The breeding biology of common terns (Sterna hirundo) and herring gulls (Larus argentatus) has been studied at colonies in the lower Canadian Great Lakes. A principal objective was to identify the biological, physical and chemical factors that influence the reproductive success of these species. With minimum modification the technique was applied to a herring gull colony and could be used readily with any colonial, group-nesting bird species. This note describes the monitoring technique and presents some initial data showing its usefulness. The technique described, if used in conjunction with visits to the colony to check operation of the equipment, provides valuable additional information to the usual demographic data collected from seabird colonies for an assessment of reproductive performance.