Effects of Patagial Tags on Laying Date and Egg Size in Common Eiders

Abstract
We examined the effects of patagial tags on common eiders (Somateria mollissima) by comparing laying date, clutch size, and egg size of the same females in 2 breeding seasons, before tagging (1987) and after the birds had carried tags for 1 year (1988). Similar data were obtained from unmarked birds used as controls in both years. Clutch size of tagged birds did not differ from controls (P > 0.05), but tagged birds laid eggs later (P < 0.001) and produced smaller eggs (P < 0.05) in 1988 than in 1987. The egg size of control females was larger (P < 0.001) during 1988. We suggest that patagial tags affect a female's ability to accumulate body reserves before breeding by decreasing feeding time, increasing preening activity, and/or increasing energy costs of diving.

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