by Tina Yerkes, Ph.D.
Mallard hens nesting in the Great Lakes states find themselves in a very different world than their relatives in the Prairie Pothole Region (PPR). The PPR will always be North America's primary duck factory, hosting not only the most breeding waterfowl on the continent, but also the greatest diversity of duck species, from pintails to gadwalls to canvasbacks. The prairie landscape was formerly the largest expanse of grassland in the world. More than 70 percent of the wetlands in the PPR have been drained or severely degraded, and the destruction continues at the rate of approximately 33,000 acres annually. In many places, are under greater siege than wetlands. For example, eastern North Dakota has lost nearly three-fourths of its original grassland, and Minnesota and Iowa have suffered even greater losses of native prairie. After years of research, Ducks Unlimited and the rest of the waterfowl-conservation community now recognize with certainty that inadequate grassland habitat is often responsible for low nest success in prairie-breeding ducks.
In the Great Lakes states, the mallard is king. Mallards are the most numerous breeding ducks in this region (encompassing Michigan, Wisconsin, Ohio, Illinois, and Indiana ), followed by wood ducks and blue-winged teal. The remarkable adaptability of mallards enables them to breed successfully in significant numbers on dramatically altered and fragmented landscapes throughout this heavily developed region.
Historically, the Great Lakes states consisted largely of mixed forest interspersed with large wetland complexes and tall-grass prairie in Wisconsin and Illinois.Unfortunately, these states have suffered extensive habitat losses to industrial, urban, and agricultural development, as well as water pollution and invasive plants like purple loosestrife and phragmites (or common reed grass). The Great Lakes states have lost 50 percent or more of their original wetlands, and many local areas have suffered wetland losses as high as 90 percent.
Despite these impacts, mallards enjoyed steady population growth in the Great Lakes states for several decades. In recent years, however, this trend appears to have reversed. Unsure of the cause of this population decline, Ducks Unlimited and the conservation community set out to clear up this uncertainty. Working in cooperation with numerous partners, DU recently completed an extensive three-year study to discover the root causes of declining mallard numbers in this region. Findings from this landmark study will help guide DU's conservation programs in the Great Lakes states for years to come.
The Great Lakes mallard study was borne out of necessity to eliminate uncertainty in habitat conservation programs for breeding mallards in these states. Modeled after extensive research in prairie Canada, this study was designed to determine what factors were limiting mallard population growth in this region. DU researchers trapped numerous hen mallards, marked the birds with radio transmitters, and followed them every day throughout the breeding season to record their movements, nest success, clutch size, brood survival, and habitat use—all things DU and its partners needed to know to determine what was limiting this population of mallards.
Beginning in early spring of 2000, field crews battled late-season snowstorms to trap female mallards as they returned and established breeding territories. Hens were collected by placing decoy traps containing tame hen mallards in the territories of wild birds. These traps work by exploiting the territorial instincts of breeding mallards. Hens attempt to drive away the intruder (in the trap) from their territory, and thus become trapped themselves. DU research scientists anesthetized the wild hens and surgically implanted a radio transmitter—roughly the size of a 12-gauge shotgun shell—in each bird's abdominal cavity, then released the ducks unharmed. This technology allowed the researchers to follow the daily activities of these otherwise elusive birds without disturbing them.
During each field season, research technicians used telemetry equipment to follow radio-marked hens from late March through the end of August, recording information such as habitat use and duckling survival. Over a three-year period, DU tracked a total of 560 mallard hens. Research sites were chosen to represent a variety of Great Lakes landscapes, from agriculturally dominated to predominately forested. Nine sites were selected throughout northwest and southeast Wisconsin; central and southern Michigan; the tri-state area of Michigan, Indiana, and Ohio; and in the Lake Erie marshes and northwestern corner of Ohio.
After extensive data analyses, DU scientists determined that brood survival, nest success, and over-winter survival, respectively, were the most important factors limiting mallard breeding populations in the Great Lakes states. Brood survival was much more of a limiting factor for Great Lakes mallards than for prairie mallards (see pie chart comparison). During the Great Lakes study, brood survival varied significantly—between 24 percent and 55 percent—but averaged only 39 percent. This means that for every clutch of 10 eggs that hatched, slightly fewer than four ducklings survived on average.
