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The story of America’s squirrels goes back to the early 19th century. At that point in time, squirrels were just another animal running around the woods, mainly useful as a source of food for frontiersmen. If you saw a squirrel in the city, it was almost certainly being kept as a pet. One escaped pet squirrel in New York City, circa 1856, drew a crowd of hundreds according to one of the city papers—which called the squirrel an “unusual visitor.”

Around the same time, a sea change in our relationship with squirrels was already underway in Philadelphia. The city had released three squirrels in Franklin Square in 1847 and had provided them with food and boxes for shelter—and the people loved it. One visitor is quoted as saying “it was a wonder that [squirrels] are not in the public parks of all great cities.” In the years that followed, the trend spread to Boston and New Haven, where squirrels soon grew so fat from humans feeding them that they were falling out of the trees. Cities even started planting nut-bearing trees so that the squirrels would have their own food source.

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– http://gizmodo.com/the-fascinating-story-of-why-u-s-parks-are-full-of-squ-1478182563