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.
An added benefit of flocking is that individual birds have a higher probability of finding and securing a mate. Concentrations of waterfowl on fall staging and wintering areas ensure that individual males and females have an opportunity to court and pair. In most duck populations, however, drakes outnumber hens, so some males will inevitably be unsuccessful in securing a mate.
While there are numerous benefits associated with flocking, there are also some potential risks to this behavior. Large aggregations of waterfowl are highly conspicuous, and the movement and sound created by a flock of ducks or geese can attract potential predators as well as hunters. In addition, concentrations of waterfowl in limited habitat can increase competition for food resources. The larger the flock, the greater the amount of food energy required to support the birds. Thus, in areas where foraging habitat is in short supply, some members of the flock may be unable to find enough food.
Disease is another potential hazard faced by flocked birds. Many diseases such as avian cholera are spread through bird-to-bird contact or ingestion of contaminated food and water, so waterfowl are at greater risk of infection when concentrated in close quarters. Habitat loss on key staging and wintering areas can exacerbate the occurrence of disease outbreaks by concentrating greater numbers of birds in fewer remaining habitats. Waterfowl are also more vulnerable to catastrophic losses caused by other calamities, such as oil spills and hurricanes, when concentrated in limited habitat.
Fortunately for us, the rewards of flocking outweigh the risks. As long as we provide waterfowl with the habitats they need to thrive, we will always be able to enjoy the annual spectacle of large flocks of ducks and geese on staging and wintering areas across this continent. Whether it's a seemingly endless skein of snow geese winging south over the prairies or a raft of canvasbacks on a large lake or estuary, flocks of waterfowl are a wonderful sight to behold for all who appreciate these magnificent birds.
Dr. John Coluccy is director of conservation planning and Kassondra Hendricks is a conservation specialist in DU's Great Lakes/Atlantic Region.
BROOD AMALGAMATIONS For the most part, waterfowl seek isolation during the breeding period. However, after the hatch it is not uncommon to see large numbers of ducklings or goslings accompanied by one or more adults in brood-rearing areas. Post-hatch brood amalgamations (also known as creches or gang broods) occur when adult birds abandon or lose their young, which are then cared for by other adult birds, or when several waterfowl families intermingle and adults cooperatively care for the young. Brood amalgamations have been documented in at least 41 waterfowl species. This behavior presumably enhances survival of both young and adults via predator detection and access to food resources.