Second North American Sea Duck Conference - Patuxent Wildlife ...

SECOND NORTH AMERICAN SEA DUCK CONFERENCE
DISTRIBUTION OF COMMON EIDERS DURING BROOD-REARING
IN THE ST. LAWRENCE ESTUARY
Hélène Diéval¹, Jean-François Giroux¹, and Jean-Pierre L. Savard²
¹University of Quebec at Montreal; dieval.helene@courrier.uqam.ca
²Canadian Wildlife Service, Quebec, Canada
Common eiders (Somateria mollissima dresseri) nest in colonies on islands of the St. Lawrence
estuary in Quebec. After hatching, females must reach adequate habitats for rearing their ducklings.
Adult birds also undertake a molt in late summer but their distribution is not well known. The aim of
our study was to determine the biotic and abiotic factors that influence the distribution of these birds
during the brood-rearing and molting periods. The number of common eiders was recorded by age and
sex bi-weekly in 2003 and 2004 at 68 sites located along a 200-km stretch of the south shore of the St.
Lawrence River. For adult females, we also noted whether they were accompanied by young or not.
In 2004, we recorded the behavior of individuals and noted their locations in relation with distance
to shore. We evaluated human disturbance, developed a shoreline sinuosity index, characterized the
nature of the substrate, and estimated food abundance. At the scale of the estuary, common eiders did
not distribute randomly and used the same sites in both years. Females without ducklings and males
were further east along the estuary, but their numbers were lower than expected based on the breeding
population. Broods preferred the westerly areas near the nesting islands. On a temporal scale, we
observed that the densities of females without ducklings increased significantly during the summer
as they move eastward. Broods spent more than half of the day time feeding while this proportion
decreased through time from nearly 40% to 10% for females without ducklings. On a local scale,
broods used mainly the intertidal zone, whereas adults without ducklings were distributed further offshore.
Densities of common eiders were exclusively influenced by food abundance with each age and
sex category distributed according to their preferred prey. Shoreline sinuosity and human disturbance
influenced the proportion of time that eiders spent in foraging activities and their location from shore.
In conclusion, our study will help to identify the best brood rearing and molting sites for common
eiders along the St. Lawrence estuary based on biotic and abiotic factors. Very few of these sites are
currently included within conservation areas and measures should be taken to insure their protection.
NOV. 7-11, 2005 ANNAPOLIS, MARYLAND, USA
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