Dennis: I am always suspicious of 'studies' by groups with a specific angle to promote. It isn't at all surprising that the RSPB should 'estimate' (guess??) such a large round number, as it suits their position and arguments and makes a good headline. Anyone who listens to 'More or Less' on Radio 4 will attest, these types of estimates are rarely made on a sound basis.
I have a feeling (perhaps a study by the cat protection league), that the biggest impact on birds, small mammals and in fact all wildlife, is the use of pesticides and the changes in farming processes.
This is acknowledged by the RSPB here:
https://www.rspb.org.uk/birds-and-wildli...n-decline/
Where they say that the decline is caused by changes in agriculture and not a mention of cats.
I remember in the 70s summers a regular job was cleaning the numerous smeared insects from the windscreen which had splattered over it on even short journeys. Now, well I can't remember when I last had a fly hit the screen and splatter. It's these insects that keep small birds alive and if they aren't there, well.
As a beekeeper there are acknowledged scientific studies which show the impact of neonicitinoids of the ability of bees to navigate (they get lost and can't get back home). They were banned across Europe for use in flowering crops as a result. Now, well the farming lobby has made a case to allow their use in beet and voila they are legal again, but only here in the UK, in year 1 as a 'trial' and now expanding.
Meanwhile global warming is allowing the Asian Hornet to migrate ever closer. It has decimated bee populations in France and has now arrived in the UK and bees are completely defenceless against it.
Who needs bees anyway...
So, whilst I agree that even us cat owners try to retrieve the occasional bird that a cat actually manages to catch, they are the least our birds, small mammals and insects need worry about.