Friday, January 11, 2013

Baltimore vs. Bullock’s Orioles

While reviewing my blog, I am surprised how little I have had to say about Baltimore or Bullock’s orioles. These species are common across North America, with the Baltimore (above from Minnesota) breeding in the East, and the Bullock’s (below from Arizona) in the West. Across the Great Plains, where their ranges overlap, the two species interbreed—even though, as you can see in these photos, the two populations look quite different.  In 1983, citing this hybridization, ornithologists merged the two populations into a single species, the Northern Oriole.

Much to the delight of North American bird listers, the merge of the Baltimore and Bullock’s orioles only lasted a little over a decade.  During this time a number of discoveries were made. The zone of hybridization between the two species is relatively narrow, only some 150 to 200 miles wide. Hybrids are only very rarely found outside this zone and pure types are often observed, even in the zone’s center. Ornithologists concluded that, although lots of hybrids can be found within the stable hybrid zone, gene flow is restricted across the zone.  Perhaps hybrids are formed only where the appropriate mates are not easily encountered. Furthermore, because of the hybridization, ornithologists have always assumed the two species are closely related. To their surprise, a number of biochemical studies indicate Baltimore and Bullock’s orioles are not each other’s closest relative. Baltimore Orioles are more closely related to Altamira Orioles, while Bullock’s are closest to Streaked-backed Orioles (Rising and Flood 1998).

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