Vol. 43, No. 5
David M. Larson
Yellow-rumped Warblers (Setophaga coronata) are widespread breeders in North America. While we are most familiar with the Myrtle Warbler here in New England, Yellow-rumps constitute a species complex consisting of four taxa (subspecies, according to the American Ornithologists Union): Myrtle (S. c. coronata) breeds in the boreal forests of North America and winters in eastern North America, Central America, and the Caribbean; Audubon’s (S. c. auduboni) breeds west of the Rockies and winters in the southwestern United States, Mexico, and Central America; Black- fronted (S. c. nigrifrons) is resident in Mexico; and Goldman’s (S. c. goldmani) is resident in Guatemala. The fact that some of these subspecies migrate—Myrtle and most Audubon’s—while others do not—some Audubon’s and all Black-fronted and Goldman’s—provides an interesting experimental model for studies on the development of migration and on what happens in hybridization zones. Toews et al. (2013) have investigated the genetics, biochemistry, and physiology of Myrtle, Audubon’s, and Black-fronted warblers, concentrating on the areas of geographic overlap and particularly on birds from the hybridization zone in the southwestern United States.
Most animal cells contain two types of DNA. Nuclear DNA (nDNA) is the genetic material in the nucleus and is derived from both parents of the organism (half from the sperm, half from the egg). This is what we usually consider as the main getriphosphate (ATP), the small molecule that provides energy for biochemical processes throughout each cell. If there is a functional link between mitochondrial genes and migration, then the prediction would be that the migratory behavior should sort with the mtDNA type— migratory individuals should have the Myrtle-type mitochondria and nonmigratory individuals should have Black-fronted mitochondria. They tested this prediction by genetic analysis of birds captured along transects through the transition zone and by estimating the individual’s migratory movements through analysis of stable hydrogen isotopes in feathers. A second prediction was that changes in the mtDNA type should result in changes in mitochondrial function. They tested this prediction by genetic analysis of mtDNA from individual birds and testing the biochemical capabilities of mitochondria from the muscle tissue of the same birds.