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Petsmart makes a wonder deterrent for the feline species.Here is a link to the device:http://www.petsmart.com/product/index.jsp?productId=2752970&f=PAD%2FpsNotAvailInUS%2FNo
Many home owners also put in motion activated sprinklers to keep cats out of the yard.
Depending on nesting area's, you could put a wide piece of aluminum or any slippery surface around tree's so the cat will not be able to climb up.
Plants that are natural repellents to cats include Coleus canina, commonly known as the "scaredy cat plant". Also, lavender, rue, and pennyroyal deter cats.
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