A large-scale seaward extension of the Port of Rotterdam will cause a decline in feeding habitat for the internationally important populations of Sandwich Tern Thalasseus sandvicensis and Common Tern Sterna hirundo breeding in the N2000 SPA ‘Voordelta’, the Netherlands. Several measures have been taken to counteract the potential negative effects on these colonies. The efficiency of these compensation measures is studied in a multidisciplinary research programme on both biotic and abiotic components. Part of this programme is focused on the breeding and feeding ecology of both tern species. This poster shows the results of the first year of this five-year programme. Diet, feeding ecology and feeding behaviour was studied from a hide placed next to an enclosed part of two colonies (Scheelhoek and Slijkplaat) during the chick rearing period of 2009. Amongst others, time absent from the colony and species and length of prey brought to the chicks were recorded. In addition 15 Sandwich and 30 Common Terns were ringed, coloured and tagged with radio-transmitters. Automatic receivers in the colonies registered presence of tagged terns continuously. Combined with aerial tracking and land-based tracking from the coast spatial distribution of terns in their feeding habitat was studied.