The effect of time and digestion constraints in Common Eiders while feeding and diving over Blue Mussel beds

1. To maintain energy intake Common Eiders ( Somateria mollissima ) in winter should compensate for reduced day-length by increasing both the proportion of time spent feeding and diving efficiency, defined as the proportional duration of a dive bout within a dive cycle. Common Eiders swallowed Blue Mussel ( Mytilus edulis ) whole with their shells where any behavioural compensation in relation to short days may be limited by digestive processes.
2. Based on time budget studies conducted from mid-December to the end of April in the Gulf of St. Lawrence, Québec, Canada, diving and feeding efficiency were compared for three seasonal periods varying in day-length (from 557 to 890 min).
3. Results showed that eiders were compensating for short days by feeding 56% of the time in mid-winter compared with 33% in spring. However, diving efficiency remained constant through the season and apparently no compensation occurred at this level of their foraging behaviour. Despite this, the daily rate of prey ingestion was much higher in mid-winter than in spring.
4. Ingestion rate values for mid-winter individuals approached or even exceeded the rate at which prey items are defecated and it was concluded that shell crushing performed by the muscular gizzard is physiologically more demanding during that period. On this basis, gizzard mass should be larger in winter when ingestion rates are higher. Data presented support that hypothesis and suggest that compensation in relation to short days can be both behavioural and physiological.