A cat does not need claws to kill a bird. Biting it's neck and shaking it until it is dead is how a cat kills a bird. A cat can pounce on and hold down a bird with it's paws (even without claws) and body. It's not so much that there is something in the saliva that will kill the bird, it is puncturing the bird's skin and maybe hitting a vein that will kill the bird, along with shaking it to break its neck. Then of course, the cat might eat the bird, so then it will really be dead. So, cutting off the ends of a cat's paws will not stop the cat from killing or injuring birds.
BTW, your saliva will kill birds also. The saliva of all mammals has bacteria that will kill birds. So, for example, you cannot take a bite of a piece of fruit and then offer it to your bird to eat.
Birds are natural prey of cats. Unless the cat was raised with and trained as a kitten to not see birds as prey (which likely will not be 100% effective anyway), birds need to be kept totally secure from meetings with cats. Natural instincts are very strong so even if you think a cat has been trained not to hunt birds, you can never be sure it won't happen.
As to your supposition that some vets might "declaw" (really should be called detoe) humanely, cutting off the ends of an animals toes is not humane, no matter who does it. Unlike humans, cats walk on their toes. When the ends of their toes are amputated, they cannot walk as God/nature created them to walk. This throws their bodies out of natural alignment, and does cause sore muscles, at first anyway. Cats use their claws to hook into something (hopefully that will be a rough textured, tall scratching post) to pull against to stretch their shoulder muscles to work out tension. Isometric exercise, it is called. Without claws, they cannot do that. The poor declawed cats then can't even work out the tension in their muscles that declawing has caused due to forcing them to walk in an unnatural alignment.
I have five cats, and many scratching posts/pads in several rooms. My cats do not scratch my furniture. You need to get some horizonal and some vertical scratchers. They need to be in the rooms the cats frequent, not just in one place. My cats love their scratchers, and use them numerous times daily. They really like the cheap corrugated cardboard scratching pads sold even in grocery stores.
I am so glad that you are seeking information before getting the cat. That is very responsible and caring.