The time-activity budget of breeding C. pusilla was studied at Barrow, Alaska, USA, in 1973 by observing several individually marked birds. Feeding, incubating and young-attending were the most time-consuming activities. About 70 and 80% of the time was spent feeding by the female during laying period and by both sexes prior to southbound migration, respectively. Up to 50% of the time was spent incubating or attending young during the incubation and the young-attending periods. The time-activity budget was converted into an estimate of energy requirements by using available estimates for energistic cost of activities. Both environmental conditions and activity patterns changed throughout the season. The daily energetic cost fluctuated between only 159 kJ/d [day] during the prelaying stage and 222 kJ/d during the laying stage for the female, and between 155 kJ/d during the prelaying stage and 167 kJ/d during the territorial stage for the male. Because females leave the area about the time of hatching, a female requires less energy from the tundra than a male; on the average, .apprxeq. 7.1 .times. 103 and 8.0 .times. 103 kJ were required during the whole season by female and male, respectively. Energy uptake was estimated by using available data on diet composition, the energy content of prey items and their digestibility, and by combining them with observed feeding rates and estimated capture rates. The resultant energy budget was generally negative for the whole season and female weights declined significantly. Either energy uptake was underestimated or the birds rely heavily upon resources obtained at wintering grounds and during migration prior to arrival at Barrow. Females, having an average daily energy requirement 15% higher than males, and entering the young-attending stage with an accumulated energy deficit, increase their fitness by deserting to better feeding grounds rather than staying with the young after they hatch. Females enhance their own survival to the next season. A preliminary comparison of the time-activity budget of C. pusilla with that of 2 rather unrelated species [Calcarius lapponicus and Phalaropus fulicarius] revealed a remarkable similarity in the pattern of time allocation.
