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With cold weather coming, officials at the National Zoo are offering city residents some “wildlife-friendly” tips for the fall season, including advice on what to do with leftover jack-o’-lanterns and fallen leaves. In short: Avoid trash cans; aim for the yard. The Zoo’s Web site advises urban dwellers to dump their raked leaves and leftover pumpkins in secluded areas of their property to create a “safe, warm haven for overwintering insects and other backyard critters.” In terms of the District’s delicate ecosystem, of course, that means rats. Dale Kaukeinin, a rat biologist, says that fall is a difficult season for Rattus norvegicus. But the zoo’s advice could make things easier by providing the District’s vermin with better ground coverage and an extra source of food. “Rats like to eat pumpkin,” says Kaukeinin. “It’s an excellent bait. I’ve used it in traps myself.” —Felix Gillette