Diurnal, Seasonal and Altitudinal Variation in Energy Reserves of Wintering Snow Buntings

Abstract

We examined the winter body mass patterns of a northern, ground-feeding passerine, the Snow Bunting Plectrophenax nivalis, which we attracted to artificial food sources in three different altitudinal habitat zones in north-east Scotland. Despite similar patterns of food availability in mid- and late-winter, Snow Buntings appeared to regulate their energy reserves by gaining body mass more quickly during the shorter days of mid-winter than during the longer days of late-winter. The increased rate of mass gain was more than sufficient to offset the longer periods of overnight fasting, so that dawn body mass peaked in mid-winter: the birds therefore showed true winter fattening. Birds also carried more reserves at higher altitudes. These seasonal and altitudinal trends suggest that Snow Buntings increase their reserves when the risk that food will become unavailable increases. However, we calculate that most birds still need to feed every day to avoid starvation.