Seabird Geographic Variation: Similarity Among Populations of Leach's Storm-Petrel

-We assessed geographic variation in 13 locality samples of Leach's StormPetrel (Oceanodroma leucorhoa), 12 from the Pacific coast and 1 from the Atlantic coast of North America. Nine phenetic characters were used with canonical variates analysis to determine similarity among samples. Regression of phenetic distance on geographic distance measured the relation between similarity and interisland distance. We found a clinal pattern of population similarity from the Aleutian Islands in the north to the Farallon Islands in the south. Populations from Los Coronados and San Benitos islands make up a distinct subset when rump color and wing and tail shape indices are included in the analysis, but these populations were part of a dominant clinal trend when only five size characters were used. On Guadalupe Island separate populations breed in summer and winter; these are strongly differentiated from the others and from each other. The North Atlantic sample was aligned closely with that from the Aleutians. The degree of geographic variation was roughly similar to that in certain migratory land birds on the North American continent, but is less than what is found for certain nonmigratory land birds on islands. Our results suggest recognizing four subspecies of Leach's Storm-Petrel in the study area, including two on Guadalupe Island isolated by time of breeding. Received 7 May 1984, accepted 3 March 1986. SEABIRDS often range over wide areas, yet return to specific island sites to reproduce. As a result, the breeding populations of many pelagic species are isolated geographically (King 1974). It is not known if the degree of reproductive isolation and the concomitant potential for divergence of island-breeding seabird populations are any greater than what might be found in land birds. A comparable mainland situation might be a migratory species with populations that nest in isolated habitat "islands." The interrupted breeding distribution of seabirds offers an attractive situation for study of geographic variation. Seabirds, however, have not figured prominently in our understanding of intraspecific evolution; most information on avian geographic variation comes from studies of land birds (e.g. Selander 1971). In a univariate analysis of geographic variation on breeding populations of Leach's StormPetrel (Oceanodroma leucorhoa) in the eastern North Pacific and the western North Atlantic oceans, Ainley (1980) found that several characters vary clinally from the Aleutian Islands in the north to the San Benitos Islands off the west coast of Baja California, Mexico. In addition, the fact that Guadalupe Island supports two morphologically distinct populations, one breeding in summer and the other in winter, led Ainley to describe the winter population as a new race. Bourne and Jehl (1982) criticized some of Ainley's interpretations (see also Ainley 1983 and Jehl and Everett 1985). We reanalyzed the morphological data by combining characters in a multivariate analysis. Information Ainley presented on vocalizations was not considered. We examined the pattern of overall similarity among populations of Leach's StormPetrel and the relationship between specific island colonies, aspects of geographic variation that were difficult to assess only through variation in single characters. We also measured the relationship between overall similarity and interisland distance to see if the pattern of geographic variation is a function of geographic distance, as is often the case with land birds.