Though largely invisible to the public, a small army of federal employees each spring descends on the continent’s most productive waterfowl lands to undertake a rigorous counting of North American breeding ducks.

The waterfowl survey has generally been conducted the same way since 1955, and its methodology to a large extent had its origins in Minnesota, under the guidance of the late Art Hawkins, among other U.S. Fish and Wildlife Service waterfowl biologists. (Minnesota conducts its own spring waterfowl survey, this year finding overall duck abundance up 47 percent. More below.)

Involved in the federal survey are specially trained pilots who fly their aircraft low and slow, while keeping an eye peeled for antennas, birds and other objects that often share the airspace 150 feet above terra firma. Traveling at only 90 miles an hour, the pilots also count ducks and geese on the left side of the plane.

An observer in the plane, meanwhile, tallies waterfowl on the right side, while also chronicling the number of ponds that lie below.

“Counting ducks is not for the faint of heart,” said Terry Liddick, a Fish and Wildlife Service biologist-pilot stationed in Spearfish, S.D.

This spring, Liddick, along with observer and fellow service employee Dave Fronczak, flew “transects” over the eastern Dakotas — exacting east-west routes that have been trekked by waterfowl surveyors for more than a half-century.

Also during May, other Fish and Wildlife Service surveyors flew over duck- and goose-producing regions as far north as Alaska, while Canadian Wildlife Service teams fanned out across Manitoba, Saskatchewan and other provinces.

Information gleaned from the counts is used to assess individual waterfowl species, perhaps mallards mostly, and is tabulated so population fluctuations can be monitored. Hunters’ waterfowl limits can be affected by the survey results.

Overseen by the service’s Division of Migratory Bird Management, headquartered in Maryland, the spring duck and goose surveys are replicated in winter in the southern U.S.

Like other survey pilots, Liddick was a wildlife biologist who after joining the Fish and Wildlife Service was trained to fly airplanes. His assigned aircraft, a Cessna 206, is popular for survey work, though some pilots who fly more remote regions of Canada employ Quest Kodiaks for their extended range.

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Important as the aerial waterfowl surveys are, they aren’t the final word on bird numbers. Instead they are supplemented by surveys conducted by ground crews who travel the same transects, counting ducks at specific intervals along the predetermined routes.

This year, Pam Garrettson, a wildlife biologist assigned to the Division of Migratory Bird Management in Patuxent, Md., headed up the four-person ground crew assigned to the eastern Dakotas. She and a fellow employee drove a four-wheel-drive service vehicle from Maryland to conduct their portion of the survey.

“This year we finished our survey in record time,” Garrettson said. “We started on May 3 and were done on May 19. Each year it’s different. It’s all weather-dependent, especially the weather’s effect on the air crew.”

Surveys are timed to coincide with the arrival of nesting birds in assigned regions.

“We don’t want to count birds that are moving through, if we can help it,” Garrettson said. “Breeding ducks come into a territory, set up and defend their territories. Those are the birds we want to count.”

Ground survey crews travel no more than two days behind their airborne counterparts, Garrettson said. Often traversing rural section roads, their job isn’t to count waterfowl on every pond they encounter. Rather, they count birds at 18-mile intervals along the transects.

“When we survey a wetland or pond along a transect we count every duck in it, even if it’s a flock of 500,” Garrettson said.

Numbers attained by the air and ground crews are tabulated to develop an estimate of breeding birds in the area.

This spring, Garrettson said, large parts of the eastern Dakotas were quite dry. Still, the region’s breeding ducks didn’t seem to vary significantly from previous years, she said.

Meanwhile, in Minnesota, Department of Natural Re­sources waterfowl specialist Steve Cordts headed up the state’s spring survey. Mallards were 18 percent higher than last year, a statistically insignificant amount. But blue-winged teal were 88 percent higher. And total duck abundance was 47 percent higher than in 2015, and 25 percent higher than the long-term average.

Significantly, DNR pilot-observer Bob Geving and Cordts had a scare during the survey when the right-hand windshield of the agency’s new (100-hour) helicopter exploded while the two were airborne, requiring an emergency landing. No one was hurt, and cause of the mishap is under investigation.

As Liddick, the Fish and Wildlife Service pilot said, “Counting ducks is not for the faint of heart.”