Morphological Variation And Species Limits In Murrelets Of The Genus Endomychura

Abstract

Species limits in Endomychura have been a matter of dispute. Recent field studies in Baja Colifornia have shown that the probable breeding ranges of two of the three currently recognized taxa are more extensive than has been realized, and that all three taxa occur on the San Benito Islands during nesting season. Morphological data indicate that Craveri's Murrelet (E. craveri) is acting as a distinct species with respect to both forms of Xantus' Murrelet (E. hypoleuca). Endomychura h. hypoleuca and E. h. scrippsi are exceptionally well-marked forms and differ sifnificantly in size and plumage characters. They apparently hybridize on the San Benitos, but the limited data suggest that interbreeding is not random, that gene flow between the two is reduced, and that isolating mechanisms are being tested. Face pattern and bill shape may be the most effective isolating mechanisms as they are sufficient to provide a rapid means of species recognition. Differences in face pattern and bill shape among the several populations may constitute a comples case of character displacement. Further studies in the area of overlap are needed to determine the extent of hybridization and whether a third species of Endomychura should be recognized. Color patterns of the downy young support the view that Endomychura is most closely allied to Synthiloboramphus.