In many parts of the European countryside, rabbit populations have been declining.

But in cities they are climbing, and fast.

And curiously, they are adapting to city-life much like people do.

In large over-populated cities, rabbits have been downsizing their homes, according to a new study of European rabbits (Oryctolagus cuniculus). 

A team led by Madlen Ziege from the University of Frankfurt in Germany compared the burrow structures of different rabbit populations in the country from rural, suburban and urban environments.

They discovered that in rural populations, rabbits lived in large burrow systems with numerous entrances – an average of around 32. They tended to stay in large social groups and lived close to each other.

In the city however, they lived in much smaller groups and were widely distributed. The average entrances for urban livers was seven and these entrances were also much smaller and closer together.

There were even single individuals which lived in single burrows, studio apartment style, something never before observed in rural populations where staying in large groups can be crucial for surviving predador attacks.

In cities there was also plenty of choice, some lived in gardens, some in parks and some beneath or between large building complexes.

"In cities we have different vegetation so the rabbits have more possibilities to establish their burrow systems, which in turn leads to higher densities," says Ziege.

They also had an abundant source of food, lots of protection from predators and therefore plenty of time to reproduce.

A warmer city climate also added to their reproductive success. "It seems they have better habitat quality in the city, so they have better survival rates and reproduce more often and for longer," Ziege told BBC Earth. 

While this particular study, published in the Journal of Zoology, was undertaken in and around Frankfurt, Ziege says these patterns persist in other cities too such as Berlin and Hamburg.

What's not clear yet is to what extent migration plays a role, if any, though a follow up study on population genetics hopes to discover exactly this.