Water Economy of Nestling Swainson's Hawks

Abstract

Six major components of the water budget of nestling Swainson's Hawks (Buteo swainsoni) were quantified to assess the influence of heat stress and dietary composition on water balance. Rates of food consumption, energy utilization, excrement and pellet production, and growth were measured in food trials. Rates of evaporative water loss and CO2 production were measured gravimetrically at 25, 35, 42, and 300C (plus direct sunlight) with an open flow system. The inverse relationship between body fat content (or energy density) and body water content in small mammalian prey has: (1) a dramatic effect on the magnitude of preformed water consumption, (2) a lesser effect on excretory water loss, and (3) a negligible effect on metabolic water production. The consequence of these relationships is that nutrient composition of the prey has a significant influence on the status of nestling water balance. Low fat diets should supply nestlings with a surplus of water, whereas high fat diets would not provide enough water to meet the minimum water needs of the rapidly growing nestlings. Water incorporated into new body tissue and water loss in excrement and pellets increased to a peak when nestlings were about 3 weeks old, an age when their rates of growth and food consumption were near maximal levels. Rates of preformed water consumption also reached peak levels during this period of maximal food consumption. Metabolic water was 10-19% of the total water input and was generated by nestling metabolic rates considerably higher than predicted by allometry for adult falconiform birds. Basal rates of evaporative water loss of 700-g nestlings at 250C were 75% higher than rates predicted by allometry for equally large adult birds. The largest nestlings (782-935 g) had evaporation rates at 25C nearly three times the rates predicted by allometry.