The Illinois Department of Natural Resources has made estimated the numbers of Canada geese at the lab at between 15,000 and 18,000 during the height of the migration season. The large areas of warm water and abundant forage and open spaces make Fermilab an ideal haven for geese during the winter. Although Canada geese are protected, they tend to be aggressive and in such large numbers, they can have a depressing effect on populations of other waterfowl.
The ideal conditions can result in problems. An overabundance of white-tailed deer has become increasingly apparent in recent years. As of 1994, the estimated deer population on the site was around 350 individuals, and the annual reproductive rate was estimated at 12 percent. An optimal density would be around 150 for the site.