Studies on small wintering birds often monitor body mass variation over days or seasons. Typically, birds are weighed manually or data are obtained using an electronic balance which displays the bird's mass during visits to a feeder. The problem with these techniques is that they require the manipulation of the animals and/or are labor intensive. We built an automated system which combines a transponder-based radio identification device (RID) and an electronic balance fitted with a weighing perch. This setup permits daily mass variation in birds to be monitored automatically without human presence. The system provides reliable estimates of inert masses (r² = 1). We also used it to monitor bird masses in an aviary. For five days birds were caught periodically and weighed manually to measure their “real” masses for comparison with the system's estimates. Again a strong relationship was found between calculated and real masses (r² = 0.92 and r² = 0.89 using two different balances). This system offers great promise and could be adapted to many forms of field work.