Factors affecting laying date in the Common Eider

Abstract

Date of laying in Common Eiders Somateria mollissima is influenced by several factors. Young (2- and 3-year-old) birds tended to lay late in the season. Females paired after mid-winter (22nd December) also bred late (or not at all), most of these were young birds. Date of laying was apparently not related to date of pairing for females paired before mid-winter. Pre-laying weights (in March) were average or above average for early breeders and average or below average for late breeders. These weights may reflect differences in feeding ability.