At 31.5 acres, Calf Island is the largest offshore island in Greenwich, CT. It is located directly south of Byram Harbor, approximately 3,000 feet from the mainland, and is connected at low tide to the Greenwich Land Trust's Shell Island.

Calf Island boasts a diverse coastal habitat including tidal wetlands, intertidal flats, rocky intertidal shore, sandy beach, mixed forest, and coastal shrubland. The island provides excellent wading bird habitat, and nearby Great Captain's Island contains one of the largest heron and egret rookeries in the Long Island Sound. Calf Island also contains several significant geological features and offers a glimpse into the geological history of the region.

In February 2003, Calf Island was acquired by the U.S. Fish and Wildlife Service and is now a unit of the Stewart B. McKinney National Wildlife Refuge. The Refuge is managed as a part of the National Wildlife Refuge System which includes almost 550 refuges located in every state in the Nation and totaling nearly 100 million acres.

The Stewart B. McKinney National Wildlife Refuge presently consists of ten separate units located along 70 miles of the Connecticut coast between Westbrook and Greenwich. The Refuge manages over 1000 acres including eight islands within the Long Island Sound, a barrier beach, tidal salt marsh and uplands,. Located within the Atlantic Flyway [view], the Refuge provides important resting, feeding and nesting habitat for many species of wading birds, waterfowl, songbirds, and shorebirds, and terns, including the endangered roseate tern.

Calf Island looks out on the Captain Islands and Greenwich Point, which were formed 20,000 years ago where the Wisconsin glacier paused as it retreated north. In contrast, Calf Island is built on bedrock or ledge, although there is plenty of eroded glacial drift around the island. The bedrock is most visible at the southerly extremities of the two areas of upland; it is a coarse-grained, highly-stratified metamorphic rock (Golden Hill Schist) created from the bottom of the ocean that separated North America from Africa 500 million years ago before the two continents came together as part of the supercontinent Pangaea.

Today, Calf Island provides roosting and foraging habitat for the waterbird colony at Great Captain's Island, located one mile from Calf Island. Although colony numbers have fluctuated over the years, Great Captain's is the site of the largest wading bird rookery in Connecticut, with approximately 300 nesting pairs of great egrets, snowy egrets, and black-crowned night-herons. Wading birds frequently utilize the Calf Island salt marsh for feeding and tall trees for roosting. Also of significance are the colonies of common terns, double-crested cormorants and gulls on nearby Bowers and Diving Islands.

Development along the Connecticut shoreline makes the conservation of Calf Island important for healthy waterbird populations. Since approximately 97 percent of Connecticut's coastline is developed, protected islands are vital to local and migratory birds as habitat in a predominantly urban setting.