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Four-Year Research Project Confirms Hen Houses Boost Mallard Production

The area around Minnedosa in the parklands of southwestern Manitoba has become waterfowl's equivalent of the "roach hotel". Ducks check in but they don't check out.

In recent years, nest-success rates have been so low that Minnedosa qualifies as a "population sink". Translation: More ducks set up housekeeping each spring than migrate south in the fall.

Yet despite Minnedosa's apparent lack of productivity, the breeding population of mallards has actually increased, climbing from 10 pairs per square kilometer prior to the 1990s to 16 pairs in the 2000s. Scientists know that given adequate wetland conditions, hen mallards often return to the area where they were hatched, but if Minnedosa is producing so few ducklings, where are all those breeding birds coming from?

That's the question University of Saskatchewan Ph.D. candidate Dan Coulton set out to answer in 2002. What four years of field research showed is that the local breeding population has been propped up by artificial nesting structures called Hen Houses.

Developed by Delta Waterfowl, Hen Houses are wire-framed, grass-lined cylinders mounted on posts and erected above the waterline to buffer hens and nests from predators.

Nest success for mallards using Hen Houses during Coulton's study was 58 percent. That's well above the 12- to 15-percent minimum necessary to sustain the population.

Mallards nesting in Minnedosa's upland cover haven't fared nearly as well in recent years. In a study conducted by Ducks Unlimited's Institute for Wetland and Waterfowl Research (IWWR), nest success on "managed upland cover" in the Minnedosa region was just 1 percent, and that was in 1998, a time when mallard populations were soaring.

Coulton's research mirrored those findings: non-tunnel nesters averaged just 2.8 percent nest success, and one year nest success was just one one-hundredth of a percent, which translates to one successful nest for every 10,000 initiated (waterfowl scientists measure nest success using a complex formula rather than a straight percentage).

Hen survival was also higher for birds nesting in Hen Houses. "Hen House females spend less time attending nests," Coulton explains. "Hens whose nests are destroyed early in the breeding-season will typically attempt to re-nest, often several times. The more time hens spend nesting, the greater their exposure to predators. Most terrestrial predators that prey on hens don't swim, thus they cannot access hens in nest tunnels."

Hen Houses, most of them erected as part of Delta's Duck Production (DDP) program, enjoyed occupancy rates ranging from 85 to 92 percent during Coulton's study.

Scientists know that nest success and hen survival are the two most important factors in determining mallard populations, and Coulton's research confirms that Hen Houses are a cost-effective tool for increasing both.

In the beginning Coulton had to consider the possibility that Minnedosa was being populated by ducks hatched in other places. The immigration hypothesis seemed plausible given Minnedosa's proximity to the highly productive Conservation Reserve Program (CRP) acres just across the border in North Dakota, but determining where a Minnedosa-nesting hen originated would be a challenge. After all, migrating ducks don't leave forwarding addresses.

To find the answer, Coulton incorporated a bit of high-tech sleuthing that sounds like a plot from one of television's crime-scene investigation shows: stable isotopes.

"Ducklings grow their first flight feathers from the food in their natal area, and stable isotope patterns from these foods are incorporated into feathers," explains Coulton. "They don't molt those feathers until after their first breeding season, so by analyzing the feather tissue of juvenile birds, we can link the stable isotope values to breeding regions and re-track which region an individual came from."

Surprisingly, these "isotopic signatures" showed most of the juvenile females captured by Coulton originated not in the U.S. prairies but in Canada's parklands. While stable isotopes cannot reveal exactly where in the parklands those ducks hatched, Coulton's banding data did confirm that Hen Houses are an important source of juvenile mallards in Minnedosa.

"My top-performing model said that tunnel recruits were important to the Minnedosa mallard population," Coulton says. "Of the banded juvenile females I recaptured during the study, 89 percent hatched in Hen Houses the previous year."

Delta and its partners have installed 3,600 Hen Houses in Manitoba, Alberta, Ontario, North Dakota, Pennsylvania, Michigan, Colorado and Utah and are launching new projects in Minnesota, Iowa and Saskatchewan. Delta anticipates installing another 1,600 Hen Houses this winter.

A native of Bay Village, Ohio, Coulton is working on his Ph.D. at University of Saskatchewan and hopes to publish this research in the near future.