9321759 Ricklefs Both energy and mass constraints are continually interacting to influence the ecology of organisms. No where is this more clearly demonstrated than in nesting tropical seabirds where there is strong selection to store energy for the incubation shift and also to maintain water balance in this hot, harsh, environment. In the Northwestern Hawaiian Islands, Wedge-tailed Shearwaters compete for nesting burrows, and those without burrows (44% of the population) must nest on the ground surface. If these sites have little protection from solar radiation, only 5-11% will produce chicks. In contrast to shearwaters, both Sooty Terns and Brown Noddies regularly nest in the open sun and 85% of the nests produce chicks. To better understand this disparity in ability to nest in a hot environment, the PI has constructed a conceptual model that links energy and material balances so that their interactions can be clearly understood. Since 1990, the PI has been collecting data to build this model and hopes to finish it this year by determining the total body water, water flux, and daily energy expenditure of shearwaters, terns, and noddies at nest sites varying in amount of shade. These determinations can be made by laboratory analysis of blood samples collected in 1991 from birds injected with deuterium or doubly-labeled water. In addition, the PI hopes to analyze the composition of meals, nasal secretions, urine and feces previously collected from the above relative importance of various energy and water balance variables. Some characteristics that may prevent shearwaters from successfully nesting in hot microclimates are long incubation shifts (9 days), infrequent drinking of seawater, and the formation of stomach oils that reduce the amount of gastric space available for water. The model of interacting balances, formed in this study, should be easily modified to evaluate the effect of other types of competing balances on the ecology of organisms. ***
