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Winter body mass and over-ocean flocking as components of danger management by Pacific dunlins

Ronald C Ydenberg1*, Dick Dekker2, Gary Kaiser3, Philippa CF Shepherd4, Lesley Evans Ogden5, Karen Rickards1 and David B Lank1

Author Affiliations

1 Centre for Wildlife Ecology, Simon Fraser University, Burnaby, BC V5A 1S6, Canada

2 3819-112A Street NW, Edmonton, AB T6J 1K4, Canada

3 402-3255 Glasgow Avenue, Victoria, BC V8X 4S4, Canada

4 Western and Northern Service Centre, Parks Canada 300 - 300 West Georgia Street, Vancouver, BC V6B 6B4, Canada

5 Centre for Applied Conservation Research, Forest Sciences Centre, 2424 Main Mall, University of British Columbia, Vancouver, BC V6T 1Z4, Canada

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BMC Ecology 2010, 10:1  doi:10.1186/1472-6785-10-1

Published: 21 January 2010



We compared records of the body mass and roosting behavior of Pacific dunlins (Calidris alpina pacifica) 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 tide period. Fat stores offer protection against starvation, but are a liability in escape performance, and increase flight costs. Peregrine falcons (Falco peregrinus) were scarce on the Fraser River estuary in the 1970s, but their numbers have since recovered, and they prey heavily on dunlins. The increase has altered the balance between predation and starvation risks for dunlins, and thus how dunlins regulate roosting behavior and body mass to manage the danger. We therefore predicted an increase in the frequency of over-ocean flocking as well as a decrease in the amount of fat carried by dunlins over these decades.


Historical observations indicate that over-ocean flocking of dunlins was rare prior to the mid-1990s and became common thereafter. Residual body masses of dunlins were higher in the 1970s, with the greatest difference between the decades coinciding with peak peregrine abundance in October, and shrinking over the course 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.


Pacific dunlins appear to manage danger in a complex manner that involves adjustments both in fat reserves and roosting behavior. We discuss reasons why over-ocean flocking has apparently become more common on the Fraser estuary than at other dunlin wintering sites.