Taxonomy
mtDNA sequence analysis (Lee et al., 2003) indicates a close relationship between the Yellow-billed Magpie and the Black-billed Magpie, rather than between the outwardly very similar Black-billed and European Magpies (P. pica); the two American forms could be considered as one species.
The Korean subspecies of the European Magpie (P. p. sericea) is more distantly related to all other (including North American) forms judging from the molecular evidence, and thus, either the North American forms are maintained as specifically distinct and the Korean (and possibly related) subspecies are also elevated to species status, or all magpies are considered to be subspecies of a single species, Pica pica.
Combining fossil evidence (Miller & Bowman, 1956) and paleobiogeographical considerations with the molecular data indicates that the Yellow-billed Magpie's ancestors became isolated in California quite soon after the ancestral magpies colonized North America (which probably happened some 3-4 mya) due to early ice ages and the ongoing uplift of the Sierra Nevada, but that during interglacials there occurred some gene flow between the Yellow- and Black-billed magpies until reproductive isolation was fully achieved in the Pleistocene.
The Yellow-billed Magpie is adapted to the hot summers of California's Central Valley and experiences less heat stress than the Black-billed Magpie.
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