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.