Digestive-Rate Constraint in Wintering Common Eiders (Somateria mollissima): Implications for Flying Capabilities

Abstract

Rates of ingestion and digestion for wintering Common Eiders (Somateria mollissima) feeding on blue mussels (Mytilus edulis) were estimated using information on average meal size, feeding- and resting-bout durations, and transit time. Rate of ingestion of mussel shells is two times higher than defecation rate; as a consequence, shells accumulate in the gut as ingestion progresses. On average, eiders shot when flying (n = 92) had 1.1% of their body mass as prey compared with 3.7 and 6.4% for eiders foraging in small and large flocks (n = 77), respectively, suggesting that eiders tend to minimize the transportation of surplus mass when flying. Wing loading of the Common Eider averages 2.0 g.cm−2, which is among the highest values determined for a bird species capable of flight. Maximum gut contents were between 8 and 11% of body mass for eiders foraging in large rafts; such mass of food significantly increases wing loading and is associated with a reluctance or a possible inability to take flight. I hypothesize that meal size in Common Eiders is regulated in response to flight limitations that result from the added mass of a meal.