DIURNAL, SEASONAL AND ALTITUDINAL VARIATION IN ENERGY RESERVES OF WINTERING SNOW BUNTINGS

We examined the winter body mass patterns of a northern, ground-feedin g passerine, the Snow Bunting Plectrophenax nivalis, which we attracte d to artificial food sources in three different altitudinal habitat zo nes in north-east Scotland. Despite similar patterns of food availabil ity in mid-and late-winter, Snow Buntings appeared to regulate their e nergy reserves by gaining body mass more quickly during the shorter da ys of mid-winter than during the longer days of late-winter. The incre ased rate of mass gain was more than sufficient to offset the longer p eriods of overnight fasting, so that dawn body mass peaked in mid-wint er: the birds therefore showed true winter fattening. Birds also carri ed more reserves at higher altitudes. These seasonal and altitudinal t rends suggest that Snow Buntings increase their reserves when the risk that food will become unavailable increases. However, we calculate th at most birds still need to feed every day to avoid starvation.