Geographic variation in plumage color between populations of Contopus sordidulus is minor. The differences in color between the races described by Burleigh (1960) is largely the result of postmortem color changes of museum skins. Variation in wing chord and length of tail between samples does not support recognizing additional races of C. sordidulus. Contopus virens is monotypic. The relationships between sordidulus and virens is unclear.
Browning, M. Ralph
"Geographic variation in Contopus sordidulus and C. virens north of Mexico,"
Great Basin Naturalist: Vol. 37
, Article 5.
Available at: http://scholarsarchive.byu.edu/gbn/vol37/iss4/5