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.