This Inland Least Tern (Sterna antillarum athalassos) is listed as endangered and prefers to nest on isolated sand bars along the lower Mississippi river.  While concentrated in their nest colonies they can easily be counted and their population status monitored on a yearly basis.  For more information on the Least Tern or other endangered species, see this newspaper article. or view this powerpoint slide show, or check out this USFWS site http://mountain-prairie.fws.gov/southdakotafieldoffice/tern.htm