Feeding Behavior of Common Eider Ducklings in Relation to Availability of Rockweed Habitat and Duckling Age

-Common Eider (Somateria mollissima) ducklings and associated females in the Bay of Fundy feed extensively on invertebrates found in association with Knotted Wrack (Ascophyllum nodosum), commonly referred to as rockweed. To assess whether commercial harvest of rockweed, which is occurring in New Brunswick, might adversely affect ducklings, I examined their feeding behavior in relation to rockweed availability and duckling age. Ducklings fed in rockweed whenever it was available. Young ducklings (less than 3 weeks old) spent more time dabbling for invertebrates in the floating rockweed canopy and less time diving than did older birds. Young ducklings fed when rockweed was most available at the surface, whereas rockweed availability had no effect on the feeding rate of older ducklings. When rockweed was unavailable, older ducklings dove for food, while younger birds still attempted some dabbling. These results suggest that if rockweed harvest lowers the canopy height and reduces the time during each tidal cycle that algae floats at the surface, it might adversely affect Common Eider ducklings during the first weeks of life, when they are less able to dive for food. Consequently, in areas where ducklings feed regularly, care should be taken during harvesting to avoid changing the height and structure of the rockweed canopy. Similarly, harvesters should avoid disturbing foraging ducklings, because it could increase the already high predation risk that these birds face in their first few weeks of life. Received 10 March 2000, accepted 16 November 2000.