Parental care influences the feeding behaviour of female eiders Somateria mollissima

We compared the feeding behaviour of lone tenders, multi-female tenders and postbreeding, non-tending eider females Somateria mollissima in the northern Baltic. Few prey species are available for eiders in the Baltic; in the non-breeding season adults prefer blue mussels Mytilus edulis, whereas small ducklings feed primarily on gammarids Gammarus spp. Infections by the acanthocephalan parasite Polymorphus minutus occur when eiders feed on gammarids and this infection may be fatal if the host’s resistance is low. The mussel beds and areas with gammarids are close together but do not overlap. Assuming that females always should prefer blue mussels to gammarids we tested the following predictions: (i) non-tending females should exclusively feed on mussels, (ii) lone tenders may be forced to feed like their young, and (iii) multi-female tenders may occasionally utilise mussels, thereby mitigating constraints associated with consumption of unprofitable gammarid prey. Tending females foraged like their young, whereas non-tending females fed on mussels throughout the brood-rearing season. Individually marked females fed on mussels immediately after losing their brood, suggesting that a conflict between female and duckling feeding needs exists during early brood-rearing. However, later in the season all females and young fed on mussels. Multi-female tenders achieved no obvious foraging benefits compared to lone tenders, indicating that creching in eiders may have primarily evolved for other reasons.

Ost, M. & Kilpi, M. 1999, Zool. Fennici 36: 195-204