Around 1 million house cats have been found to do that.
Step-by-step explanation:
The exorbitant number of cats that are killing the birds in the out are seemingly in the numbers that may not be possible.
There are close to a few million cats that are domesticated in homes as of now but there is also a substantial number of them in the roads suburbs and the wild.
These animals are the ones that would be hunting most of the birds.
This is not to say that it is not happening in the other domesticated ones too as they are wont to be the predators here too.