Seasonal Wetlands Water Indicates Brood Survival

Center: Over 90 percent of broods survive when more than 50 percent of seasonal wetlands filled

Article taken from The Jamestown Sun, September 20, 1997
by
Scott Kraus, Sun Staff Writer

Gadwall ducks like these were a part of a study by the Northern Prairie Science Center near Jamestown in the late 1980's. Researchers found they could estimate duckling survival mainly by counting the number of seasonal wetlands with water, and also by looking at rainfall.

The Northern Prairie Wildlife Research Center near Jamestown has found a new way to estimate brood and duckling survival that is faster and more accurate than the existing aerial and ground counts, a researcher said.

Wildlife biologist Gary Krapu said the system — which is based on research done from 1988 to 1994 — uses the percentage of filled seasonal wetlands and weather information to predict duckling survival. Krapu, who completed the study with co-researcher Pam Pietz, said these predictions are important for setting hunting bag limits and managing wildlife areas. "We've been looking to find a way to estimate duckling survival over a large area and accurately, he said. "The findings in this study make major progress in that direction."

Traditional aerial and ground surveys that try to count the number of broods can't estimate how many died before the survey and can miss some ducklings that are under cover. The new system, however, relies heavily on how many seasonal wetlands have water. They are easier to count. Personnel can just run a transect line across the countryside and count them along the line, he said. Other possibilities include counting filled seasonal wetlands from aerial video or even satellite pictures.

Seasonal wetlands are important because they have a lot of tiny swimming invertebrates that hens need for protein to lay eggs and that ducklings need to grow. Research on a total of 400 radio-tagged mallard and gadwall ducklings found that over 90 percent of broods survive when more than 50 percent of seasonal wetlands have water in them. But less than 40 percent survive when under 25 percent of seasonal wetlands have water in them. More wetlands means predators can't focus on the ducks in just a few spots, like in dry years. So more ducklings survive. Ducklings also don't have to travel as far to wetlands. So they spend less time on land, where they're more vulnerable to predators.

That's one of the main reasons why when we get wet cycles like this on the prairie, duck production really rebounds," Krapu said. There was a lot of water in 1993 and 1994, so brood survival was extremely high.

The study found that mink killed the most ducklings. But losses to mink dropped markedly when the drought ended in 1993, and more wetlands filled, according to Krapu. Another factor in duckling survive is cold rain right after they hatch. The rain can kill them because they have poor body temperature control during their first week out of the egg, he said. Rain, however, has little impact when fewer than half of seasonal wetlands are filled because predators kill so many ducklings anyway. Ducklings were also three times more likely to die during the first week after they hatch.

"If they can survive the first couple weeks of life it's very likely they'll survive to flight stage," he said. Another interesting finding of the research was that even when a hen is killed, ducklings can travel up to a mile to find water on their own. It's uncertain how they locate the water that far away, he laid.

The research was done on sites near Jamestown and Kulm, in McLean County, and near Hitterdal, Minn. "It's a step toward coming up with a rapid means of estimating survival...so that a better prediction can be produced," he said.