Two Modes of Clutch Size Determination in Passerine Birds

Abstract

In the Pied Flycatcher clutch size was found to be tied to the date, which confirms earlier results (v. Haartman 1967). In the Starling clutch size was correlated to the order in which the females laid, the relatively early and relatively late clutches having a constant average, independent of the earliness of the year. As a consequence of these modes of clutch size regulation, the mean annual clutch size of the Pied Flycatcher was correlated to the mean laying time of the population, but no such correlation was found in the Starling.