Shamrock Island is the remainder of a re-curved barrier spit that once extended southwestward from Mustang Island into Corpus Christi Bay. Shamrock Island was formed when Hurricane Celia breached this re-curved barrier spit in 1970. Since the breaking of the land bridge, Shamrock Island has become one of the most important colonial bird nesting islands on the Texas coast. Up to 21 bird species, including the threatened reddish egret and white-faced ibis, nest on the island.
The island includes high quality beach habitat used by four species of terns (gull-billed, Sandwich, Caspian and royal). In some years, royal terns number close to 8,000 on Shamrock Island, making it by far the most important nesting site for this species on the Texas coast. Black skimmers also use the shell beach sites. The preserve's mudflats are ideal wintering habitat for piping plovers and other shorebirds. The aforementioned shorebirds and abundant waterfowl are an attractive food source for peregrine falcons during winter. Northern pintails, redheads, common goldeneyes and buffleheads are commonly recorded waterfowl on the preserve.
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