Posted on 6/3/2009 by Heath M. Hagy
Each year, millions of waterfowl (i.e., ducks, geese, and swans), travel thousands of miles and undergo critical physiological events which help them fulfill their annual life cycle. This cycle can be divided into four phases which are all critical for survival: (1) Breeding / Reproductive Season–late Spring and Summer, (2) Fall Migration–Fall and early Winter, (3) Pair Formation / Wintering–Winter, and (4) Spring Migration–late Winter and Spring. Each species of waterfowl differs slightly in the timing and the specific activities during these 4 major stages, but generally waterfowl in North America follow temperate weather south in the fall and back north in the spring, spending the time between these migrations preparing to breed, breeding, and finding enough food resources to survive. We will follow waterfowl through this magnificent journey describing some of the critical events, processes, and influences which shape the behavior and adaptations within the annual cycle of waterfowl in North America.
The air is cool and crisp as she descends upon the remnant prairies of the Dakotas. She is a female mallard who, just last year, hatched north of Ipswich, South Dakota. As temperatures have warmed and food resources have been depleted on the wintering grounds of the north Mississippi Delta, where she spent the last three months, she began leap-frogging back north using the forested wetlands and rice fields in eastern Arkansas. After resting and feeding on moist-soil and a few remaining agricultural seeds, she headed north, stopping briefly in the boot hill of Missouri where millions of other mallards and species of waterfowl rest and feed during migration. From there, it was only a few hundred miles north to the southern extent of the Prairie Pothole Region (PPR) in South Dakota and northern Iowa.
She flew over the landscape dotted with grasslands and wetlands, but dominated by agriculture, driven by some unclear honing ability to the place where she successfully hatched almost one year ago. Then, small, temporary wetlands were abundant in the grasslands and crop fields of northeastern South Dakota. Winter snowfall had been good that year, filling many small basins with water and flooding agricultural fields. She had been raised on a tract of land preserved by the Nature Conservancy called Ordway Prairie. Several hundred bison and thousands of cattle roam 135,000 acres of a perpetual grassland easement keeping this section of the prairie coteau in mixed-grass prairie and perfect for upland nesting ducks.
Surrounding Ordway Prairie, conservation groups like Ducks Unlimited work diligently with the U.S. Fish and Wildlife Service, South Dakota Department of Game, Fish, and Parks, and the Prairie Pothole Joint Venture to enhance, restore, and create habitat for waterfowl and many other species of wildlife. This mallard hen was brooded on lands secured by funding provided by the North American Wetland Conservation Act and directed by the North American Waterfowl Management Plan to protect, restore, and enhance wetlands and waterfowl habitats in North America. Along with provisions in the Farm Bill discouraging producers from draining wetlands and cultivating native grasslands as well as paying incentives for restoring waterfowl habitats, these groups and policies help save the PPR from destruction and provide waterfowl and many other species with critical habitat.
She continues to fly north over South Dakota looking for an abundance of small wetlands, but finds none. She stops briefly on a Waterfowl Production Area wetland, which was purchased and restored using federal duck stamp dollars provided by hunters and avid outdoorsmen, but only a few inches of water persist and invertebrate foods have not yet begun to emerge due to the warm, dry winter. She continues north, looking for an interspersion of grasslands and wetlands on which to begin her first clutch. Soon after crossing I-94 into south-central North Dakota, the landscape begins to change. Early spring snowfall and rains have filled many permanent basins to the point of overflow and hundreds of seasonal and temporary wetlands dot each square mile. She and her mate, a mature drake mallard, descend on a very promising landscape near Woodworth, North Dakota.
Here she will rest for a period of just a few days to a few weeks before selecting a nest site, creating a nesting bowl, and laying eggs. Her mate may remain nearby for a short time, ready to defend her from other males attempting to breed when she leaves the nest. During the 26-30 days of egg-laying and incubation, she will have to be fortunate enough to evade predation from fox, raccoon, skunk, and small mammals and be even luckier to have her eggs evade depredation by a slough of similar predators.
