Effect of food availability on arrival and departure decisions of Harlequin Ducks at diurnal feeding grounds

We investigated three types of decisions made by Harlequin Ducks (Histrioni cus histrionicus) in moving between nearshore feeding and offshore resting areas: when to move, whether to move synchronously, and whether to form den se flocks on the roosting grounds. We used the spawning of Pacific herring (Clupea pallasi) as a natural food-supplementation experiment. Birds arrive d at nearshore feeding areas a few minutes later and departed almost an hou r earlier relative to sunrise and sunset when spawn was available than befo re and after. Cloud cover and high winds resulted in earlier departures, es pecially during spawning. Arriving, departing, and offshore groups consiste d most frequently of two ducks, and birds showed little tendency to synchro nize movements or to form dense flocks when resting. Results indicate that Harlequin Ducks avoid crepuscular and nocturnal periods near shore when not constrained by food availability and the length of daylight in which to fe ed.