Did it seem like there were fewer cottontail rabbits around this winter than there were a few years ago? It certainly seemed that way to me. Rabbits are known to have cyclic populations, reproducing rapidly, increasing to enormous densities and then crashing to a low before rising yet again. Population peaks usually occur about once every decade, but there can be ups and downs in between as well.

As it turns out, we had significantly fewer rabbits this year than when we started our urban rabbit research project in 2010. The trend is most likely due to these natural fluctuations in the population. We estimated 179 rabbits on the main grounds of the zoo in winter 2010 compared to 103 rabbits last winter.

How do we know this? The zoo’s Urban Wildlife Institute has been studying the local rabbit population for the last three years. We're interested in learning how wildlife coexists with humans in urban areas like Chicago. To this end, each winter we use box traps to capture rabbits, afterward releasing them again with ear tags. We then conduct visual surveys of rabbits to see how many tagged and untagged rabbits we can detect. Since we know how many rabbits are tagged, the ratio of tagged to untagged rabbits gives us the total number of rabbits, with some defined margin of error.

This method of marking animals and then conducting surveys to see how many marked and unmarked animals are observed is frequently used in wildlife studies—it’s referred to as “mark-resight.” Researchers have used this technique to obtain population estimates for all kinds of species, from mountain goats to beluga whales. In our case, this technique works well because rabbits tend to have small home ranges and are relatively easy to spot, once you get the hang of it.

With the relatively small rabbit population observed in 2012, it seems the plants in our gardens had a bit of a reprieve this winter. However, if the rabbits follow the expected pattern, it’s only a matter of time before they'll bounce back! And if you head to Lincoln Park Zoo for Easter weekend, chances are you'll be able to spot a bunny or two hopping through the tulips.

Vicky Hunt