Relation of "Clutch-Size" to Number of Ova Ovulated by Starlings

Abstract

THE CLUTCH SIZE of birds has been widely used as a measure of biological processes. The data can be obtained without killing the bird or performing any technical manipulation. However, the relation of the number of eggs actually ovulated to the number in the nest has not been examined except incidentally. The purpose of this contribution is to compare the number of eggs ovulated by Starlings (Sturnus vulgaris) with the number in the nest box. Problems of interpretation of clutch size will be discussed. Clutch to an ornithologist means the number of eggs in a nest that are being incubated. It is generally assumed that the loss of eggs from accidents or predation is negligible and that the number in a clutch is essentially the same as the number ovulated. Yet, it has been known for a long time that eggs are occasionally found on the ground. Some species such as Rheas (Rhea americana) commonly scatter eggs for a week or so before starting to lay in a nest. Others such as ducks lay eggs in nests of other ducks or even pheasants. Pheasants (Kabat et al. 1948) have been shown to lay only part of their eggs in a nest. Among passerine birds the number of eggs lost has been considered to be negligible and the clutch size has been assumed to be a good measure of the egg production, although Davis (1955a:274) raised some doubts. Interest in this problem arose from an attempt to determine whether Starlings are determinate or indeterminate layers. Davis (1955b) had shown that the clutch size for Barn Swallows and Magpies was no more or no less than normal when eggs were removed from or when eggs were added to the nests. Experiments of this type were begun on Starlings in 1953 and it was soon suspected that the number of eggs in the nest might not be a good measure of the effect of removing or adding eggs. Comparison was then made with the number of eggs ovulated; as determined by counting the number of post-ovulatory follicles.