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Baltimore orioles.
Checklists of North American Birds published by the American Ornithologists’ Union listed these as separate species in 1957. Then in 1983, the AOU listed them together in a single species called the Northern oriole. Then in 1995, the AOU listed them as a separate species again. Why all the name changes?
The main reason for the confusion is that Baltimore and Bullock’s orioles commonly interbreed where their breeding ranges overlap in the Great Plains. This has been recognized since the early 1900s and represents one of the best-known examples of a “hybrid zone” in ornithology.
Studies suggest that hybrids form readily in large part because there is little evidence for mating preferences based on species. Hybrids also commonly backcross with parental species, and all hybrid forms appear to be as viable as the parental species.
The high frequency of hybridization among these orioles suggests a close
See ORIOLES, Page B2
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