Yes but isn't the quantity of birds relevant only when taking the total population of that breed of birds into account.
Cats kill birds, I accept that no problem. I've checked google and for every page I found saying cats were damaging to wild life, I could find another saying that they weren't; that they were not a problem or rodent focused etc. None of the web pages looked that authoritative enought to quote here to be honest. So I remain unconvinced that they are a "danger to wildlife". Certainly my cat brings way more rodents to the doorstep than birds (like one bird to every twenty small rat/big mouse and/or pigmy shrew), so maybe one bird a year. Nobody is getting too worked up about the rodent situation here. My mother's cat brought home a hare not so long ago but that is very rare too and he ate that pretty much whole, so by the hunting for fun/food debate he should be in the clear. Obviously my cats are not representative of the whole cat population of the world, but it is as good as a lot of the googled pages I read. If I didn't have a cat, I would more than likely have a rodent problem where I live in the countryside and given the close proximity of grain stores at the next door farm. So what then traps or poison?
Even on the pages that are more pro-bird than cat, they don't recommend use of a bell, just in case anyone reading this is feeling conscience-bound to inhibit their cat's hunting instinct, since the sound of a bell is not something that would naturally alarm a bird. These pages recommend keeping your cat inside. So we're back to square one then, if you believe them.
It appears to me, that people simply prefer some animals to others and are happier to restrict the activities of certain animals (e.g. mice, cats) to promote that of (e.g. birds). I don't buy into that. I believe that they are all equal and that a balance can be struck. For example, I feed the birds in my garden and make sure to do it in such a way as to keep them clear of the cats. I do what another poster recommends and wait til well after the dawn-chorus and after the blackbirds and thrushes have got their worms before I let the cats out after the night inside. Cats are by nature nocturnal so even by keeping them in at night you are constricting them a little.
I actually tried keeping my cat in all the time for selfish reasons (I didn't want him to get killed/lost etc) but changed my mind for a couple of reasons. Firstly, I didn't think it was right doing it for myself, secondly the kitty litter factor. Mined clay (the most common and cheapest) kitty litter is dangerous to a small cat, it is also a major contributer to landfill as it is not biodegradable (in less than 200 years or something). Wood-based kitty litters are not nearly as effective, are much harder found (where I live) and are over twice as expensive but are biodegradable (if you can put them in your garden, cos I don't think you should put cat poo in your compost). I came to the conclusion that on balance it was better for the cat, the environment and me for the cat to go outside during the day and inside at night.
Rebecca