Analysis of Audubon Society Christmas bird count data and certain environmental variables shows the degree to which the Black-billed Magpie is a bird of cool arid regions in North America. The abundance and distribution of this species appear to be limited by two major climatic barriers: increasing summer temperatures in the Southwest and increasing summer humidity and precipitation on the central plains.
Bock, Carl E. and Lepthien, Larry W.
"Distribution and abundance of the Black-billed Magpie (Pica pica) in North America,"
Great Basin Naturalist: Vol. 35
, Article 2.
Available at: http://scholarsarchive.byu.edu/gbn/vol35/iss3/2