Scientists Confirm Rare Mourning Warbler × Yellowthroat Hybrid
A newly published study has confirmed the identity of an unusual hybrid warbler captured in New York – a cross between a Mourning Warbler and a Common Yellowthroat – providing fresh insight into how plumage traits are inherited and why some hybrid combinations occur more often than expected.
Hybrid warblers are not unusual across the Americas, but pairings between distantly related species are typically scarce and often difficult to confirm. Until now, all records of Mourning Warbler × Common Yellowthroat hybrids were based solely on appearance and behaviour, with at least 14 suspected individuals reported since the 1950s. None had been genetically verified.
In this new study, a team analysed a male bird captured in 2017 at Baltimore Woods Nature Center in New York State. Its appearance raised suspicion at the time: the olive body and bright yellow underparts were strongly reminiscent of a Mourning Warbler, yet the throat was strikingly pale, lacking both the black mask and rich colour expected in either parent. A song similar to that of a Common Yellowthroat further fuelled interest.
Using whole-genome sequencing, mitochondrial DNA analysis, and detailed examination of plumage, the researchers confirmed the bird as a first-generation hybrid – a true 50:50 offspring. Crucially, the mitochondrial DNA showed that the mother was a Mourning Warbler and the father a Common Yellowthroat. This matches the pattern suggested by earlier field observations, in which most suspected hybrids behaved and sang more like male Yellowthroats.
The study highlights the extraordinary variability shown by these hybrids. Some individuals exhibit nearly full black masks; others show only faint remnants. Some display splotches of black across the chest, while others have almost none. The bird analysed in this study showed a pale white throat with scattered dark flecks, a mix of traits that do not occur together in either parent species.
By comparing the hybrid’s plumage with that of both parental species, the researchers were able to explore how different types of pigments – melanin and carotenoids – are inherited. In many bird hybrids, the less pigmented parent tends to dominate carotenoid-based traits. That pattern held in the bird’s head, producing the distinctive white throat. But the body plumage told a different story: the hybrid was surprisingly rich in olive and yellow tones, resembling the more colourful Mourning Warbler rather than the duller Yellowthroat.
This contrast suggests that different genetic mechanisms may govern carotenoid deposition in different parts of the body. It also hints that some plumage traits may be controlled by regulatory switches rather than pigment genes alone – an area still poorly understood in birds.
The researchers propose that the most likely hybrid combination is a male Common Yellowthroat pairing with a female Mourning Warbler. Several factors may underpin this pattern, including mate choice, habitat overlap, or genetic compatibility between parental genomes. Interestingly, nearly all known suspected hybrids sing songs more typical of Yellowthroats, hinting at a predisposition for learning or producing the paternal species’ vocalisations.
One unusual past record – a bird singing a Mourning Warbler-like song – may represent not an F1 hybrid but a backcross. That possibility underscores the value of modern genomic techniques for distinguishing first-generation hybrids from later-generation individuals.
Ultimately, the study argues that occasional hybridisation between distantly related warblers may play a larger evolutionary role than previously recognised. Such rare crosses may help shuffle traits, spreading distinctive plumage patches between species or influencing the early stages of diversification.
As genetic tools continue to advance, the authors suggest that more “uncommon mourningthroats” are likely to be identified in the future – and each new discovery may help untangle the complex genetic basis of colour and pattern in North America’s warblers.
November 2025
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