Precipitation, whether in liquid or solid form, has an especially big influence on waterfowl food availability and, consequently, habitat use. Ducks respond to changing water levels. Generally speaking, declining water levels are not as favorable for waterfowl as rising water, which creates gradually expanding wetland margins and newly inundated food resources. Constant attention to approaching weather systems can help hunters anticipate possible changes in precipitation as well as water levels and waterfowl use. After a major front accompanied by significant precipitation passes, ducks will be much less likely to feed in the same places they fed the day before.
Of course, water in solid form as snow or ice also triggers waterfowl migration, bringing the practical end to the hunting season in many areas and considerable changes in hunting conditions in others. Ducks either migrate south or change local movement patterns in response to declining open water and food availability. Snow buries food or at least makes it less available. And ice cover significantly reduces habitat available to ducks and often to duck hunters as well. Ultimately, these wintry conditions force the departure of all but the hardiest waterfowl, bringing flights of new birds to waterfowlers down the flyways. No wonder duck hunters pay particularly close attention when snow and cold temperatures occur to their north—new birds are coming!
When waterfowl migrate also often depends on wind direction and velocity. Flight is energetically expensive, and birds can migrate more efficiently when they have a tailwind. In spring, southerly winds, falling barometric pressure, and increasing temperatures are optimal conditions for waterfowl migrating northward. In fall, waterfowl migrations, which often occur at night, are usually associated with north to northwest winds, falling temperatures, and high-pressure systems. As a result, predictions of wind direction and velocity can be used by waterfowl hunters to gauge the likelihood of new ducks arriving in their area as well as where to hunt relative to the prevailing wind direction.
As waterfowl hunters, we know, depending on where we hunt, approximately when we can expect birds to begin showing up, when their numbers will likely peak, and in many respects when they will depart. What we can't predict is exactly when they will move or how short-term changes in weather and habitat conditions will affect duck distribution. This can make it difficult to plan vacation time in advance of hunting season. If possible, waterfowlers should focus their hunting efforts when migrations typically peak in their area, leaving some flexibility to hunt on specific days when weather conditions—particularly wind, temperature, and precipitation—are most favorable.
Have you ever wondered why you never see flocks of migrating coots? While it may seem like these waterbirds mysteriously appear out of thin air, coots migrate primarily at night and rest and feed in marshes during the day.
Why Some Waterfowl Don't Migrate
While most of this continent's waterfowl migrate, there are a few notable exceptions, such as the mottled duck (below), which lives year-round on wetlands along the Gulf Coast and in parts of the south Atlantic region. Migrant waterfowl expend considerable energy flying long distances to and from their breeding and wintering areas. Resident waterfowl eliminate these energy demands by staying put year-round. The trade-off is that these sedentary birds have less flexibility in finding breeding sites, avoiding predators, and responding to changing food availability and habitat conditions. When the going gets tough, migratory waterfowl simply fly to areas where habitat is more favorable, while resident species must make do with what they have.
Photo: Ron Bielefeld
In a number of waterfowl species, post-breeding males, nonproductive birds, and even females whose broods have already fledged make what is known as a "molt migration." These birds depart brood-rearing habitats in late spring and early summer, freeing up food resources for remaining females and their young. These ducks and geese usually fly hundreds of miles to more northerly molting areas. Molt migrants usually seek refuge on large, relatively undisturbed wetlands with adequate cover and food to sustain them when they are flightless during their summer molt and to prepare them to continue their annual cycle of migration.
Dale D. Humburg is chief biologist at DU national headquarters in Memphis.