Horse Island, at 17 acres (69,000 m²), is the largest of the Thimble Islands off Stony Creek, a section of Branford, Connecticut. It is owned by Yale University and is maintained as an ecological laboratory by Yale's Peabody Museum of Natural History. It was purchased and donated to the university in 1971 as a convenient addition to the Yale Coastal Field Station in nearby Guilford, which has its own dock and boats.  Since then, however, the thrust of biological research at Yale and elsewhere has turned in the direction of molecular biology[ citation needed], and research conducted on the island is less active.
A large house on the island has been kept in good repair and can serve as a base for experimenters or others remaining on the island for overnight stays or longer.
The origins of the island's name are not known. Some say that at some point a cargo of horses may have found its way to the island from a capsized or wrecked ship, but no physical or documentary traces of such an event exist. Others speculate that the island's horseshoe shape may have influenced its name. For many years it was owned by an executive of Standard Oil named Clark, and was called "Clark's Island" by everyone. When James and Esther Rettger purchased the island in 1946 they restored the original name as found on the geodetic survey maps.
- Laskin, Daniel (14 December 1971). "Biology Field Station Buys Island". Yale Daily News. Retrieved 23 April 2014.
- Wealthy Widow Buying Up Thimbles, "New Haven Register", January 22, 2006, page A1
- Half a Mile Off the Coast; Stacey Stowe; "In the Region/Connecticut", New York Times, July 30, 2006; Real Estate page 10.
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