From a paper published in Nature
last year but featured by the BBC today
Cats are one of the top threats to US wildlife, killing billions of animals each year, a study suggests.
The authors estimate they are responsible for the deaths of between 1.4 and 3.7 billion birds and 6.9-20.7 billion mammals annually.
The abstract to the Nature
Un-owned cats, as opposed to owned pets, cause the majority of this mortality. Our findings suggest that free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single greatest source of anthropogenic mortality for US birds and mammals. Scientifically sound conservation and policy intervention is needed to reduce this impact.
A nice reminder in yet another ecological arena that anthropogenic doesn't mean 'human controlled.' The perturbatory ripples issuing out from our actions rapidly attain shapes, patterns and magnitudes almost unrecognisable to us as they are amplified by processes and agents with aims and objectives entirely their own.