A Distributional History of the Black-crested Titmouse

Abstract

The following account, supported by distributional, paleontological and archaeological data, is presented as a conjectural explanation of the origin and present occurrence of the black-crested races of Parus bicolor. Grey-crested ancestral populations reached northeastern Mexico from eastern Texas via a wooded coastal corridor under humid conditions in late Wisconsinian time. The initial isolation coincided with the early Holocene expansion of the Great Plains climate to the Gulf coast of southern Texas. Differentiation in thorn forest or related arid tropical growth is suggested by the plumage traits of the derived form which promote visual signalling against a vernal vegetational background that is dense and grayish. Once adapted to semiarid conditions, the black-crested titmice spread northwestward in foothill or montane woodlands to the mountains of western Texas, and thence eastward into central Texas. Fragmentation of pinyon-oak-juniper woodlands, leading to the isolation of weakly differentiated subspecies, occurred coincident with increasing aridity no later than 4000 years ago. Hybridization with the gray-crested populations (the tufted titmouse) in central Texas, demonstrating incompleteness of speciation, probably began soon afterward.