by Dale Humburg
You'd think a marsh would be a serene and peaceful place after dark. But having spent a few nights trying to sleep in a duck boat, I can assure you this isn't the case. With the frequent whistling wings of ducks overhead, the incessant clucking of coots, and the din of migrating snow geese—not to mention the muskrat on the bow of the boat eating a cattail tuber—I found the marsh to be quite an active place at night.
And on many occasions, I've left a marsh late in the day with hardly a duck in sight only to return to the same spot early the next morning and flush what seemed like half the flyway's birds ahead of me. These mysteries raise questions like "where were the birds yesterday, when did they arrive, and what were they doing all night?"
Migrations and Local Movements
Most waterfowl migrations occur at night. Studies indicate that migratory movements intensify shortly after sunset, peak in the middle of the night, and decline thereafter. The result can be an impressive increase in local waterfowl numbers overnight. These nocturnal migrations often explain why a marsh can be void of waterfowl one afternoon and then teeming with birds the next morning.
But waterfowl also make shorter, local movements at night. On migration and wintering areas, the daily activity of ducks and geese is influenced by the birds' energy demands, weather and habitat conditions, and disturbance from hunters or natural predators. Being highly mobile, waterfowl respond quickly to changes in their environment by moving from one habitat to the next. And ducks and geese often make these local shifts in distribution at night.
Loafing and Roosting
Waterfowl spend many hours a day loafing, sleeping, and performing basic maintenance and comfort movements like preening and stretching. The birds select loafing and roosting sites based on the temperature, humidity, wind speed, and sky conditions. On warm, sunny days, for example, ducks and geese will loaf in open areas where they can warm themselves in the sun. At night, waterfowl often roost in more sheltered habitats where the birds can conserve body heat and save energy. By moving among a variety of different loafing and roosting sites, the birds can maximize their energy savings under different weather conditions and at different times of day.
Habitat selection is especially important for waterfowl during extended periods of unfavorable weather when even big ducks like mallards and black ducks have to rely on stored fat reserves to survive. Energy costs to waterfowl are highest on clear nights when heat loss is greatest in open habitats. On cloudy nights, differences in heat loss are less pronounced among different habitat types.
Studies conducted in the Mississippi River floodplain showed that, at the same temperature, flooded willow wetlands with dense woody cover provided a more favorable microclimate for roosting ducks than flooded corn or deep-water habitats. At night, the closed canopy of flooded willows shielded the birds from heat loss as well as avian predators like great horned owls.