Intraclutch variation in egg component mass and determination of clutch size in House Sparrows.

Abstract

Intraclutch variation in mass of dry yolk, dry albumen, dry shell and water content of House Sparrow eggs were examined during egg laying of the third brood of the summer of 1992 at three farms in northern lower peninsula Michigan. In 5-egg clutches, while there was no significant variation due to egg sequence observed in dry albumen, dry shell, or water content, there was significantly less dry yolk mass in first laid eggs than later laid eggs. I argue that this variation in dry yolk mass is evidence that the extent of fat reserves in a female two days before laying of the first egg plays an important role in a proximate mechanism for the determination of clutch size. Evidence is also presented linking interclutch variation in fresh egg mass to variation in water content.