Using low-coverage whole-genome sequencing, analysis of vocalizations, and inferences from natural history, we document a first-generation hybrid between a rose-breasted grosbeak (Pheucticus ludovicianus) and a scarlet tanager (Piranga olivacea). These two species occur sympatrically throughout much of eastern North America, although were not previously known to interbreed. Following the field identification of a putative hybrid, we use genetic and bioacoustic data to show that a rose-breasted grosbeak was the maternal parent and a scarlet tanager was the paternal parent of the hybrid, whose song was similar to the latter species. These two species diverged >10 million years ago, and thus it is surprising to find a hybrid formed under natural conditions in the wild. Notably, the hybrid has an exceptionally heterozygous genome, with a conservative estimate of a heterozygous base every 100 bp. The observation that this hybrid of such highly divergent parental taxa has survived until adulthood serves as another example of the capacity for hybrid birds to survive with an exceptionally divergent genomic composition.

T.J. audio-recorded vocalizations of the individual between 5:40 and 8:00 AM on the morning of June 11, 2020, using a Wildtronics Pro Mono microphone mounted in a 22” Wildtronics parabolic reflector and a Sound Devices MixPre-3 digital audio recorder. Files were recorded as lossless .WAV files, and lightly edited using the iZotope RX 6 audio editor. The 3 recordings of the bird totaled approximately 5.5 minutes of recording of song and calls. Two of these recordings contained the putative hybrid singing alone (n=25 songs). The third recording captured a counter-singing interaction between the putative hybrid and a scarlet tanager.

We used Raven Pro Sound Analysis Software v1.5 (K. Lisa Yang Center for Conservation Bioacoustics 2014) to assess the characteristics of the n=25 of recorded songs, excluding the two songs captured in the counter-singing interaction. For each recording we generated a spectrogram, a visual representation of the sound with time on the horizontal axis, frequency on the vertical axis, and amplitude (or “loudness”) represented by the darkness of the pixel (Fig. 2). We used the annotation feature of Raven Pro to identify the time and frequency boundaries of the syllables within the recorded songs. We then compared measurements of the songs’ frequency range, number of syllables, and duration of the putative hybrid’s song to those of previously published measurements of those attributes in scarlet tanager and rose-breasted grosbeak songs (Table 2; Mowbray 2020; Wyatt and Francis 2020). While we did not include the songs in the counter-singing recording in this assessment, we did annotate which songs belonged to which individual.