Winter Body Mass and Over-Ocean Flocking as Components of Danger Management by Pacific Dunlins

Background: We compared records of the body mass and roosting behavior of Pacific dunlins (Calidris alpinapacifica) wintering on the Fraser River estuary in southwest British Columbia between the 1970s and the 1990s.‘Over-ocean flocking’ is a relatively safe but energetically-expensive alternative to roosting during the high tideperiod. Fat stores offer protection against starvation, but are a liability in escape performance, and increase flightcosts. Peregrine falcons (Falco peregrinus) were scarce on the Fraser River estuary in the 1970s, but their numbershave since recovered, and they prey heavily on dunlins. The increase has altered the balance between predationand starvation risks for dunlins, and thus how dunlins regulate roosting behavior and body mass to manage thedanger. We therefore predicted an increase in the frequency of over-ocean flocking as well as a decrease in theamount of fat carried by dunlins over these decades.Results: Historical observations indicate that over-ocean flocking of dunlins was rare prior to the mid-1990s andbecame common thereafter. Residual body masses of dunlins were higher in the 1970s, with the greatestdifference between the decades coinciding with peak peregrine abundance in October, and shrinking over thecourse of winter as falcon seasonal abundance declines. Whole-body fat content of dunlins was lower in the 1990s,and accounted for most of the change in body mass.Conclusions: Pacific dunlins appear to manage danger in a complex manner that involves adjustments both in fatreserves and roosting behavior. We discuss reasons why over-ocean flocking has apparently become morecommon on the Fraser estuary than at other dunlin wintering sites.