Early dabblers leave near freezing while mallards wait for snow

STEP 1 · The Score

Mallard numbers dropped once the running index cleared 7.2

Schummer and colleagues in the Journal of Wildlife Management set duck counts from Missouri Conservation Areas against Historical Climatology Network temperature and snow for autumn and winter 1995 to 2005. Support was strongest for the formula that changes the sign of the Celsius daily mean so a reading under freezing adds points and a reading over freezing takes points away. Add the number of days in a row that the mean has held at or below 0°C (32°F). Multiply snow depth in centimeters by 0.394 so that term is in inches. Add the number of days in a row with at least 2.54 cm (1 inch) of snow. Their 2014 restatement tied a daily score of 7.2 or higher to a greater chance that mallards left mid-latitude areas. That paper mapped with 8 as a slightly stricter line and treated the mallard response as similar across roughly 33°N to 49°N.

Mallard counts on those areas were still climbing while the air neared freezing. Pooled counts for the remaining dabbling ducks were already dropping. Steep mallard declines arrived with a longer spell of cold and snow after the index cleared 7.2.

Our rule is to total several mornings before shifting a hunt. When no true daily mean is posted take the midpoint of the high and the low. A mean of 23°F sits 9°F under 32°F. Nine times 5/9 equals 5 and that matches the term for a mean of -5°C. On freezing day number three in the streak add 3. A two-inch snowpack held for two days contributes 2 as depth and 2 as the streak. The four terms total 12 and that total clears both 7.2 and 8. A 50°F mean on bare ground gives a temperature term equal to -10. That warm bare day sat outside the Missouri mallard leave zone.

STEP 2 · Who Leaves

Dabbler dates on the Great Lakes run weeks ahead of mallards

Notaro and colleagues in PLOS ONE set departure dates inside the Great Lakes band from 40°N to 50°N using a weather severity equation fit to each species. They defined that date as the first autumn or winter day when cold and snow carry a 50 percent or higher chance across years of a falling local population rate. Late in the 20th century those dates were northern shoveler on 2 October, green-winged teal on 15 October, American wigeon on 16 October, northern pintail on 4 November, gadwall on 5 November, mallard on 9 December, and American black duck on 10 December.

Averaged across six models the dates move later. Pintail delay is the smallest. Mallard delay is the largest at near 19 days at mid-century and near 40 days or no crossing by late century.

In Wildlife Biology Thurber and colleagues drew on Canadian Wildlife Service aerial counts spanning 1968 to 2011 across 90 shoreline sectors of the lower Great Lakes. Blue-winged teal peaked in early September. Green-winged teal and American wigeon peaked in mid-October. Gadwall and mallards peaked in late October. American black ducks peaked in early November. Across the 43 years the mallard peak shifted 14 days later and the American black duck peak shifted 18 days later. Both shifts carry about a week of slack on either side at 95 percent confidence. Gadwall shifted 11 days later and wigeon shifted 13 days later. The slack on each was 11 days to either side. Neither teal species changed its peak date. An October count peak and a December weather exit measure different things.

Across 1950 to 2008 the 2014 paper found mallard weather for November through January in eastern North America mildest during the 2000s. In a 2017 Wildlife Society Bulletin abstract Schummer and colleagues reported that Mississippi and Atlantic Flyway ground with weather harsh enough to push dabblers south shrank from 1979 to 2013. Wigeon, green-winged teal and shovelers lost that severe ground during October through December. Mallards, black ducks and pintails lost it during December and January.

In Movement Ecology O'Neal and colleagues used weather radar from 1995 to 2009 on a 14,431 ha central Illinois complex of wetlands and backwater lakes including the Emiquon Preserve. Those departures show a mixed flock getting out under open sky. Departure nights totaled 216 of 723. The paper called that share about 30 percent. The winning model kept a tailwind, dry conditions, and a sky short of full overcast. When all three were favorable the modeled chance of departure that night was 0.76. Tailwind outweighed the other two conditions. Local weather severity dropped out of the winning model. Pintails, green-winged teal, wigeon, gadwall and shovelers were in the sample. The authors wrote that those birds often leave the marsh ahead of a freeze. Their text says mallards often go only after severe weather wrecks the site.

STEP 3 · Marked Birds

Snow days lined up with most tagged mallard departures

In Movement Ecology Weller and colleagues followed satellite-tagged mallards through the Mississippi Flyway during 2004 to 2007 and again during 2010 to 2011. After filters 43 birds remained and 39 were female with 4 male. They logged 73 moves headed south of at least 30 km (about 19 miles) from 1 September to 31 December. Only a winter-conditions model with inputs of temperature, snow depth and snow duration stayed competitive. It named the true departure day on 60 percent of choices. Choosing blindly inside a set of seven days would have landed on 14 percent. Of the departures the model assigned 65 percent traced to snow in the recent window and 30 percent traced to temperature that day. Their snow day was at least 1 inch and even one sent the relative chance of a departure sharply higher, as did depth past 7 cm (about 2.8 inches). Where recent snow was thin or missing the chance of a departure rose once the air dropped under 5°C (41°F). Cloud cover, wind, rain and pressure lost.

In Ecology and Evolution Pearse and colleagues put transmitters on juvenile mallards across North Dakota and South Dakota during 2018 and 2019. The movement work covered 97 birds. They classed 25 to 310 km (about 16 to 193 miles) as regional hops and anything past 310 km as migration. The first migration tracked the daily low and stayed unlikely while the minimum held above -5°C (23°F). The chance climbed 29 percent with each drop of 1°C. A favorable wind also lifted odds on that opening migration. Every additional 1 cm (about 0.4 inch) of snow lifted the chance of a later migration by 80 percent. Those later moves were also 2.0 times as common on weekend days. The authors tied that weekend bump to people pushing birds. Distance flown on migration grew by 166 km (103 miles) for every 1°C (1.8°F) colder. Median spots for mid-December through January sat north by roughly 250 km (155 miles) in 2018 to 2019 relative to 2019 to 2020 and west by roughly 300 km (186 miles).

STEP 4 · The Card

Score four terms each morning and split the water by species

Score all four terms on the water you hunt. Any day with a mean above 32°F resets the freeze streak. The snow streak resets once depth drops under 1 inch. Our read from the Missouri counts is mallard water turns uneasy after the sum clears 7.2. Passing 8 also satisfies the stricter 2014 map cutoff for likely mallard departure areas. The pooled other dabbler counts were already falling as the air neared freezing on our reading. With snow still absent that reading sits near an index of 0.

Shovelers, teal and wigeon land early on the Great Lakes weather clock. Teal and wigeon also reached early peaks on the Ontario surveys. Shovelers were outside that survey species list. For juveniles on a first long move off the northern prairie weigh a low near 23°F ahead of snow depth. After that move weigh fresh snow more and expect more of those later trips on weekend days.

Cold in the Missouri index raises energy demand and snow plus ice cut birds off from food. Use it as a departure watch. Ducks per hunter and the leading species in each state bag are covered in the per-hunter bag paper. Where duck harvest states and goose harvest states part is in the duck-country and goose-country paper. Season length and bag limits differ by state and zone so check the current wildlife agency page before hunting.