Cold Weather and Snow Cover Drive Mallard Migrations
Dr. Michael Schummer is a scientist with Long Point Waterfowl in Port Rowan, Ontario. Before migrating north to work in Canada, he conducted post-doctoral research at Mississippi State University with Dr. Rick Kaminski. Specifically, in 2008 and 2009, Schummer studied how weather affects mallard
movements, and if the timing of mallard migrations had changed over the years. "Southern hunters were worried that mallards weren't showing up in the same numbers and as early as they once did. They also wondered if these changes were related to the weather, and if this trend was going to continue. We set out to answer those questions," Schummer says.
The first step was determining what causes mallards to migrate. "We developed a weather severity index for mallard migrations," he explains. "Using different combinations of temperature and snow depth data and waterfowl counts from Missouri conservation areas, we correlated changes in numbers of mallards with weather events to determine which conditions are most likely to push these ducks down the flyways
"We learned that mallard migrations are affected mostly by a combination of weather conditions, specifically how cold it is, how many days in a row the average temperature stays below freezing, how deep the snow is, and how many consecutive days at least one inch of snow is on the ground," Schummer says.
"What it comes down to is this: when it gets cold and stays cold for a long time, ducks start burning up body fat reserves. Prolonged cold can also increase ice coverage on wetlands, which results in less food being available to mallards. Then, if there's snow covering their food in dry fields and they're having difficulty replenishing this lost energy, they reach a threshold that makes them migrate.
"By correlating duck numbers with weather conditions on the Missouri
conservation areas, we came up with a weather severity index that would predict when ducks would move farther south. Then we calculated the weather severity index at 60 different weather stations in the Atlantic and Mississippi flyways from 1950 to 2011 to see if the severity of weather known to spur mallard migrations during fall had changed."
Schummer says the data suggests that there has been no long-term, sustained change in the weather severity index from 1950 to the present. "Granted, we have had strings of years with warmer or colder weather patterns," he adds. "For instance, we had more severe weather in the 1970s and 1980s, while we had a string of milder weather in the 1990s and 2000s. What many duck hunters in the southern states of the Mississippi Flyway
remember are the 1970s and 1980s, when mallards seemed to come early and in good numbers nearly every year. However, to a mallard the 1990s and 2000s were some of the mildest fall seasons on record. But there were also several mild years in a row back in the 1950s, and the 1960s had a mixed bag of mild and severe winters. Few hunters can remember back that far, though.
"Overall, from 1950 to 2011 we haven't seen continually milder weather and later migrations with fewer mallards arriving on their Deep South wintering grounds. Instead, each year's migration appears to be dependent on the severity of that year's fall and winter weather. Yes, we will have cyclic ups and downs, and some cycles will be more severe than others. In years with cold fall and winter weather, the ducks will still come down as early and in as great numbers as before. But during the warm cycles they will probably come later."
So what's the application in all this for hunters? "They can watch the weather up the flyway and be able to predict fairly accurately when ducks will move south," Schummer says.
"We learned that other dabbling ducks besides mallards begin to move farther south when the daily average temperature approaches freezing. However, it often takes several more days of freezing temperatures and snow cover to push mallards south from northern and mid-latitude staging areas. You can watch temperatures and snow lines and predict fairly accurately when this will happen."