By J. Brian Davis, Ph.D.
The survival and recovery of wood duck populations in North America are renowned wildlife success stories. By the late 1800s to early 1900s, extinction of the wood duck seemed imminent. Sentiments like, "Becoming scarce, likely to be exterminated," and "Almost exterminated, only few breed" reverberated among ornithologists in eastern portions of the continent.
Destruction of bottomland hardwood forests—the bird's primary breeding and wintering habitat—and market hunting were the two major factors that contributed to the species' decline. Because of their extensive breeding ranges in eastern North America, where most people lived in the early 20th century, wood ducks were probably the most hunted waterfowl species prior to 1918. Wood ducks were hunted from September to April. In 1918, however, the Migratory Bird Treaty Act prohibited the hunting of wood ducks nationwide. Wood duck populations recovered, and seasons were reopened in 1941. Fortunately, some lowland habitats composed of impenetrable vegetation and subject to prolonged flooding withstood man's environmental barrage, providing safe havens for wood ducks in parts of their range. These areas provided the remaining wood duck population that eventually expanded into its current range in North America.
Population Surveys and Management
Each spring and summer in selected parts of Canada and the United States, biologists fly airplanes along transects encompassing millions of acres, enumerating the numbers of May ponds, breeding pairs of ducks, and numbers of duck broods. These counts help estimate the population sizes of nesting ducks and are used to formulate waterfowl hunting season frameworks and daily bag limits.
Estimating the population size of wood ducks from aircraft, however, is basically impossible because the birds prefer to nest in forested wetlands and other densely vegetated habitats. Counts have even been attempted from helicopters, but to little avail. Thus, biologists primarily rely on four techniques to monitor wood duck populations and other biological trends of the species: 1) the annual Breeding Bird Survey (BBS), 2) long-term monitoring of wood duck nest boxes, 3) examining wings of harvested birds at annual Wing Bees, and 4) hunter harvest surveys and recoveries of wood duck bands.
The BBS is a roadside survey conducted annually in June to estimate the relative abundance of breeding birds across Canada and the United States. Monitoring nest boxes across large geographic areas, such as statewide or by flyways, allows biologists to monitor nesting attempts and the nesting success of breeding wood ducks. This information is useful for biologists to assess long-term trends in nesting success and the general health of local wood duck populations.
The annual Wing Bee is a meeting where biologists examine the wings of ducks harvested by hunters to determine the species, sex, and age of the birds. This technique allows biologists to estimate the ratio of the numbers of young to adults in the population as an index to productivity. Lastly, bands from harvested wood ducks reported by hunters to the U.S. Fish and Wildlife Service provide critical information, such as the age, sex, and location of a bird when it was harvested. These data are vital to estimating waterfowl species' annual survival rates and other important population trends.
Wood duck populations are currently considered to be increasing or remaining stable throughout much of their range. From 1959 to 1986, wood duck populations seemed to grow in the Atlantic and Mississippi flyways and elsewhere. Dr. Frank C. Bellrose, considered the world's authority on wood duck ecology and management, estimated that from 1959 to 1986 populations grew by 9 to 16 percent per year in the Mississippi Flyway and from 7 to 9 percent in the Atlantic Flyway. Respectively, approximately 1.07 million and 1.65 million pairs of breeding wood ducks were estimated in the Atlantic and Mississippi flyways during the 1980s. Band recovery data from the mid-1980s to 1999 indicate that wood duck populations may have increased in the Mississippi Flyway in recent years. However, much of this growth may have been in peripheral areas recently exploited by breeding wood ducks. Estimated numbers of breeding wood ducks in the Pacific (60,000) and Central flyways (76,000) are much less, but populations appear to be increasing there, primarily because of nest box programs.
Several factors may have contributed to the overall population growth of wood ducks during the past few decades. Although drainage and clearing of bottomland hardwood forests continued, there was an increase in abundance and size of trees suitable for nesting by wood ducks, and a great increase in the numbers of beaver ponds and farm ponds. The wood duck's favorite habitats are bottomland hardwood forest and other densely vegetated areas, but the bird has also exploited habitats of "secondary quality." For example, wood ducks expanded their range into some habitats lacking ample bottomland hardwood forest, such as the northern parts of the Great Lakes states, areas of the Great Plains, and higher elevations of the Appalachian Mountains. Habitats such as sluggish prairie streams, marshes, and other locales that provide aquatic insects and some cavity trees or nest boxes can be beneficial to wood ducks. Because of severe population declines once incurred by wood ducks, it simply has taken time for the bird to reoccupy some habitats where it was formerly abundant, and to expand gradually into secondary habitats.
Despite existing techniques used to monitor wood duck populations, it remains difficult to know with a degree of precision how many wood ducks exist. The wood duck has a highly fragmented, forested breeding habitat across eastern North America. Also, wood ducks are extremely philopatric, meaning they tend to return to the same breeding location each year. Thus, these factors contribute to the difficulty of monitoring some subpopulations of wood ducks.
