The role of male mate-guarding in pre-laying Common Eiders Somateria m. mollissima in the northern Baltic sea

Abstract

During the two weeks prior to laying on the Finnish breeding grounds in the central Gulf of Finland, Baltic Common Eider males escorted their females constantly. Towards the start of laying, males increased their vigilance by reducing their feeding rate from 70% to 0%. Concurrently, they lost weight. During the same time period, females kept their feed-ing rate constant at around 75%. Upon laying start, the initially even sex ratio of the popu-lation temporarily became male-biased when females disappeared to nests and emanci-pated males were still around. Yet, experimentally widowed females, feeding alone at this stage, were not harassed by the emancipated males and kept their clutch size and body weight similar to those of their previous nesting years. However, the hatching rate of the widows showed progressive decrease with the number of days from mate loss to laying. The viability of their mates'sperm varied from 1 to at least 20 days and this correlated with the proportion of viable eggs in their clutches. We suggest that male mate guarding is for fertilization assurance rather than for female protection during the pre-laying period at northern breeding grounds. – We did not observe any instance of paired males actively pursuing extra-pair copulations (EPC). We suggest that an inter-sexual conflict over cop-ulations exists in our study population. Females may need repeated copulations because of variation in the duration of sperm viability. Males apparently rely on intense mate guarding as a means to safeguard paternity, resulting in a high rate of male refusals over mate solicitations (40%).