The Cover Connection

New DU research has provided more evidence that conserving large blocks of grassland is critical to prairie waterfowl populations

by Matt Young

On a windy spring morning in North Dakota, two ATVs carrying waterfowl researchers creep across a vast expanse of native prairie, dragging a 200-foot chain between them to flush ducks nesting in the lush green grass. I am accompanying Scott Stephens, a Ducks Unlimited research biologist and Ph.D. candidate at Montana State University, and research technicians Ed Penny and Andrew Puls on a nest-searching mission in the heart of the Missouri Coteau.

This rugged, glacier-carved region, dimpled with millions of prairie potholes surrounded by large tracts of unbroken grassland, contains some of the continent's most productive waterfowl breeding habitat. During wet years, some areas of the coteau support more than 100 breeding pairs of ducks per square mile.

It doesn't take long to find a nesting hen in such prime upland cover, and a female pintail flutters into the air just ahead of the chain, betraying the location of her nest. We hop from the four-wheelers and ease through the dew-soaked vegetation to the area where the hen flushed. Parting the high grass with his hands, Stephens discovers the small bowl-shaped nest holding a clutch of seven off-white eggs. He lifts one of the eggs and inspects it through a black plastic tube, noting the silhouette of the developing duckling, which is faintly visible through the opaque shell. This procedure, known as candling, reveals how long the hen has been incubating the eggs, and, consequently, when they will hatch.

"Looks like this one has about five days to go," Stephens says. "We've been finding lots of pintail nests on our study sites this spring. There appears to have been an especially large influx of pintails, perhaps from the west, that have pioneered into this area in response to excellent wetland conditions." Stephens carefully places the egg back in the nest and covers it with down to keep the eggs warm until the hen returns.

The technicians record the location of the nest, the number of eggs, their estimated hatching date, and the height and density of the surrounding upland cover. Then Stephens pushes a long willow branch flagged with orange tape into the ground a few yards from the nest to serve as a visual marker. The researchers will return to check the nest within a week to see if the eggs have hatched or if they have been lost to a predator.

While searching the remainder of the 160-acre pasture, we find more than a dozen duck nests, including those of mallards, blue-winged teal, gadwall, scaup, and shovelers. As we move to other study sites throughout the morning, we find dozens more. By the end of July, Stephens and his research crews will monitor the fate of more than 2,000 duck nests scattered across more than 33,000 acres of native prairie and planted Conservation Reserve Program (CRP) cover.

This monumental effort is part of an ongoing study examining how the abundance of upland nesting cover influences waterfowl nest success across vast prairie landscapes. Stephens' research has significant implications for DU's Grasslands for Tomorrow initiative, which is working to permanently protect 2 million acres of the most productive waterfowl habitat in coteau regions of North and South Dakota.

Dr. Jim Ringelman, director of conservation planning at DU's Great Plains Regional Office (GPRO), explains, "The main objective of our research is to test our assumption that duck nest success and the amount of grassland habitat are directly related. We are particularly interested in native prairie because of our programs that are targeted at protecting these critical grasslands."

Before beginning his field research, Stephens used satellite land-cover imagery to select 18 different study sites-each encompassing an area of four square miles-in five counties in the Missouri Coteau of northwestern North Dakota. Sites were chosen with varying levels of upland cover, ranging from intensively farmed landscapes with very little natural vegetation to nearly solid tracts of native prairie and planted CRP cover. Stephens also considered the abundance of upland cover in a larger, 36-square-mile block surrounding the study areas.

"We are interested in the landscape characteristics at two different scales," Stephens says. "We want to find out if the level of upland cover at a four-square-mile scale is more important than the amount of cover at that bigger scale and how it affects nest success in the core area."

Stephens and his two research crews went to work locating nests on the study sites during the spring of 2000. They searched fencerows, ditches, and other small patches of cover on foot. In larger areas, such as CRP fields and native pastures, they used ATVs and a chain. Blessed with excellent wetland conditions, they were able to obtain a large sample of duck nests, which they monitored throughout the nesting season.

The researchers also monitored the nests of any other grassland-nesting birds that they discovered, including sharp-tailed grouse, harriers, short-eared owls, and several species of shorebirds. By the end of the nesting season, it was clear that 2000 was a banner year for duck production in the Missouri Coteau. "Of the more than 2,200 duck nests that we monitored on all the study sites, we had an overall nest success rate of 22 percent," Stephens says. "Twelve of the 18 sites had nest success of more than 15 percent (the level required to maintain duck populations), including some sites that had very little cover. The most productive areas were those with more than 70 percent grass cover, which collectively had an average nest success of 32 percent. One of these sites had a phenomenal 43 percent nest success."

