Barrow’s (Bucephala islandica) and Common (B. clangula) goldeneyes winter in large numbers in the St. Lawrence estuary and little is known of their distribution and diet. The objective of the study was to characterize how these two similar species, co-existed in the St. Lawrence estuary during the non-breeding season. Their local distribution, diet, and dive efficiency were compared. There was little overlap in habitat use by both goldeneyes. Their distribution was not correlated in the autumn (r = 0.04) but was in the spring (0.68). Autumn and spring distributions were more correlated in Common Goldeneyes (r = 0.82) than in Barrow’s Goldeneyes (r = 0.49). Both species moved to the north shore of the estuary during January and February 1999 as south shore intertidal areas froze. Barrow’s Goldeneyes foraged in larger flocks than Common Goldeneyes (x̄ = 20.3 vs 9.8 birds) and flocks were larger in autumn than spring. Both species fed on amphipods but differed in their use of gastropods (Barrow’s Goldeneye) and polychaetes (Common Goldeneye). Dive duration was similar in both species but varied between areas. Pause duration was shorter in Common Goldeneyes than in Barrow’s Goldeneyes but was not affected by area. Dive efficiency was higher in Common Goldeneyes than Barrow’s Goldeneyes and varied between areas. The predominance of polychaetes in the diet of Common Goldeneyes and of amphipods in the diet of Barrow’s Goldeneyes had not been highlighted before. These species provide a good example of niche differentiation between closely related species.
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Vol. 30 • No. 2