Fourmile Island Rookery State Natural Area is a protected 15-acre, forested island in the southern half of Horicon Marsh within the Horicon State Wildlife Area, about 3 miles north of Horicon in Dodge County, Wisconsin. One of the largest heron and gull rookeries in the Midwest, it also supports a major heron and egret nesting colony in trees such as oak, basswood, elm, aspen, and cottonwood. Reached only by the Rock River, it is closed to the public from April 1 through August 24 to protect nesting birds. A July 1984 windstorm, Dutch elm disease, and heron guano reduced nesting trees, leading to the addition of artificial platforms.