Aggregative response of Harlequin Ducks to herring spawning in the Strait of Georgia, British Columbia

We determined the scale of aggregative response of Harlequin Ducks (Histrionicus histrionicus) to seasonally and locally superabundant prey at Pacific herring (Clupea pallasi) spawning sites in the northern Strait of Georgia, British Columbia, in 1995–2002. Aggregations of 3400–5500 birds gathered at a small number of sites along the same 8-km stretch of shoreline each year that spawn was available there. Aggregations occurred in only a small fraction of the habitat area where spawn was available. Duration of stay at spawning sites averaged 2–3 weeks and many birds returned to their wintering grounds afterwards. Birds moving to spawning sites represented 55–87% of the total wintering population. The proportion of local wintering populations that moved to spawning sites was negatively related to the distance they had to travel, and few birds travelled farther than 80 km. The decline in proportions moving with increasing distance suggests that more distant individuals may be constrained by a lack of information or that there are trade-offs between the benefits of exploiting spawn and the costs of movement. This raises a conservation concern because the temporal and geographic range of herring spawning in British Columbia is contracting and some wintering waterbird populations may be losing access to this important late-winter food.