Most of the island is covered with trees. Tall hackberry trees make up most of the canopy, with an understory of poison ivy 12-feet tall. Great Solomon's seal reaches seven to nine feet in height, and a great variety of ferns, wildflowers, mushrooms, and other plant life abound. The island is composed of glacial fill over a limestone shelf. West Sister Island's rookery is a host for great blue herons, great egrets, snowy egrets, black-crowned night herons, and double-crested cormorants. The water around the island is too deep for the wading birds to feed in, so they must travel an 18-mile round trip to the mainland marshes to hunt for food for themselves and their young.