In 1997, the Wood Duck Population Monitoring Initiative was conceived. The objective of the initiative was to develop methods to 1) identify and estimate the size of subpopulations of wood ducks, 2) measure productivity of the birds, and 3) improve banding programs for wood ducks. Identification of subpopulations of wood ducks in North America is important so that region-specific management plans can be developed. One goal of the initiative is for biologists in eastern North America to leg-band more than 10,000 wood ducks annually, to derive meaningful survival estimates of the species. The objective of this goal for the eastern two flyways is apparently currently being satisfied. However, significant expansion of banding efforts would have to occur to provide reliable estimates for subpopulations of wood ducks within all of the flyways. Other techniques, such as monitoring of nest boxes and expansion of BBS routes, may be too logistically and financially prohibitive, at least at this time. Thus, there is current discussion over how wood duck population monitoring efforts will be enhanced in the future.
Wood ducks comprise approximately 10 percent or more of the annual duck harvest in the United States. Wood ducks were second, only to mallards, in total harvest in the Atlantic and Mississippi flyways in 25 of the past 30 years. From approximately 1959 to the late 1980s, total wood duck harvest increased in the United States, and was believed to be indicative of a general population increase. Approximately 560,000 and nearly 260,000 wood ducks were harvested in the Mississippi and Atlantic flyways, respectively, during that time; as few as 32,000 were harvested in the Pacific Flyway. In the 2000-2001 hunting season, between 1 and 1.3 million wood ducks were harvested in the United States. Thus, wood ducks continue to be especially popular among waterfowl hunters.
Establishing effective harvest strategies for wood ducks is challenging because the species' population size is difficult to estimate. Because of the great decline in numbers of wood ducks in the early 20th century, and ongoing difficulty with estimating their population size, daily bag limits have been conservative. In 1941, the first year that wood ducks could be hunted legally in the United States since 1918, one or two wood ducks per hunter per day were permitted in parts of the Atlantic and Mississippi flyways. Most states, though, did not allow legal hunting of wood ducks until 1959 because of possible population declines. For the next several decades and through the present, conservative one- to two-bird bag limits have prevailed. However, in some years and states, bag limits have been more liberal, such as a legal daily limit of seven wood ducks in the Pacific Flyway during the 1975-76 hunting season.
Harvest management of wood ducks remains relatively conservative in the United States. In 1995, the U.S. Fish and Wildlife Service (USFWS) developed a waterfowl harvest management system termed Adaptive Harvest Management (AHM). AHM has several goals, but basically it seeks to provide a more systematic approach to establishing waterfowl hunting regulations. Hunting seasons of 20, 30, 45, or 60 days are established based on numbers of breeding pairs of mallards and prairie wetland ponds counted each year in May. During much of the 1990s, when breeding pairs of ducks and pothole wetlands were high, 60-day duck seasons existed from 1997-1998 to 2002-2003. That many consecutive lengthy seasons had not occurred in the two eastern flyways since the 1950s.
Biologists have been curious about how the liberal duck seasons might be affecting wood duck populations. In 2002, waterfowl researchers H.W. Heusmann and John McDonald analyzed wood duck population and harvest data from the Atlantic and Mississippi flyways. They subdivided the two flyways into six regions, based on the latitude of an individual state (i.e., Minnesota, New York, and others considered "northern," Pennsylvania, Iowa, and others were "middle," and North Carolina, Tennessee, Arkansas, and those farther south were "southern"). The researchers also examined numbers of hunters, wood duck harvests, and banding data. They determined that total wood duck harvests increased in both flyways during the liberal 60-day hunting seasons. Although this might have been expected, it was interesting because the increased harvest of wood ducks probably benefited southern hunters more than northern hunters. This is because northern wood ducks migrate earlier in fall than some other waterfowl species, and many wood ducks were probably present in southern states for the duration of the 60-day seasons.
The next logical question asked by Heusmann and McDonald was whether the increased harvest harmed wood duck populations. They evaluated the BBS and other breeding surveys and concluded that the long-term (1966-2000) and more short-term (1980-2000) data indicated maintenance or growth of wood duck populations in the Atlantic and Pacific flyways. Information available from the 1997-2002 liberal seasons suggests "population leveling" or even a reversal of the population trend. However, despite an apparent changing trend, there does not appear to be any indication that liberal hunting seasons have harmed wood duck populations.
Regardless of future harvest strategies, regulations will remain rather conservative because of the lack of efficient large-scale methods to assess the population status of wood ducks.
