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Issue link: http://bg.aba.org/i/737370
52 Birder's Guide to Listing & Taxonomy | October 2016 Check-list Supplement Redux, v. 2016 the three-way split. Identification of these four forms is complicated by the fact that all of them have variable rump patterns. Even leu- corhoa can have a totally dark rump, as has been observed on breeders from the Far- allones, but light-rumped leucorhoa usu- ally have a dark mark down the center of the rump—something not seen in many white-rumped Ainley's and Townsend's. Field identification should be approached cautiously, to say the least. If you are confused, don't feel bad. The issue is extremely complicated, and there's a reason we're only now starting to under- stand what's going on. For more details, see Steve N. G. Howell's Petrels, Albatrosses, & Storm-Petrels of North America, the ar- ticle by Howell et al. in North American Birds (vol. 63, p. 540), and the article by David Ainley (for whom Ainley's Storm- Petrel is named) in the Jan./Feb. 2005 is- sue of Birding. Split of Green Violetear –––––––––––––––––––––––––––––––––– • Mexican Violetear (Colibri thalassinus) • Lesser Violetear* (Colibri cyanotus) This split separates birds of central and northern Middle America (Mexico through Nicaragua) from those that are found from Costa Rica to Bolivia. Besides being larger, Mexican Violetear, as "our" species is now to the Baja California mainland. Compared to leucorhoa, chapmani is smaller on aver- age and usually has a darker rump. It may also appear to have a more deeply forked tail. "Chapman's Storm-Petrel", as it is called, can be fairly common off the south- ern California coast in summer. Townsend's and Ainley's storm-petrels are sympatric; they both nest on small islands off the southern tip of Guadalupe Island, which is itself well off the west coast of Baja California. They both aver- age smaller than chapmani and leucorhoa, with a more subtle carpal bar. Townsend's tends to be smaller and darker than Ain- ley's, with a larger white rump patch, but some Townsend's are completely dark- rumped, and some are intermediate. The two differ substantially from one another, and from Leach's sensu stricto, in vocaliza- tions. Townsend's nests in summer, and Ainley's nests in winter, so the two cannot interbreed, even though they are allopat- ric. This phenomenon is referred to as tem- poral isolation, and it is another reason for Townsend's Storm-Petrel. Photo © Todd McGrath Suspected range of Townsend's Storm-Petrel in the ABA Area. Ainley's Storm-Petrel*. Photo © Tom Blackman "Chapman's" subspecies of Leach's Storm-Petrel. Photo © Todd McGrath Range of Leach's Storm-Petrel in the Pacific Ocean portion of the ABA Area. (Includes "Chapman's" Storm-Petrel.) Pink = breeding season. Purple = year-round. Yellow = migration/wandering. Note the indigo patch on the underparts of this Mexican Violetear. Photo © Lila Theis leucorhoa only both subspecies chapmani only