By John M. Coluccy, Ph.D., and Kassondra Hendricks
Seeing large concentrations of waterfowl in the marsh is a thrill for waterfowlers and wildlife watchers alike. In fact, the propensity of waterfowl to gather in spectacular concentrations on migration and wintering areas is among the most compelling aspects of the birds' behavior
Most waterfowl are downright unsociable during the breeding
period, but are drawn together for the remainder of the year. Following the breeding season, waterfowl become more gregarious, gathering on large wetlands and shallow lakes north of breeding areas to undergo the wing molt. As summer ends and fall progresses, the birds continue to mass on northern staging areas in preparation for fall migration. On rare occasions, when conditions are just right, waterfowl migrate south en masse in a phenomenon known as a "grand passage." Such an event occurred in November 1995 when a powerful cold front and storm system hit the northern Great Plains, where large concentrations of waterfowl were staging. The flocks of ducks and geese moving ahead of this storm were so dense that radar systems at several major midwestern airports couldn't distinguish the birds from airplanes, forcing dozens of commercial flights to be grounded or rerouted.
As waterfowl migrate
south toward their wintering grounds, the birds become even more gregarious, foraging and roosting together in great numbers on traditional staging and wintering habitats. In general, waterfowl do not engage in activities that are not beneficial to their survival, and there are indeed many benefits for individual birds in being associated with a flock. A group of waterfowl is more likely to detect predators and other potential threats than a single bird, and large numbers of birds may be able to confuse or overwhelm predators by presenting them with a variety of possible targets, increasing the odds of survival for all the members of the flock. Moreover, migrating in large flocks has advantages for individual birds. Flying in a characteristic V formation helps waterfowl conserve energy during long-distance flights. In addition, young birds benefit from the past experience of more seasoned adults, who are familiar with migration routes
as well as good places to feed and rest along the flyways.