Genetic confirmation of an “Uncommon Mourningthroat” (Geothlypis philadelphia x G. trichas): a rare but persistent hybrid warbler

For much of the history of American ornithology, hybrid birds discovered
outside of known hybrid zones have presented an identification
challenge—given the likely parental species are typically identified based
on inferences from plumage patterns—but also a source of insight into
evolutionary processes. With the advance of modern genetics, the tools are
now available to definitively identify hybrids for which genetic samples
are available and, in many cases, determine the identity and the sex of
each parent species as well as F1 versus backcross status. Typically,
these kinds of hybrids are rare; however, one apparently common hybrid
wood-warbler (family Parulidae) are putative crosses between Mourning
Warblers (Geothlypis philadelphia) and Common Yellowthroats (G. trichas),
which have been reported at least thirteen times between 1955 and 2024,
though never confirmed genetically. Here we describe the results of
genetic and plumage analysis to identify a putative G. philadelphia x G.
trichas hybrid captured in July, 2017 in New York, USA. We used genotype
likelihoods derived from whole-genome sequencing data of the putative
hybrid and several species in Geothlypis and Oporornis (the most likely
genera of the parents) to confirm bird’s identity as an F1 hybrid between
G. philadelphia and G. trichas. We sequenced one mitochondrial gene and
used publicly available sequences of several related species to identify
the maternal parent as G. philadelphia. The data and code presented here
and in GenBank are intended to permit reproduction of the analyses that
made up the study, including processing reads, making trees, and running
PCA and admixture.