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

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.