There are many good reasons to keep your cat out of the window sill: The window gets dirty, the curtains get furry and most importantly, it can lead to a dangerous fall. The American Society for the Prevention of Cruelty to Animals notes though cats are known for their great climbing and balance skills, their instincts make them vulnerable to high-rise syndrome, the trend of cats falling out of high-rise windows, balconies and other high perches.
Consider what is so alluring to your cat as she watches out the window: Birds are flying, squirrels are climbing, tree branches are swaying. Any number of things may draw her attention and tempt her to pounce right out the window. If a cat falls from a high perch, she may not land squarely on her feet, potentially causing a host of injuries. Apartment building cats aren't the only ones at risk from injury from a fall out the window; a spill from one or two stories also can be dangerous because the cat doesn't have ample time to position her body to absorb the fall.
Window Sill Deterrents
The first order of business is to discourage your cat's interest in the window sill. Remove the temptation by keeping the curtains or blinds drawn so she won't be enticed to the window sill. The ASPCA advises using spray bottles or sharp commands may only frighten a cat instead of teaching her to stay away from the off-limits area. Instead, the organization recommends devices that discourage her behavior, whether you're home or not. Motion sensitive deterrents sense her presence and respond with a quick spray of harmless gas or a mild static charge, such as the Scat Mat does, banishing her from the sill.
If you can, consider setting up a cat tree in front of an interesting window that can remain closed, allowing your cat the opportunity to engage with the outside world safely. Cats love to climb and see the world from a high vantage point. A high tree with multiple platforms and sleeping spots will provide a safe playground and viewing station for your cat.
You may have a particularly stubborn cat. If that's the case, keep the tempting spot secure just in case your deterrents aren't effective. All window screens should be sturdy and snugly in place. Cats can slip through childproof window guards, making them an unreliable method of keeping your cat in the window and off the outside ledge. If your cat is especially determined to pass the day in your window sill, no matter what roadblocks you put in her way, keep the window closed for her safety.