Intergradation

Under intergradation mixing two or more subspecies of a species is understood in a regional area. An example is the intergradation of the two subspecies of the carrion crow (Corvus corone ), which takes place along a line of Schleswig -Holstein across eastern Germany to eastern Austria. In the area of Schleswig- Holstein and East Germany, the Elbe makes this line. East of it incubates the hooded crow (C. c. Cornix ), west of the line, the pure black carrion crow (C. c. Corone ). The Intergradationszone, also called contact zone in which to mix the two subspecies is between 40 and 150 km wide. In appearance the descendants step on the characteristics of the parents in continuous variation. Another even longer contact zone exists in North America between the members of auratus group and those of the cafer group of gold -spotted woodpecker. It extends from southern Alaska to Texas.