Loads of pet cats in the south of England are being tagged to find out how their hunting habits are affecting different species of birds.
Scientists are fitting more than 200 cats in Berkshire with electronic harnesses so they can track where they go and what animals they kill.
It's thought cats kill a staggering 92 million animals every year in the UK, including about 27 million birds.
The birds thought to be most affected are blackbirds, robins and sparrows.
Sparrows are one of the species most commonly killed by cats
When the cats' hunting habits have been studied, researchers will be able to tell how big a part the killer cats are playing in the falling numbers of the birds.
Rebecca Dulieu, who's carrying out the research at the University of Reading, said there are about nine million pet cats in the UK, mostly living in urban areas.
She said that because there were so many cats they could be having a big influence on the number of birds.
"For example, house sparrow numbers in urban areas have declined by 60% since the 1980s, most likely due to changes in urban habitats, but this is also one of the species most commonly killed by cats."