The egg size variation of the Treecreeper Certhia familiaris in south-western Sweden

Abstract

Egg measurements were in 1994—1996 collected on a Treecreeper population nesting in artificial nest sites erected in deciduous forests in south-western Sweden. The mean egg volume was 1.21±0.091(SD)cm³ (n=533) and the mean weight was 1.26±0.100g (n=133). The mean volume decreased as the clutch size increased from 1.26±0.091cm³ (4-egg clutch) to 1.16±0.079cm³ (7-egg clutch). Within clutches, the volume showed on average an 8% increase from first to last laid egg, and the size and laying order coincided for about 50% of the eggs on each separate day in the laying sequence. The egg sizes of neighbouring clutches with laying start on the same day tended to vary in parallel from first to last egg, indicating a constraining influence of environmental origin on egg formation. The size of an egg was positively correlated with the ambient temperature of the first two or three days before laying. This relation was less clear during periods of falling temperature, probably because the temperature influence was counteracted by the tendency of the female to successively increase egg investments in the laying sequence. The consequences and adaptive value of this trait are discussed.