Grassland songbirds often nest in the same grassland-wetland complexes as waterfowl, particularly in a certain part of those complexes, namely, upland habitats surrounding wetlands. Although some wildlife management procedures directed at waterfowl, such as habitat enhancement or restoration, may also benefit songbirds , the impact of others, especially the control of waterfowl predators, remains difficult to predict. For example, most predators of waterfowl nests prey opportunistically on songbird nests, and removing these predators could directly increase songbird nesting success. Alternatively, small mammals such as mice and ground squirrels are important in the diet of many waterfowl-nest predators and can themselves be important predators of songbird nets. Thus. Removing waterfowl-nest predators could affect songbird nesting success through subsequent increases in small-mammal populations.
In 1995 and 1996, researchers trapped and removed certain waterfowlnest predators. primary raccoons and striped skunks, then observed subsequent survival rates for songbird nests. Surprisingly. They observed no significant effect on songbird nesting success. This may be due to several factors. Neither raccoons nor striped skunks consume ground squirrels, which are important predators of songbird nests. Thus, their removal may not have led to significant increases in populations of smaller predators. Additionally, both raccoons and striped skunks prefer wetlands and spend little time in upland habitats; removing these species may not have increased the nesting success of songbirds in the uplands enough to allow detection.
The passage suggests that removing Waterfowl-nest predators could possibly Have a negative effect on songbird populations because

key words: removing waterfowl-nest predators.
locate to the beginning of the last paragraph: "researchers trapped and removed certain waterfowl nest predators, primary raccoons and striped skunks, then over served subsequent survival rates for songbird nests"
then the author wrote: "surprisingly, they observed no significant effect on songbird nesting success". Thus, we can infer the researchers thought that the removal might decrease the nesting success.
The author went on to explain this unexpected phenomenon, speculating that the removal did not lead to increases in populations of smaller predators. Therefore, we can infer that the supposedly negative effect on the nesting success may result from the fact that the removal would increase the populations of small predators that prey on songbirds.