Brood survival was most closely related to wetland vegetation and forest cover in the surrounding area. Ducklings survived at much higher rates in vegetated wetlands than in those that had been plowed and farmed to the water's edge. Especially valuable wetland vegetation included emergent plants such as cattail or bulrush, and wet meadow grasses and sedges. Other important brood habitats included wetlands rimmed by scrub-shrub vegetation, typical of the Midwest. Proximity of forest cover appeared to negatively affect brood survival, likely because of a greater abundance of raptors such as great horned owls and red-tailed hawks, which can be significant predators of hens and ducklings. Overall, wetland conditions appear to have the greatest influence on brood survival in the region, but the quantity of brood habitat may also be important as ducklings may have higher survival on landscapes with more high-quality wetlands.
The study found that nest success had less of an impact on mallard populations in the Great Lakes states than on the prairies (see pie chart comparison). Radio-marked hens in the Great Lakes study had nest success of about 16 percent overall, slightly above the 15 percent level required to sustain a population. The observed variation in nest success—between 10 and 24 percent—appears to have been closely related to the intensity of agriculture within a three-square-mile area of nest sites. The more acres of row crops within this area, the lower the nest success. Nest success fell below 15 percent on landscapes where approximately 46 percent or more of the area was planted in row crops, such as corn, wheat, or soybeans. Pastures and hayfields, however, did not appear to have an adverse effect on nest success. Interestingly, older hens had higher nest success on average than those attempting to nest in their first year.
A third limiting factor for Great Lakes mallards was over-winter survival of hens. This had a much greater effect on mallard populations in the Great Lakes states than on the prairies (see pie chart comparison). Although this was not directly measured in this study, DU researchers had to determine annual and seasonal survival rates to conduct complete life-cycle analyses of the birds. Band-return data were used as well as survival estimates from radio-marked hens monitored in the study. Overall, the researchers found that annual survival among female mallards from the Great Lakes states was approximately 51 percent, slightly lower than the 58 percent annual survival estimated among prairie-nesting mallards. Breeding survival among female mallards during the summer also was not really that different—75 percent in the Great Lakes states compared to 72 percent on the prairies. However, the difference in over-winter survival between the two groups was dramatic. Only 69 percent of hen mallards from the Great Lakes states survived the winter, compared to 80 percent of prairie-nesting mallards. Generally, two factors have the greatest influence on over-winter survival: wintering habitat conditions and harvest.
What does all this mean for waterfowl habitat programs in the Great Lakes states? In the Prairie Pothole Region, inadequate grassland habitat is a major factor limiting duck production. In the Great Lakes, the foundation for breeding habitat delivery has always been based on research from the prairies, which led to a heavy emphasis on grassland restoration accompanied by wetland restoration. Now, findings from the Great Lakes study have prompted DU to change its habitat-delivery philosophy in the region to focus more on wetland restoration and less on grassland conservation. This does not imply, however, that grasslands are unimportant, because on some landscapes where nest success has fallen below 15 percent, the restoration of grasslands will be needed to improve mallard production.
To help field biologists deliver the best habitat conservation programs in specific areas, DU has used data from the study to develop the Habitat Evaluation Network, affectionately known as HEN. This is a complex modeling program using geographic-information-systems technology that recommends the best habitat-management scenario, based on current landscape conditions. For example, when a landowner asks DU what he or she should do for the ducks, HEN will provide DU biologists with several key pieces of information, including whether or not there are former wetlands on the landowner's property that can be restored, the composition of the surrounding landscape, and what is likely the best strategy to improve mallard production on the property. In a nutshell, HEN will provide DU with the information required to customize habitat restoration and protection plans for individual landowners throughout the region.
Although Great Lakes mallards face the same limiting factors and threats as their prairie brethren, the relative importance of these factors is not the same. We now know how to address these challenges through habitat conservation programs improved by science. Instead of Grasslands for Tomorrow, a crucial DU conservation program on the prairies, the Great Lakes region needs Wetlands Forever!