A list of 284 species was compiled in the mid-1990s by Robert Winkler, with contributions from Richard Soffer, Frank Mantlik and Charles Barnard Jr. The list published in 1996 is available here
Additions since 1996 (shown in red): Ross’s Goose, Yellow Rail, Marbled Godwit, Black Skimmer, Cave Swallow, Dicksissel, Clay-colored Sparrow, Smith’s Longspur, Yellow-headed Blackbird, Boat-tailed Grackle, Western Meadowlark and Yellow-throated Warbler – all Very Rare. And also: Nelson’s Sharp-tailed Sparrow, common in Fall.
Note: Robert Winkler’s birding experiences at Sherwood Island are recorded in his book Going Wild and in essays available at his website.