Jos Stratford said:
Sounds a bit gun-crazy the last couple of suggestions from the USA ...first I think you'd scare the birds further than the cat, second probably be illegal in the UK! And regardless of how much I like birds, I know what my reaction would be if someone took a pot shot at my cat!
First, I think name-calling ("gun-crazy") is inappropriate, regardless of your opinion on the use of guns of any kind. I have probably used my BB gun fewer than a dozen times in the nearly 3 years I've lived here. Hardly "crazy" behavior by any standards. Also, I'm sure you are aware that BB and pellet guns are airguns, right? Not firearms. They're still "weapons" per se and need to be handled with respect because they can injure.
Secondly, as I alluded to in my response to PaulG, wildlife are adaptable to varying circumstances. Our yard "regulars" may flush if I happen to use the BB gun when they're around, but they also immediately (within seconds) return to feeding. They somehow know I'm not shooting AT them. The same holds true when I shoo megaflocks of, e.g., pinyon jays, who will invade the yard by the literal hundreds (150-300) for weeks at a time. My loud clapping disperses the feeding frenzy, and within seconds, the smaller birds -- who were, BTW, displaced by the megaflock -- return to the yard.
Thirdly, if you have a pet -- cat or dog or horse or snake -- keep it under control. If your animal is allowed to wander onto others' property and do damage (pooping, digging, scaring/killing other pets/wildlife, etc., etc.), you shouldn't be surprised if others will do the controlling for you. Besides, as I said, I don't shoot or even aim AT the trespassing, bird-killing (not just bird-scaring!) cats -- the noise of the gun alone scares them off as does the sound of the BB smacking into the tree under which they're hiding.
Edit: Forgot to mention the reason I was so protective of the Lewis's woodpecker brood: They're listed as a "vulnerable" species and, in some areas, are a "management indicator species." Otherwise, I would have let Nature take its course vis-a-vis the invading common ravens.