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I don't need one, I'm too slow to catch birds
Aren't you like a vegan anyway? It might be annoying but you've got to weigh that up against dealing with dead or dying animals in your house and the potential ecological impact* that your cat has. For some people it's worthwhile. Just a suggestion, you don't have do it if you don't like it
* RSPB says the impact on bird populations in the UK is fairly minimal. In the US I think cats eating birds is quite a big problem.
You can put a bell on your cat's collar to decrease its chances of catching stuff. Birdologists (technical term) will thank you