A genomic perspective on the classic hybrid zone between Baltimore and Bullock’s Oriole

How much has this hybrid zone changed over the past decades?

In the introductory chapter of my PhD thesis, I used a quote by John Michael Crichton (who wrote Jurassic Park): “If you don’t know history, you don’t know anything. You are a leaf that doesn’t know it is part of a tree.” This quote can be interpreted in two ways. On the one hand, it refers to the importance of reconstructing the evolutionary history of a species to understand present-day patterns. This idea was nicely captured by Theodosius Dobzhansky (Dobby for the friends): “Nothing in biology makes sense, except in the light of evolution.” On the other hand, the quote by John Michael Crichton can also refer to the importance of the history of science. Ideas do not pop into existence in a vacuum. They are the outcome of countless hours of observations, experiments and thinking by numerous scientists. Diving into the scientific literature and reading “old” papers can lead to new insights and can help you to understand complex concepts.

With regard to hybridization in birds, for example, there is a long history of hybrid zone studies. During my PhD, I read classic papers on avian hybrid zones by Jürgen Haffer (in South America), Julian Ford (in Australia) and Charles Sibley (in North America). Several of these hybrid zones have been revisited with modern genomic techniques. Recently, Jennifer Walsh, Shawn Billerman and their colleagues provided a genomic perspective on the hybrid zone between Baltimore oriole (Icterus galbula) and Bullock’s oriole (I. bullockii) in North America.

Oddly Plumaged Orioles

The Baltimore and Bullock’s orioles hybridize along the riparian corridors that cut through the Great Plains of North America. The scientific history of this hybrid zone goes back to Sutton (1938) who described hybrid specimens as “oddly plumaged” orioles. In the 1960s and 1970s, Charles Sibley and James Rising (among others) published several studies on the interactions between these orioles, providing a detailed overview of the hybrid zone structure.

The application of genomic data can refute hypotheses based on morphological inferences (see for example this blog post about Brewster’s and Lawrence’s Warbler). In the oriole case, however, the researchers found that “classically scored plumage traits are an accurate predictor of pure vs. hybrid genotypes.” This allowed them to compare past and present hybrid zone dynamics. What has changed since the first description of this hybrid zone?

The distribution of Baltimore Orioles (orange) and Bullock’s Orioles (yellow) across North America. From: Walsh et al. (2020) The Auk.

To the West

The new analyses supported previous studies suggesting that the center of the hybrid zone has moved to the west. The exact mechanism behind this westward shift remains unclear, but could be related to the expansion of suitable woodland habitat. Alternatively, climatic changes might allow Baltimore orioles to expand their range westwards. Regardless of the underlying mechanism, the westward spread is also apparent in the genomic patterns. When hybrid zones move, selectively neutral genes are expected to flow from the receding species (Bullock’s oriole) into the advancing species (Baltimore oriole). This theoretical mechanism can result in a “tail of introgression” that lags behind the moving hybrid zone. And that is exactly what we observe in the oriole case.

In addition to the westward movement, the hybrid zone has also become narrower over time. The width of a hybrid zone is an indication for the strength of selection against hybrids. If there is weak selection, hybrids will frequently backcross with parental species, resulting in a geographically widespread sharing of genetic material. However, if there is strong selection against hybrids, they will not be able to backcross with parental species and the genetic admixture will be concentrated in the center of the hybrid zone. Hence, the narrowing of the oriole hybrid zone suggests that reproductive isolation between Baltimore and Bullock’s oriole has strengthened.

Geographic cline analyses revealed that the hybrid zone has moved westwards and became narrower. From: Walsh et al. (2020) The Auk.

Species Concepts

The study of Baltimore and Bullock’s oriole hybrids was also relevant for their species status. In the 1960s, Charles Sibley argued that hybridization was extensive and that there was little evidence for selection against hybrids. Based on this information, the American Ornithologists’ Union decided to lump both taxa in the “Northern Oriole”. The refinement of Biological Species Concept (taking into account hybrid fitness) and additional information from genetic studies has provided a new perspective on the species status of these orioles. The authors concluded that “Our interpretation of these patterns is that Baltimore and Bullock’s orioles are best considered distinct species, with strong selection likely acting to restrict the expansion of the hybrid zone, which is in turn evidence of substantial post-zygotic reproductive isolation despite widespread admixture within the hybrid zone.”


Walsh, J., Billerman, S. M., Rohwer, V. G., Butcher, B. G., & Lovette, I. J. (2020). Genomic and plumage variation across the controversial Baltimore and Bullock’s oriole hybrid zone. The Auk137(4), ukaa044.

Featured image from Sutton (1938).

This paper has been added to the Icteridae page.

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