You're sitting in your easy chair, engrossed in a book, while your cat suns himself on the windowsill. Suddenly, he startles you from your reading when he begins yowling and swatting at the window, obviously reacting to something or someone outside.
When your housecat sees another cat outside the window, it can bring up feelings of aggression. After all, unlike cats who live outdoors, your housecat had his territory thrust upon him when you brought him home. He had no choice but to assume that everything within his line of sight automatically was his domain. If he sees a strange cat or dog through the window in "his" yard, his first reaction might be to warn off the intruder with a menacing yowl, accompanied by a few low, guttural growls, some hissing and a few smacks on the window with his paw to show he means business.
Watch Out for the Other Signs
If an interloping cat or dog visits your yard regularly, mostly for the purposes of irritating your cat, who obviously can't get out of the house to make good on the threats he's made, it can push your normally mild-mannered house kitty to the point of acting out. He might become aggravated in general, but more so when he spies his nemesis in the yard. This could cause him to attack other pets in the house randomly and possibly humans, too. Your perfectly potty-trained puss also might take to marking his territory, which in this case is your previously fine-smelling home. If your cat resorts to spraying in your house, you might notice the smell before you see the stains on the walls under the windows and maybe even near the door.
A Heated Concern
Maybe your yowling cat is female. There's still a chance that she could be territorial, but with a female cat who hasn't been spayed there is another reason she may yowl when looking out the window. Girl kitties in heat have the need to tell the world that they are available and looking for a mate. Whether the window is open or not, if your cat is in heat you're likely to find her sitting on the sill and announcing her status to the entire neighborhood -- especially if she happens to spy a potential suitor just outside.
"Fixing" the Problem
Whether your cat is male or female and the cause of the windowsill yowling-fest is aggression-based or is brought on by a heat cycle, you usually can fix the problem by fixing the cat. Once a cat has been spayed, she will no longer go into heat and won't have the need to call out to any and all available males. Additionally, the majority of cats who spray, male and female, drop that habit entirely once they are altered because territorial aggression and the drive to protect their domain typically subsides. The earlier a cat has the operation, the less likely it is that aggression or heat-driven caterwauling will even become problems. Using the same type of anesthesia that doctors use when operating on human babies, kittens as young as eight weeks old can be spayed or neutered. A cat can have the alteration at any time in his life, but vets recommend having it done by six months of age.