CLUTCH SIZE DETERMINATION IN COMMON EIDERS - AN EGG REMOVAL AND EGG ADDITION EXPERIMENT

Egg addition and egg removal experiments in Common Eider, Somateria mollissima were carried out during three breeding seasons to examine their effect on final clutch size. When the third egg and all subsequent eggs were removed during the laying cycle, females did not respond by laying more eggs. This result is consistent with the hypothesis that egg-production is limited by body reserves; however the experiment may have been carried out too late in the laying cycle to make further egg-laying possible. When three eggs were added to nests containing two eggs females produced fewer eggs than a normal clutch. This suggests that females respond to egg parasitism by reducing their own clutch size. A reduced host clutch size suggests that availability of body reserves is not the only control of clutch size in Eiders, but that costs to incubate and/or to care for chicks may increase with increasing clutch size.