This study quantifies the relative importance of factors affecting selection of brood-rearing areas by Common Eiders (Somateria mollissima) according to duckling age. A total of 1,431 h of observation was conducted from June-August on the New Brunswick mainland in 2000 and on Grand Manan Island in 2001. The number of breeding pairs of Great Black-backed Gulls (Larus marinus) on colonies near study sites appeared to be a better predictor of duckling abundance in sites than the number of Common Eider nests on those colonies. Neither degree of exposure of site nor availability of rockweed (Ascophyllum nodosum) explained variations in duckling abundance. However, sites with a gradual slope, which offer increased surface-feeding area, supported 43-85% more ducklings than sites with a steep slope. Rockweed-harvested sites with a steep slope supported the least number of ducklings on Grand Manan Island, and duckling numbers on the mainland decreased faster in harvested than in control sites. Predation was unimportant, with only nine successful predator attacks on ducklings occurring. Of those, Bald Eagles (Haliaeetus leucocephalus) were responsible for six duckling losses. However, Common Eiders may have avoided Herring Gulls (L. argentatus) in duckling-feeding sites, possibly to avoid klepto-parasitism on eiders by gulls. Human disturbance was also a minor factor.
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Vol. 31 • No. 4