Communal roosting by White Wagtails (Motacilla alba alba L.) in glasshouses is now known to occur in the Netherlands since at least six years. It has been recorded from one place in the province of Gelderland and from at least 6 sites in Noord-Brabant. The numbers of birds on then roosts varied from tens’ till at least 500 per night. The peak may be reached either in August or in the first half of September. The nurserymen disliked the birds because they fouled the tomatoe plants. The best way to prevent this damage is to keep the windows shut during about three hours around dawn. May be recoveries of some of the 160 birds caught by ringers in two of the glasshouses will give us some indication of the origine of these wagtails.