Timing of Cranial Pneumatization in White-Throated Sparrows

To assess correlates of the timing of cranial pneumatization in wintering Whitethroated Sparrows (Zonotrichia albicollis), we repeatedly determined the stage of pneumatization of marked individuals in the field following autumnal migration. There were pronounced differences among individuals and among years in the timing of cranial pneumatization. Pneumatization occurred later in larger individuals, as indicated by wing length, but the timing of pneumatization was not related to dates of arrival in wintering areas nor to social dominance.