from Guatemala, Honduras, El Salvador, and the southernmost Mexican state of Chiapas, while all Type 2 recordings were from the northern Mexican states of Sonora, Chihuahua, Coahuila, and San Luis Potosi. Type 3 recordings came from the same northern states with the addition of Guerrero, Mexico City, and the State of Mexico in the center of the country. In other words, Type 1 is geographically isolated from Types 2 and 3, which overlap substantially across the Sierra Madre. This pattern suggests that Type 1 could represent a distinct subpopulation or even subspecies, though the authors caution that more data are needed to verify it. Of the eight unsorted flight call recordings, seven came from states not represented among the sorted types, hinting that additional call types likely await description with a larger sample.

Beyond the flight calls, the study revealed a repertoire apparently richer than that of the Evening Grosbeak. Hooded Grosbeak songs contained plastic elements clearly distinct from social calls, found regularly across recordings from different regions, indicating that individualized song is a widespread component of the species’ vocal behavior. This is notable because Evening Grosbeaks are among the few passerines that appear to lack a complex reproductive song with elements distinct from social calls, instead possibly relying on sequences of stereotyped flight calls in place of a typical song. One possible explanation, the authors suggest, lies in reproductive life history: Type 4 Evening Grosbeaks in Colorado apparently arrive on breeding grounds already paired and do not defend territories, so reduced song complexity could reflect a reduced need for competitive or territorial functions. The closely related Hawfinch (Coccothraustes coccothraustes) and the majority of Nearctic fringillids possess distinct songs, making the simplification or possible absence of Evening Grosbeak song a potential innovation unique to that species. Hooded Grosbeak trills, meanwhile, were longer and tended to contain more modulated peaks than Evening Grosbeak trills, though the small sample of 13 recordings prevented formal sorting into types.

The authors are candid about the limitations of working with such a scarce dataset. Without direct field observations connecting recordings to specific territorial or social behaviors, classification of vocal elements rested on comparisons with related taxa and behavioral cues discernible from the recordings themselves, such as changes in distance from the microphone suggesting a bird in flight. It remains possible that a small number of vocalizations classified as flight calls represent unusual song variations, or that other variants were mislabeled as song elements. Nonetheless, drawing on many collective years of field experience with Evening Grosbeaks and other finches, the team is confident that the study approaches the most accurate account of the species’ vocalizations possible from available data. Elucidating differences between putative Types 2 and 3, both present in northern Mexico, would require targeted field research in the Sierra Madre north of the Isthmus of Tehuantepec and in the apparent overlap zone in Hidalgo, Mexico, Queretaro, and San Luis Potosi.

The broader implications reach well beyond a single species. Call types have been used to describe new cryptic species and to illustrate ecological or geographic boundaries between subpopulations, and the discovery that discrete flight call types are not limited to Evening Grosbeaks and the Red Crossbill complex suggests that the phenomenon may be widespread across the more than 200 species of Fringillidae, most of which occur outside the regions with the largest audio libraries, Europe and temperate North America. From nearly 150 recordings of this neotropical finch, the researchers were able to distinguish at least three putative vocal subpopulations, and with additional field work, many more subpopulations or call types are likely waiting to be described. Recognition of such infraspecific groups expands understanding of each species’ ecology, evolution, and conservation, and field studies on flight call variation, genetics, and ecology of the Hooded Grosbeak and other Neotropical finches are clearly needed to illuminate the nature, extent, and implications of vocal diversity in this unique family. For now, the cloud forests of Mesoamerica keep their secrets, but the voice of the Hooded Grosbeak is beginning to give them away.

Subject of Research: Ornithology and Bioacoustics

Article Title: Flight call variation and vocal repertoire in the Hooded Grosbeak Hesperiphona abeillei

Article References: Centanni, C. T., Kirsch, W. M., Young, M. A., & Robinson, W. D. (2026). Flight call variation and vocal repertoire in the Hooded Grosbeak Hesperiphona abeillei. Frontiers in Bird Science, 5, Article 1770412. https://doi.org/10.3389/fbirs.2026.1770412

Image Credits: AI Generated

DOI: 10.3389/fbirs.2026.1770412

Keywords: Hooded Grosbeak, bioacoustics, flight calls, call types, Fringillidae, vocal dialects, Sierra Madre, community science, cryptic populations, bird vocalizations, Flight, call