This particular hen is fortunate to hatch a successful clutch. She has timed her egg laying synchronously with the first flush of invertebrates in nearby wetlands. Her brood will feed on freshwater shrimp, midge larvae, water beetles, and any other protein-rich forage that they can find for the next 45-60 days. By that time, each duckling will have developed flight and body feathers utilizing the abundant invertebrate foods rich in protein and will be able to move to more distant wetlands to find food. Meanwhile, the hen mallard is beginning to go through physiological changes of her own. She has been feeding on protein-rich foods to help improve body condition, as more than 30% of her weight was depleted by spring migration, egg laying, and incubation. Soon, she will molt or lose her worn feathers, replacing them with new ones before fall migration. Though some waterfowl species share parental responsibilities, her mate has likely moved elsewhere, perhaps traveling farther north into Canada to molt with other males and build up his own reserves for fall migration. Analogs to the Conservation Reserve Program in Canada such as Greencover Canada and Alternate Land Use Services are helping to protect habitat throughout Canada for environmental quality and wildlife benefits. Even within the breeding season, waterfowl depend on a variety of habitats, including different wetland types for food, brood rearing, and protection; thus programs and policies which create and protect habitat in the United States, Canada, and Central America are all extremely important for migratory birds.
Just before fall migration, the mallard hen feeds heavily to build up reserves needed for her journey. She will likely wait until cold weather and strong north winds give her an incentive to fly south. Along the way, she will stop at temporary wetlands, permanent water, and man-made or maintained wetlands such as reservoirs, sewer lagoons, industrial filtration and cooling ponds, hunting clubs, river systems, aquaculture ponds, and other waters to rest and forage. These wetlands are called stopover sites and may be used for a short duration or for several weeks. Some stopover sites are actively managed by private individuals for hunting and by state and federal agencies for provision of waterfowl habitat (i.e., National Wildlife Refuges) and commonly include moist-soil, flooded agricultural, and deep water units. River systems and areas protected by the Clean Water Act, legislation assigning the Army Corps of Engineers to protect navigable waters and those connected by a significant nexus from dredging, filling, and diversion, are also important stopover areas for migratory waterfowl.
During this season, hunting replaces small mammalian and avian predators as one of the greatest threats to survival for the mallard hen and other ducks like her. From late September until she arrives on wintering grounds, she will have to evade hunters and other predators, find forage, and avoid disease in order to survive. Adaptive Harvest Management (AHM) and legislation such as the Migratory Bird Treaty Act ensure that waterfowl populations are managed correctly and are protected during most of the year. In particular, AHM allows federal agencies to set harvest restrictions based on the number of breeding waterfowl in a given year. This helps ensure that over harvest does not negatively affect population levels and under harvest does not leave populations susceptible to disease and other density-dependent factors.
Nonbreeding / Wintering Season
After a long journey from North Dakota down through the Mississippi Flyway, the mallard hen has arrived in southern Missouri. Here, private, federal, and state lands provide important habitat for wintering waterfowl, the latter purchased with conservation sales tax dollars. Throughout the Mississippi Alluvial Valley, state and federal agencies and private organizations pay incentives for landowner provision of waterfowl habitat through the Conservation Reserve Program (i.e., WRP, CP 23, CP 9, swampbuster, etc.), easements, and cost shares. The variety of conservation programs helps to maintain a diverse suite of habitats important to waterfowl and other wildlife throughout winter. As the season progresses, she will initially begin to replace the fat reserves lost during fall migration with high energy foods such as agricultural waste grains and seeds, then slowly reduce her intake, in a "coasting" and survival mode until spring migration. She must continue to forage throughout winter, but her primary focus transitions from migration and feeding to finding a mate and pair bonding. She will move between the available habitat complexes, though like most other regions, many natural wetlands have been lost to agriculture or development (>50% loss in the U.S.), searching for a suitable mate and simply surviving.
