One of the midwest's largest heron colonies, the Baker's Lake Heron Rookery in Illinois, was once a tree covered small island. It took about 20 years for all of the trees to die from the bird droppings. As the trees died they were replaced in the off-season by man-made nesting structures. Today the island has no trees but still is home to hundreds of occupied nests. See the attached link for photos. Allan Welby Ridgefield http://www.dailyherald.com/article/20110516/news/705169957/