 On migration and in winter , a wider range of wet habitats may be used , including flooded thickets or bracken . Freezing condition may force birds into more open locations such as ditches , rubbish dumps and gardens , or even out onto exposed ice . A Welsh study suggested that individual winter territories overlap , with each bird using a significant proportion of the reed bed . After site desertion in freezing weather , birds return to their former range . A density of 14 birds per hectare ( 6 @.@ 6 per acre ) was recorded . Birds wintering in Iceland rely on warm geothermal streams , and may access streams through tunnels under the snow . When not feeding , they may shelter in holes and crevices in the solidified lava . This species sometimes wanders well outside its normal range and vagrants have been found in the Azores , Madeira , Mauritania , the Arctic , Greenland , Malaysia and Vietnam . 
