A Balance for Weighing Ground-Nesting Birds

Abstract

SUMMARY A nest balance for weighing incubating birds repeatedly in the field has been success- fully used with herring gulls (Larus argentatus) and barnacle geese (Branta leucapsis). Three methods of recording data from the balance are discussed. Periodic weighing of animals is easily and profitably performed in the laboratory but is often more difficult in the field. The technique presented here allows the weight of a bird to be calculated during the incubation season by placing its nest on a tray on a balance buried in the ground from which remote readings of weight may be made. Successful calculation of the weight of the bird depends on knowing the weight of the nest. This may be obtained directly when the bird leaves the nest, or indirectly by allowing for changes in moisture content of the nest that have occurred since an earlier measurement. The construction of the balance is shown in Figs 1 and 2. The accuracy of the system depends critically on four factors: that the centre-lines of the springs be equidistant from the centre-line of the potentiometer, that the springs and the flexible coupling hang vertical, that the springs have the same elasticity, and that the length of the flexible coupling be unaffected by moisture. Twelve balances were made and were found to have a linear response over a 5 kg range, with a standard deviation about the regression line of approximately 15 g (range 7-30), which represents the limiting accuracy of the balances we constructed. Balances were assembled and calibrated prior to use. Installation of a balance beneath a nest takes about an hour, and with care can be done with little change visible to the human eye. The first few feet of the cable leading from the potentiometer to the recorder were buried. In 1976 ten, and in 1977 nine balances were placed under herring gull (Larus argentatus) nests in a fenced study area in the large mixed colony of herring and lesser black backed gulls (L. fuscus) on Walney Island, Cumbria. Measurements were taken auto- matically every 5 min by data-logger in a cottage 200 m from the birds. To correct for variation in moisture content one balance was loaded with an artificial nest and the amounts of nest material on the other balances were adjusted to be roughly the same. The operation of the balances was checked and lost eggs were replaced by fresh ones once a day.