Rabbits have the ability to breed and continue to breed in almost any environment.
After observing that there seemed to be more rabbits than usual in the Denver metropolitan area, one CPR News listener asked Colorado Wonders, “Is Denver’s rabbit population growing?”
And as a member of a multi-generational Colorado family that has lived in Denver since the 1920s, I’ve come to a chicken-and-egg question, so to speak, regarding foxes in this city. Denver used to have a lot of foxes, but at least this year, they don’t seem to be as noticeable or as numerous as they once were.
As it turns out, both observations about urban wildlife are correct… maybe.
Gunnar Zollinger, natural resources specialist for Arapahoe County Open Space, said he has set up small cameras in the county’s open spaces and has also noticed an increase in rabbits and fewer foxes than usual.
In reality, almost everything is guesswork, since no one is doing censuses on wildlife in these urban areas, but they are guesses based on science.
Zollinger hypothesized that the severity and length of the 2022-2023 winter could be affecting animal populations today.
CPR listeners also asked: “Why is the city’s rabbit population growing?”
The reason for this begins with the long winter.
The Front Range saw higher than normal snowfall, about 8 inches above normal, according to the National Weather Service. It was also the cloudiest winter in the state’s history, with a record 56 days of snow on grasses and fields (and front yards). For comparison, the winter of 2023-24 saw 23 days, according to the weather service.
That meant a cold, long and snowy winter in 2022-2023, which likely had a negative impact on the foxes, Zollinger said.
Foxes have a lifespan of four to five years, and if the cold weather continues for that long, the older and weaker foxes will not survive, and neither will the younger ones once the snow starts to fall. Foxes mate once a year, usually in December and January, and give birth to litters of three to five pups. The reduced mating during that season may be due to the need to prioritize survival, as the cold and snow on the ground hides their natural food sources.
But what does this have to do with rabbits?
Kevin J. Beatty/Denver Site A small hopper discovered at Centennial, June 17, 2019.
It has to do with the food chain: Fewer predators would give cottontails — the small grey urban rabbits you find in parks, gardens and trails — more opportunities to thrive.
Furthermore, in contrast to carnivores higher up the food chain, rabbits reproduce much faster, being able to mate up to seven times a year and giving birth to 3-8 baby rabbits per litter.
What further helped the rabbits that winter, Zollinger said, was that the snow finally melted, bringing with it lots of green grass and shrubs in wilderness areas — all of which rabbits love to eat. And with unlimited food available, the rabbits were able to give birth to even more babies than they normally would.
“I think it has to do with a predator-prey cycle,” says Monique Rivera, a federal wildlife biologist at Rocky Mountain Arsenal National Wildlife Refuge east of Denver. “It waxes and wanes over time. With the amount of rain we’ve had this year, we’re seeing a lot of vegetation, which is why we’re seeing a lot of cottonwolves. We’ve also got foxes. More prey means more predators.”
Rivera said this is good news for fox lovers.
Rivera noted that just this summer, a new species of fox has been spotted in the area at Arsenal.
“This is actually an indicator that our ecosystems and grasslands are healthy,” Rivera said. “We weren’t actively managing for foxes. Often, if you improve habitat, species will naturally come back where resources are available.”
Vickie Vargas Madrid, wildlife program manager for Denver Parks and Recreation and Mountain Parks, said the defining feature of urban animals — all wildlife — is that they’re endlessly adaptable.
Kevin J. Beatty/DenverSite A baby urban rabbit might be too short to jump the curb at 16th Street Mall, but an incredulous nearby photographer saved the bunny from an oncoming car on May 24, 2018.
She said urban wildlife will change over time as different areas experience different things throughout the seasons and years, such as new development, increased population density, the death of some trees, etc. For example, when new apartment buildings are built in place of surface parking lots, certain animal populations may end up dispersing.
Madrid said that doesn’t necessarily mean the animals are gone, just that there are fewer of them visible.
“These are urban species; they’ve adapted to the urban environment; they have what they need: food, water, shelter,” she said. “It’s hard to say why you’ll see one species more than the next from one year, but from a management standpoint, it usually has to do with the habitat that’s available to them.”