She will likely be attracted to managed wetlands that have an equal interspersion of emergent vegetation (i.e., moist-soil plants, bulrush, and cattail) and open water nearby flooded agricultural fields and large reservoirs (forming a habitat complex). This hemi-marsh is important to provide landing and feeding areas which are somewhat isolated from other ducks. Further south, forested wetlands including greentree reservoirs, coastal marshes, and other types of emergent habitats provide this type of cover. During mid-winter, waterfowl hunting seasons close, easing the pressure on all wetland wildlife. As temperatures have remained warm and mid-latitude states have remained ice-free, the hen has been able to remain in the northern tier of the Mississippi Alluvial Valley, not risking the flight further south to Mississippi or Louisianna. She will select a mate and spend the remainder of winter pair bonding and simply surviving until spring migration. As the day length increases and temperatures rise, she and her mate will increase their food intake to increasing body mass before spring migration.
Changing seasons will queue many waterfowl, including the hen mallard, to begin their long migration back north to breeding grounds. With her mate in tow, she will navigate between stopover areas, slowed only by the availability of open water and her breeding strategy. National wildlife refuges and temporary wetlands (i.e., sheet water left in shallow depressions from snow melt) are important stopover and feeding sites during the spring. Other than snow geese, hunting is closed during the spring which eliminates a major mortality threat compared to fall migration. Some waterfowl species migrate more directly in the spring than the fall and many mix between the flyways as the lack of available food and the urgency to breed necessitate a fast migration. Successful migration is partially dependent on body condition achieved on wintering areas and may be influential on breeding success, illustrating the influence of cross-seasonal effects within the entire annual cycle. Migrants who wintered in areas where food was abundantly available and were able to build reserves without major harassment or predation may arrive on breeding areas more ready to begin the process of recruitment.
As long as she survives, the hen mallard will continue to progress through each of these life-stages or physiological events, not as four independent parts, but as a continual cycle where body condition and factors affecting one stage have consequences in subsequent stages. Waterfowl that are best adapted to a situation or utilize the resources given during the entirety of the annual cycle will gain fitness (i.e., successfully recruit the next generation of waterfowl). Waterfowl life-cycles are complex, yet driven by a simple principle—survival and fitness.
Though I provided only the example of a hen mallard, the processes, factors, and external influences which drive waterfowl populations and life-history strategies are all related. Instead of taking short flights, letting ice cover, temperature, day length, and food availability push them south, snow geese ascend as high as 20,000 feet, and travel as much as 17,000 miles (70 hours) at a time. Most ducks, including the mallard hen, conversely, travel in short bouts, stopping to feed and rest along their migration routes. These adaptations or strategies have evolved over hundreds of thousands of years to allow many different species of waterfowl to inhabit a finite number of habitats and utilize all of the available resources. Each species fulfills a particular niche whether it is a mallard stopping regularly during migration to dabble on natural and agricultural seeds, snow geese traveling in one long flight to arrive and grub on rice in Louisiana, canvasbacks and other diving ducks using deep pools along the Mississippi River to forage on aquatic plants and invertebrates, or Canada geese traveling in medium-length migration flights and stopping to forage on a wide variety of agricultural seeds and winter wheat. Waterfowl, as a whole, have partitioned out these niches and strategies to maximize habitat use and biodiversity during each part of the annual cycle.
Continual habitat conservation, enhancement, restoration, creation, and research will allow future generations to experience, first hand, the story of the hen mallard and other waterfowl. Of particular concern, most wetlands throughout North America remain unprotected. Recent court decisions restricting the extent of the Clean Water Act and uncertainties surrounding the Farm Bill continue to threaten waterfowl habitats. Furthermore, high commodity prices and the cellulosic ethanol industry threaten the integrity of grasslands in the Northern Great Plains and Canada. It is imperative that these habitats and others associated with each stage of the annual cycle be conserved and that conservation efforts recognize local wildlife management and habitat provision within the realm of landscape scale conservation planning.