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Split of Leach’s Storm-Petrel
- Leach’s Storm-Petrel (Oceanodroma leucorhoa)
- Townsend’s Storm-Petrel (Oceanodroma socorroensis)
- Ainley’s Storm-Petrel* (Oceanodroma cheimomnestes)
You may be wondering, “Which of these have I seen?” The answer is, “Probably not more than one, and that one is probably Leach’s.”
In North America, Leach’s Storm-Petrel (sensu stricto) now consists of two subspecies. The nominate is found in the Atlantic Ocean and the Pacific Ocean. Any Leach’s seen north of Santa Barbara, California, can probably be safely assumed to be leucorhoa. The subspecies chapmani breeds on islands (such as San Benito and the Coronados) fairly close to the Baja California mainland. Compared to leucorhoa, chapmani is smaller on average 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 southern 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 average smaller than chapmani and leucorhoa, with a more subtle carpal bar. Townsend’s tends to be smaller and darker than Ainley’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’ssensu stricto, in vocalizations. Townsend’s nests in summer, and Ainley’s nests in winter, so the two cannot interbreed, even though they are sympatric. This phenomenon is referred to as temporal isolation, and it is another reason for the three-way split.
Identification of these four forms is complicated by the fact that all of them have variable rump patterns. Even leucorhoa can have a totally dark rump, as has been observed on breeders from the Farallones, but light-rumped leucorhoa usually 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 understand what’s going on. For more details, see Steve N. G. Howell’s Petrels, Albatrosses, & Storm-Petrels of North America, the article 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 Issue of Birding.
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