The Hybrid Zone Between Northern and California Spotted Owls in the Cascade–Sierran Suture Zone

Abstract

Both the geographic center and the shape of the hybrid zone between the Northern Spotted Owl (Strix occidentalis caurina) and California Spotted Owl (S. o. occidentalis) have been unknown because of a lack of samples from the transition zone between Mt. Shasta and Lassen Peak in northern California. However, the position of the boundary between these taxa, and their associated taxonomic rank, have important biogeographical, management, and political implications. We used sequences of the mtDNA control region from owls captured in that region to infer the quantitative structure of this contact zone. The proportion of Northern Spotted Owl haplotypes to total haplotypes fell below 50% approximately 24 km south of the Pit River; the 95% confidence interval for this transition extended from just south of the Pit River to just north of Lassen Peak. We estimated the standard width (20%–80%) of the hybrid zone, which appeared symmetrical, to be 94 km. We rejected the prior hypothesis that the boundary between Northern and California Spotted Owls was the Pit River; rather, the center of the zone did not appear to be closely associated with any obvious physical barrier to gene flow. This is the first described instance of a secondary contact zone between avian taxa that is clearly associated with Remington's Cascade—Sierran suture zone. This hybrid zone was too narrow to be consistent with neutral introgression; it is likely that there is a selective, behavioral, or demographic barrier to gene flow across the Pit River region.