Genetics have confirmed another case of long-suspected split within Xantu's Murrelet, a small auk from the southern west coast of North America.
Analysis of a large number of samples of both a mitochondrial gene and a microsatellite (measuring between species and between population differences, respectively) from all 13 breeding sites off the south-western coast, has shown a significant differentiation between the two currently acknowledged subspecies of Xantus's Murrelet. The small auk is currently divided into nominate Xantus's Synthliboramphus hypoleucus hypoleucus and Scripps's Murrelets S h scrippsi, and these have long been suspected of being different species by birders and ornithologists alike.
Microsatellites showed that gene flow within colonies of the different subspecies was very high, but that there was little if any between colonies and none at all between colonies where both subspecies occurred.
Scripps's Murrelet is midway in appearance between Xantus's and the closely-related Craveri's S craveri Murrelets, with only a small white wedge below its eye compared to Xantus's incmplete eye-ring, and with an entirely white throat contrasting with Craveri's back chin. The suggestion of reproductive isolation by the parallel non-interbreeding populations of either (former) subspecies is confirmed by their genetics, and the two should almost certainly be described as separate species.
With the most recent estimate of a world population of 39,700 in their limited breeding areas of continental shelf islands off the coasts of California, USA, and Baja California, Mexico, Xantus's Murrelet sensu lato was already an endangered species. Scripps's takes a place as the more numerous northern species, but Xantus's is now a species of urgent conservation concern, having less than 5,000 individuals in existence. It is under threat from feral cats on Guadaloupe Island, Mexico, and its highly limited range makes it especially vulnerable.
Reference
Birt, T P, Carter, H R, Whitworth, D L, McDonald, A, Newman, S H, Gress, F, Palacops, E, Kepke, J S and Friesen, V L. 2012. Rangewide population genetic structure of Xantu's Murrelet (Synthliboramphus hypoleucus). The Auk 129: 44-55.
