A Contact Zone between Coastal and Interior Fox Sparrows in South-Central Alaska

Abstract

I report observations, supported by museum voucher specimens, photographs, and audio recordings, from an area of breeding contact between two divergent groups of subspecies of the Fox Sparrow (Passerella iliaca) in upper Cook Inlet, south-central Alaska. In this area, covering ~50 km2, the interior iliaca group comes in contact with the Pacific coast unalaschcensis group in lowland mixed boreal forest. Phenotypically pure P. i. zaboria and P. i. sinuosa (subspecies representing the iliaca and unalaschcensis groups, respectively) occur in approximately equal abundances and outnumber intermediate phenotypes. These subspecies co-occur on a fine scale, males of zaboria and sinuosa often holding adjacent territories. I conclude that some form of pre- or post-zygotic isolating mechanism between these two subspecies-groups is hindering free interbreeding.