WELCH, K.C.*; SUAREZ, R.K.; Univ. of California, Santa Barbara; Univ. of California, