This island is a small, treeless, granite dome in southern Penobscot Bay.
This site was once a significant nesting site for Double-crested Cormorants with over 300 nesting pairs in the mid-1980s. The most recent estimate in 1999 revealed a decline to only 70 nests. This also is the northernmost nesting location for Snowy Egrets. However, the most recent breeding record for Snowy Egret was in 1995 and the site has not been surveyed since 1999.
Typical of other islands in this area, threats from an oil spill and other forms of pollution are the greatest concern. Steep rock faces surrounding much of the island serve as a deterrent to visitors landing at this site.
Owned by the State of Maine, the Maine Dept. of Inland Fisheries and Wildlife has management authority for Marblehead Island as part of the Coast of Maine Wildlife Management Area. This island is closed to visitation from April 15 to July 31 each year.