Mass Audubon staff in partnership with USGS have used satellite telemetry to gather information on nighttime roosting locations for Long-tailed Duck (LTDU) (Clangula hyemalis), especially with respect to Horseshoe Shoal, the proposed site of a 130-turbine wind farm. Hundreds of thousands of LTDUs overwinter in Nantucket Sound. These LTDU Nantucket display commuting behavior: each day at dawn, thousands of LTDU exit the Sound traveling to feeding sites on Nantucket Shoals, returning to the Sound at dusk. Mass Audubon and the applicant for the Cape Wind project have conducted extensive aerial surveys of Nantucket during daylight hours when the vast majority of LTDUs are absent from the Sound. Previously there was little direct information available regarding the nighttime roosting locations of LTDUs, especially in relation to Horseshoe Shoal. In particular, we needed to know whether Horseshoe Shoal is used by significant numbers of LTDUs as a nighttime roosting site or staging area as they exit or return to Nantucket Sound. If the answer to this question was yes, then the LTDUs would be potentially at risk of collision with the wind turbines when they enter, exit, or otherwise move within the project area. A principal goal of our avian monitoring has been to obtain a reliable baseline estimate of the relative abundance of wintering sea ducks that will, in future surveys, allow us to detect and interpret changes in bird abundance and distribution in Nantucket Sound.