Illinois Natural History Survey - University of Illinois

Don't Blame It All on the Raccoons

Much of the natural habitat of the midwestern U.S. has been converted to agricultural use. In east-central Illinois, for example, row-crop agriculture covers about 75% of the land area and dominates the landscape. The remaining natural areas are also typically highly fragmented, creating large amounts of edge habitat. Recent conservation literature has focused primarily on the negative aspects of such habitat fragmentation, but some species thrive in these heterogeneous areas. In fact, medium-sized mammalian predators, such as coyotes, raccoons, and opossums, have increased to what are probably historic high densities in Illinois in the past few decades despite extensive conversion of natural habitats to agriculture. These species tend to be very opportunistic in their choice of food and habitat, as long as certain basic requirements, such as suitable den sites, are met. Some recent studies have shown that raccoons reach their highest numbers in landscapes with extensive agricultural edges, in wooded remnants in areas with extensive corn cover, and in fragmented landscapes with a high diversity of cover types, especially where there is proximity to water.

The good news for some kinds of animals can be bad news for others, though. Predators may use habitat edges as travel lanes or forage more intensely there, elevating rates of predation on songbird nests. In some areas of the Midwest where habitat fragmentation is extensive, rates of predation and nest parasitism by cowbirds can be so high that nesting songbirds do not fledge enough young to maintain stable populations. These areas may be population sinks for songbirds, meaning that populations must be maintained by constant immigration. Many kinds of predators take songbird nests, but at least two studies in the Midwest have implicated the raccoon as the major predator in agricultural regions. So far, though, few data show how different types of predators use habitat edges.

At the Middle Fork Fish and Wildlife Area in Vermilion County, we radio tracked 15 raccoons and 4 opossums to determine whether they used habitat edges preferentially. We also conducted experiments using wicker nests baited with commercial quail and Zebra Finch eggs to determine if rates of predation differed in fields of different sizes (are predation rates lower in large fields?) or at different distances from habitat edges (are predation rates higher closer to the edge than out in the middle of the field?). This research is part of a Federal Aid in Fish and Wildlife Restoration project, funded through the Illinois Department of Natural Resources, being conducted by Scott Robinson, Jeff Brawn, Ed Heske, and other INHS biologists to study factors affecting nest predation on edge-, shrubland-, and grassland-nesting songbirds.

We tracked radio-collared raccoons and opossums at night from two vehicles with antennae mounted on the roofs. Coordinating our positions and timing by CB radio, we recorded simultaneous bearings for each animal several times per night between about 9 p.m. and 4 a.m. during three-week tracking sessions in June and August 1997. Using a differential global positioning system (GPS) to pinpoint precisely the locations from which we took our bearings and a computer-based geographic information system (GIS) to map our data onto aerial photos, we could then plot the nightly movements and habitat use of our study animals at the Middle Fork. From these data, we could clearly show that raccoons and opossums used habitat edges extensively during their nightly activities, and were located less frequently than expected out in open fields away from edges. So, does that mean we can blame edge effects in nest predation on raccoons?

A fox snake, one of several predators of songbirds.

Our experiments with artificial nests showed there was a moderate tendency for predation rates on the quail eggs to be lower in larger fields, and a weak trend for predation rates to be higher near edges, as we expected. However, predation rates on Zebra Finch eggs showed no pattern. Further, only about 35% of the quail eggs we set out were depredated, whereas the total predation rate when the Zebra Finch eggs were included reached about 75%, close to the rate for natural nests at our study site. Because the quail and finch eggs were together in the same nest and we assume that a predator like a raccoon takes both eggs if it finds a nest, those cases where the finch egg was eaten but the quail egg was left indicate that some other predator was at work. For example, mice and chipmunks can prey on small eggs like finch eggs (and most songbird eggs), but can't break the larger, harder quail eggs.

A preliminary analysis of nest predation on over 1,800 natural songbird nests found at the Middle Fork by other biologists working on this project also did not show any clear spatial patterns relating to edges. Other aspects of our research are showing that natural areas embedded in this agricultural landscape attract and support high densities of a diversity of potential nest predators. Although some predators like raccoons may use edges extensively, some small mammals and snakes (which we are also radio tracking) may prey on nests far from edges. We hope to identify patterns of habitat use by different kinds of predators, and patterns of nest predation in different kinds of habitats and landscapes, that may point to management practices that could improve the nesting success of songbirds. In the meantime, don't blame it all on the raccoons.

Ed Heske, Dan Rosenblatt, and Julianne Newton, Center for Wildlife Ecology

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