Harvest mice are getting some rather unusual homes built for them - using tennis balls!
The mice are the smallest rodents in the UK and their habitat is under threat because it's being cut down to grow food for humans.
A tennis ball is ideal because it's the same size as a harvest mouse's nest. It's also brightly coloured so they can be easily tracked to see how many mice move in.
Hayley finds out how the 'mouse houses' are built here.