[All photographs copyright, Gary Nunn 2015] – On pelagic trips here in San Diego, California, any storm-petrels that we come across with an extensive white rump are investigated carefully. Particularly if they look small sized and shorter tailed. On 23 August 2015 we found this one just inside San Diego County waters along the east escarpment of the Thirty Mile Bank. The location was about 26 miles distant from the nearest landpoint at La Jolla. This bird fits well the known field marks of Townsend’s Storm-Petrel Oceanodroma (leucorhoa) socorroensis, currently recognized by the AOU as a form of Leach’s Storm-Petrel. This enigmatic small storm-petrel is only known to breed at islets surrounding Isla Guadalupe, which is about 200 miles southwest of San Diego, far off the coast of Baja, Mexico, and from where it ventures to Southern California waters.
In the photographs below the dark blackish coloration of this bird is quite evident and, in direct comparison with the browner colored Chapman’s Storm-Petrel O.(l). chapmani with which it simultaneously appeared, it looked almost snappily black and white. I think the more rounded shorter wings and the very shrunken look of the tail, compared to other forms of Leach’s Storm-Petrel, are also very noticeable. The bright white rump patch is sort of pointed in shape with perhaps a faint median dark mark when seen from behind. The white rump patch looks about equal length to the black tail. The white coloration of the rump patch does wrap a little way onto the undertail coverts which is characteristic of this form. The primaries and remiges do look quite worn away at the tips, but, even accounting for this wear, it presented a more bat like impression in flight than the other forms of Leach’s Storm-Petrel and looked more like a Least Storm-Petrel flying. I think the rear of the bird sort of hangs down a little bit in flight while the front is hunched up with the highest point at the shoulders leading to the squat neck and head. In several photographs this “rear down” flight profile is evident. You can also see in the photographs that the legs do not extend past the tail and look relatively short.