Everyone knows I love birds. I would not be doing this if I did not. Many people do not know I also love cats. My daughter also loves animals. I'm certain she inherited love of animals from both her mother and me. She wanted to save the world actually. She's an adult and belongs to a sea turtle rescue organization. She has, over the years, rescued five feral cats and somehow they have ended up in my backyard. Spayed and neutered of course, and well fed and taken care of. So the question is how can a lover of both birds and cats reconcile the activities of both?
There are some organizations that promote keeping cats indoors. This really is a good idea for domestic cats but for cats that have grown up feral, and there are admittedly too many, that's not an option. If you live in a neighborhood with neighbors close by you'll always have a few that let their cats roam and there's little you can do about that either. So what does someone who likes birds do when there are feral cats around or when the neighbor's cat comes into your yard looking for a snack?
Ideally you should place your birdhouse as high into the bird's habitable zone as you can. What I mean by a habitable zone is that height off the ground that a bird prefers. For example a Violet-Green Swallow prefers to be 5-15 feet above the ground while the Indigo Bunting prefers 2-10 feet. If you're trying to attract the Swallow place the house 15 feet above the ground and for the Bunting 10 feet. This puts as much distance between your birds and any marauding cats as possible.
If you're mounting your birdhouse on a pole you have other options. Imagine if you have a 4 "x 4" post in the ground and 8 feet tall. Go to your local hardware store and purchase a can of foam filler and a PVC pipe 5 "in diameter and 8 foot long. Before mounting the birdhouse put the PVC pipe over the post and sink it 6" into the ground. Then use the foam to seal the exposed end of the pipe so water can not get into it and it's not a trap for small birds that might fall from the nest. The foam will expand and seal the pipe. It can also be trimmed and painted to suit your landscape. This will prevent cats, and even squirrels, from climbing to your birdhouse and attacking your birds.
Even with the best precautions you're going to have fatalities. Be prepared for them. While we do not like it it's a fact of life and there's only so much that can be done to prevent it. We all love nature and most of us do not flinch when an Eagle or Osprey snatches a fish from the water. Yes, they are hunting and no one is feeding them like they do domestic cats. However cats can not ignore their nature. It's what they do. Take all the precautions you can and be satisfied that you are doing your best to help our feathered friends survive and thrive. When you do have the unfortunate loss see if there's more you can do to prevent it, maybe put up another birdhouse. Do not dwell on it because you are doing all you can.