Cats love bird watching. We are lucky to live in a building with big trees right outside in perfect view from our windows. Our cats watch squirrels and all kinds of birds going about their day. On occasion, a bird might even fly to our air conditioner or ledge. Typically, the cats are frozen, watching intently when that happens.
Putting a cat tree in front of windows is a great way for cats to hang out and relax watching the goings on outside. To ensure safety, make sure you have screens on your windows.
Cats need more mental stimulation than most people think. Watching birds is like Cat TV. It's a great way to enjoy some down time.