Geographic and Seasonal Variation in the Occurrence of Incompletely Pneumatized Skulls in the House Sparrow

Abstract

The process of skull ossification or pneumatization, by which most songbirds obtain a double-layered cranium during the first year or so of life, has received detailed documentation for the House Sparrow (Passer domesticus) (Nero 1951; Harrison 1960). House Sparrows commonly achieve the fully double-layered, adult cranium by the age of 6 months, with incomplete pneumatization persisting in occasional individuals for a somewhat longer period (up to 221 days in one sparrow seen by Nero). During each of four seasons in 1968-69, I collected P. domesticus at five localities on an east-west transect running from the eastern edge of the Great Plains at Lawrence, Kansas, into the southern Rocky Mountains at Gunnison, Colorado (table 1). These specimens were preserved as flat skins and cleaned skeletons with entire skulls. I collected only individuals which had completed the postjuvenal molt. This restriction, coupled with the observation that all birds collected during spring and summer were in breeding condition, reasonably assures that individuals taken in spring and summer were in at least their second calendar year of life. The occurrence of a few such individuals with skulls not completely ossified (this condition is hereafter abbreviated as snco, the fully pneumatized condition is termed sco) prompted me to assess the proportions of snco's in each seasonal locality sample. Packard (1967) has studied morphological geographic variation in P. domesticus along essentially the same transect. Chi-square tests of proportional differences in twoway contingency tables provided the statistical foundation for all of the following analyses.