| Summary: | This study is an examination of geographic variation and evolutionary history of Stelgidopteryx swallows. These swallows comprise two recognized species: Northern Rough-winged Swallow (S. serripennis) and Southern Rough-winged Swallow (S. ruficollis). A third species, Yucatan Rough-winged Swallow (S. ridgwayi)is also commonly recognized. The species are largely allopatric, except for a contact zone in Costa Rica. Using plumage and molecular variation, I examined the likelihood of two (or three) different species of rough-winged swallows, the genetic interrelationships among taxa, and their biogeographic history.
Geographic plumage variation reveals a latitudinal cline across the genus from North to South America. Specimens from throughout the range of Stelgidopteryx show that most published subspecies should be synonymized as clinal variants.
Molecular data (mitochondrial cytochrome-b sequences, microsatellite allele frequencies) show a pattern consistent with a species-level division between S. serripennis and S. ruficollis . The data also suggest that the Yucatan swallow is distinct. Phylogenetic trees from the sequence data divide Stelgidopteryx into northern and southern clades consistent with S. serripennis and S. ruficollis. The Yucatan clade, S. ridgwayi, is sister to the northern group. Microsatellite data indicate allelic frequency differences between the groups, but none fixed.
Population genetic analyses among individuals within S. serripennis and S. ruficollis reveal genetic structure possibly worthy of taxonomic recognition. Measures of the population parameter theta indicate high allelic diversity in each group and suggest S. ruficollis is the younger population. The Northern Rough-winged Swallow exhibits broad clinal variation, but little subspecific subdivision. The Yucatan Rough-winged Swallow, is phenotypically and genetically dissimilar to the other groups, further supporting species-status. The Southern Rough-winged Swallow contains at least two subspecies, including decolor on the Pacific slope of Central America, and ruficollis throughout the rest of its range.
The widespread distribution and contact zone in Costa Rica are consistent with secondary contact between the northern and southern swallows. The emergence of the Panamanian landbridge could have contributed to the formation of the contact zone. Pleistocene climatic shifts could also have played a role in isolating birds on opposite slopes of Central America allowing divergence between S. r. uropygialis and S. r. decolor.
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