Answering this question with genomic data and acoustic analyses.
The Eastern Meadowlark (Sturnella magna) and Western Meadowlark (S. neglecta) are in secondary contact across North America. It seems plausible that these species hybridize. However, hybridization is difficult to assess because they look alike. To circumvent this issue, Wesley Layton set up a captive breeding program with Eastern and Western Meadowlarks. In 1979, he presented his findings in the journal Nature:
I now report that between 1966 and 1978 I was successful in inducing 25 captive meadowlarks to pair and produce 44 clutches of 158 eggs. Mixed matings among non-hybrids resulted in 90% fertility, not significantly different from the 87% fertility among eggs from pure matings, whereas the fertility of eggs from pairing of hybrids was only 10%. All eggs resulting from pairing the one surviving backcross hybrid were infertile.
These captive crosses clearly indicated that hybrids between Eastern and Western Meadowlark are mostly sterile. This level of reproductive isolation, in combination with differences in songs, contributed to the decision to treat them as separate species.
Although the species-level taxonomy is well-established, it remains unclear how many subspecies there are. The variation in songs and plumage has led to the description of no less than 17 subspecies within the Eastern Meadowlark. More research is needed to bring some clarity in this taxonomic turmoil. A recent study in the journal Ornithology took a first step and focused on two subspecies – S. m. lilianae and S. m. auropectoralis – that are collectively known as Lilian’s Meadowlark.
Genes and Songs
Johanna Beam and her colleagues sequenced the whole genomes of 32 museum specimens: 22 Eastern Meadowlarks (covering 5 subspecies) and 10 Western Meadowlarks. Phylogenetic analyses revealed a surprising pattern. The subspecies S. m. lilianae and S. m. auropectoralis did not belong to the Eastern Meadowlark, but represented a distinct evolutionary lineage. Moreover, there were no signs of gene flow between any of the taxa. Lilian’s Meadowlark might thus be a separate species.
This conclusion was corroborated by acoustic analyses. The researchers studied the song characteristics in 85 recordings of the different taxa. They found “significant differences in starting, minimum, and median frequencies [which] all indicate strongly divergent song between all taxa.” As song might be an important trait in species recognition, these results suggest that Lilian’s Meadowlark is reproductively isolated from Eastern and Western Meadowlark.
Taken together, Lilian’s Meadowlark seems to be a distinct species. It is genetically separated from Eastern and Western Meadowlark, and it sings a different song. Hence, the researchers suggest to elevate this taxon to species level. This study nicely shows how different lines of evidence – such as genomics and acoustics – can be combined to make solid taxonomic decisions. A pluralistic approach to avian taxonomy is clearly the way to go.
Beam, J. K., Funk, E. R., & Taylor, S. A. (2021). Genomic and acoustic differences separate Lilian’s Meadowlark (Sturnella magna lilianae) from Eastern (S. magna) and Western (S. neglecta) meadowlarks. Ornithology, 138(2), ukab004.
Featured image: Eastern Meadowlark (Sturnella magna) © Mike’s Birds | Wikimedia Commons