Nearly all duck species had good production in 2000 on Stephens' study areas. Pintails led the pack with an average nest success of 30 percent, followed by blue-winged teal (26 percent), gadwalls (20 percent), and mallards (19 percent). The only species that failed to hatch enough young to achieve population growth was the scaup. Stephens noted that their preference for nesting along wetland margins seemed to make the birds more vulnerable to predators such as skunks and foxes.

When the researchers returned in 2001, wetland conditions once again were excellent, enabling them to locate and monitor another large sample of duck nests. As the season unfolded, however, it became clear that predators were taking a much greater toll on nesting ducks than the year before. When Stephens analyzed the final data, he discovered that nest success on his study sites had plummeted to 8 percent overall. Only six of the 18 sites had nest success higher than 15 percent. All six of the successful sites had more than 70 percent upland cover.

He suspects that the general decline in nest success was due to a sharp drop in the abundance of small rodents, the primary food source for many predators. When mice and voles are scarce, predators may be forced to forage over much larger areas to find food and thus encounter more duck nests during their travels. In 2001, small rodent population samples conducted in conjunction with Stephens' research revealed that mice and voles were almost nonexistent on the study areas (rodent data wasn't available for 2000). "We conducted more than 13,000 trapping nights and found only 45 mice and five voles. On one site, nest success fell 39 percent from one year to the next. It's pretty hard to believe that predator density would change that much in one year. I think the predators just didn't have much else to eat out there except nesting birds."

Despite the dramatic differences in nest success, Stephens found several consistent trends in the data from 2000 and 2001. During both years, most of the sites with more than 70 percent upland cover had nest success higher than 15 percent. "We have learned that there are dramatic fluctuations on the prairies, not only in water conditions and the number of ducks that attempt to breed, but also in how successful the birds are. However, it appears that in areas where grass levels exceed 70 percent, the ducks can hold their own, even in poor years."

Stephens explains that there are several reasons why nesting ducks fare better in areas with abundant upland nesting cover than on more intensively farmed landscapes. In large blocks of native prairie or planted CRP cover, ducks have ample room to disperse and hide their nests from predators.

In addition, the predator community on grassland-dominated landscapes is largely comprised of coyotes and badgers, which have relatively large home ranges compared to those of smaller predators, such as skunks and foxes. Larger home ranges mean fewer predators within a given area in search of duck eggs. Also, some studies suggest that rodent populations are more robust in areas with abundant grass cover, providing more buffer prey that could improve the odds for nesting birds.

The data also revealed a direct correlation between the abundance of upland cover and nest success within the four-square-mile study areas, while the level of grass in the surrounding 36-square-mile area seemed to have much less of an impact on nest success. This is good news for DU because it is much easier to positively influence land use on this smaller scale.

Interestingly, Stephens discovered that the type of upland cover in which ducks nested had little impact on their success. During both years, nest success for ducks in CRP and grazed native pasture was almost identical. He also observed that several shorebird species, including willets, marbled godwits, upland sandpipers, and Wilson's phalaropes, had very specific nesting cover preferences. "We didn't find shorebirds in anything other than native prairie," Stephens says. "If we continue to lose this habitat, those birds are going to disappear because, unlike most duck species, they won't use planted cover."

This research underscores the urgency of DU's efforts to save large remaining blocks of grassland and small wetlands, which are presently being lost at an alarming rate across the prairies. It also confirms the immense benefits CRP provides to waterfowl and many other prairie-nesting birds. "CRP fields clearly are very productive for breeding waterfowl in areas where large blocks of land have been enrolled in the program and where CRP fields augment extensive tracts of native pasture," Stephens says. "In many cases, CRP fields fill in the gaps between existing blocks of upland cover. If you lose those chunks of planted cover, nest success could plummet throughout surrounding landscapes."

Although Stephens' graduate research concluded in 2001, he will continue to monitor nest success on many of his study sites over the next five to 10 years while serving as research biologist for the GPRO. DU also will have two full-time technicians sampling rodent populations, and plans are under way to track the movements of predators fitted with radio collars, including foxes, coyotes, and skunks.

Ringelman concludes, "We've learned a lot about how the abundance of upland cover affects waterfowl nest success, but more research will be needed to determine how interactions between predators and alternate prey species impact nesting ducks in different areas. By monitoring these complex natural systems over several years, we'll continue to gain a better understanding of what types of landscapes will produce the most ducks year after year."