Nest Box Programs
The 1930s marked the budding of the wildlife management profession. It also was the time when Dr. Bellrose and Arthur Hawkins initiated studies of wood ducks nesting in man-made nest structures (nest boxes) at Chautauqua National Wildlife Refuge in Illinois. Since then, nest box programs have been popular among biologists and conservation-minded private citizens for studying and propagating wood ducks.
Today, it is estimated that 300,000 wood duck nest boxes produce 100,000 ducklings annually in North America. The use of nest boxes varies among regions, and basically is influenced by the availability of suitable natural tree cavities, the proximity of wetlands valuable to wood ducks, and other factors. In 1994, Dr. Bellrose estimated that states such as Iowa, Missouri, Illinois, and Ohio contain 7 percent of the wetlands important to waterfowl in the Mississippi Flyway, but only 17 percent of that land comprised hardwood forest. Alternatively, northern states such as Michigan, Minnesota, and Wisconsin contain 41 percent of important wetlands in the Mississippi Flyway, but 31 percent of that landscape is covered by forest. Thus, the use of nest boxes generally is greater in mid-latitude or southern areas because of fewer natural tree cavities.
Regardless of geographic location, nest boxes that exist among quality brood habitats and that are properly maintained can increase local wood duck populations.
Basically, wood ducks need a cavity in which to nest, and an adjacent (one to three miles) wetland habitat that contains some trees, shrubs, or dense herbaceous vegetation that provides quantities of aquatic insects for ducklings. Generally, where such a substantial food base occurs and natural tree cavities are sparse, there may be significant use of nest boxes.
In studies partly supported by Ducks Unlimited that I conducted at Mississippi State University from 1994 to 2001, I determined that nest box maintenance during the breeding season significantly increased wood duck production. Oftentimes, managers of wood duck nest houses only visit and inspect structures in late summer after the birds have completed nesting. Wood ducks do not bring material (e.g., sticks) to a site to construct nests. In natural cavities, they deposit their clutch of eggs in their own down feathers, dead wood, and other material in the cavity. In nest boxes, we should provide three to four inches of material, such as wood shavings, to provide excellent nesting conditions. Early in the nesting season, wood ducks may frequently abandon nests for various reasons or lose their eggs to predators. Thus, the removal of dead eggs and refurbishing nest boxes with clean shavings can increase wood duck production significantly. For example, at our study sites in Mississippi and Alabama, we found that 38 to 65 percent of total wood duck ducklings produced occurred in second or third nesting attempts. It is likely that duckling production may have been greater had abandoned and destroyed eggs been removed from the boxes. Thus, our and other studies show that nest box maintenance during the breeding season (i.e., monthly inspections) could tremendously boost duckling production. However, remember that federal law prohibits removing eggs of migratory birds from nests. Thus, one must be certain that wood duck nests are no longer viable before initiating nest maintenance activities.
In the same study, we addressed survival of wood duck ducklings exiting man-made nest houses. To do so, we equipped 135 female nesting wood ducks and more than 400 ducklings with radio transmitters. We monitored survival and other aspects of duckling ecology from March through July at two study areas from 1996 to 1999. We determined that duckling survival was low overall at about 20 percent. However, we discovered that, when ducklings traveled more than a mile per day to densely vegetated shrub habitats that did not contain nest boxes, their survival was as great as 60 percent. Nest houses typically have been installed in high densities and in visible locations over water in North America. As attractive as this may be to nesting wood ducks, it also may be beneficial to predators of ducklings. Thus, we recommend that nest boxes be scattered amidst scrub-shrub habitat, and not scattered in open environments, which may attract predators.
DU Habitat Efforts
Habitat is one critical factor that influences waterfowl populations in North America. Ducks Unlimited prides itself on partnering with various governmental and other agencies to protect and restore habitats critical for waterfowl, wetlands, and other environments. Because of the extensive breeding and wintering range of the wood duck, DU's habitat projects in nearly every state could benefit wood ducks at some time during the birds' annual biological activities.
In the Mississippi Alluvial Valley (MAV), the region extending from southeast Missouri to southern Louisiana, more than 24 million acres of bottomland hardwood forest were once present; only 4.5 million acres remain today. Thus, DU continues to partner with several agencies to deliver habitat restoration projects. The Wetland Reserve Program, administered by the U.S. Department of Agriculture's Natural Resources Conservation Service, has been invaluable to wood duck and other waterfowl habitat restoration efforts. To date, DU has helped plant 33.9 million seedlings of bottomland hardwood forest tree species on 160,000 acres in the MAV.
In addition, 31,000 acres of wetland hydrology have also been restored there. In Mississippi, Arkansas, and Louisiana alone, 16 million tree seedlings have been planted since the 1990s. Restoration of bottomland hardwood forest and associated wetlands will provide abundant food, natural nesting cavities, space, and critical environmental benefits. Such habitat restoration efforts are measures to help ensure the welfare of wood ducks and myriad other species of North American wildlife.