Abstract

Temperature and diet quality are two of the most important factors affecting the dynamic regulation of animal energy budgets. Because hummingbirds are very sensitive to energy stress, we used Green-backed Firecrowns (Sephanoides sephaniodes) to test the dynamics of their energy budget under different energetic challenges in chronic conditions (20 days). Experimental groups were: HQ-TNZ (high quality diet and thermoneutrality), HQ-LT (high quality diet and low temperature), LQ-TNZ (low quality diet and thermoneutrality), and LQ-LT (low quality diet and low temperature). Analysis of behavioral, morphological, and physiological variables revealed that thermal and dietary factors affect time and energy budgets independently. Hummingbirds increased energy intake during the first day of acclimation at LT, but after second day, the LQ-LT group did not maintain their energy intake and began to loose body mass. Moreover, diet quality affected digestive organs. The intestine, gizzard, liver and kidneys all increased in size when food quality was lowest, probably to obtain more food per feeding event and to more efficiently process the ingested food. Exposure to low ambient temperatures affected the most metabolically productive organs such as the heart, lungs, and muscular mass. Lower temperature increased basal and daily energy expenditure, and changed the time budget. Sephanoides sephaniodes spent more time perching when their energy balance was close to be negative. We suggest that energy budget regulation in hummingbirds does not reside exclusively in the energy output nor in the energy-input but in both pathways. © 2002 Elsevier Science Inc. All rights reserved.