Subspecies status of the western Yellow-billed Cuckoo (Cuculidae: Coccyzus americanus occidentalis) : using cytochrome b to elucidate the enigma

Abstract

This study documents sequence variation in three mitochondrial DNA (mtDNA) genes in the Yellow-billed Cuckoo {Coccyzus americanus) to re-examine the subspecies status of the western population (C.a. occidentalis). Described traditionally as occurring from southwestern British Columbia, western Washington, northern Utah, central Colorado, and west Texas south and west to southern Baja California, Sinaloa, and Chihuahua in Mexico (Hughes 1999), the western population was first given the designation of subspecies by Ridgway (1887) based on morphological differences between eastern (C. a. americanus) and western forms. Since this initial designation, controversy has ensued over its validity as recent attempts to address the question have produced contradictory results (Spiller 1988; Banks 1988, 1990; Franzreb and Laymon 1993; Pruett et al. 2001). The western population has declined rapidly within the last century with an overall range contraction, as their riparian forests have been removed for fuel, agriculture and grazing, and urban development (Laymon and Halterman 1987). Furthermore, lower water tables resulting from river damming and irrigation projects have claimed much of the remaining preferred habitat, leaving only isolated pockets of suitable habitat (Gaines 1974). Consequently, petitions have been submitted to the U.S. Fish and Wildlife Service (USFWS) since 1986 to list the western population under the Endangered Species Act (ESA); unfortunately, they have been largely ignored due to the lack of resolution inherent in the various taxonomic debates. In addition, the USFWS cannot recognize a subspecies or species for listing unless it meets certain mandated criteria for designation as a Distinct Vertebrate Population Segment (DPS). The western Yellow-billed Cuckoo, as currently described, fails to meet these criteria; hence, it is not eligible for conservation and recovery programs mandated for threatened taxa by the ESA. A recent study by Pruett et al. (2001) addressed the subspecies question in the Yellow-billed Cuckoo by analyzing mtDNA sequence variation in cytochrome b. They identified four fixed base pair (bp) differences between the eastern and western forms. Although compelling, the work of Pruett et al. had several limitations including small sample size with poor geographic representation of the overall species’ distribution, and analysis of only one gene.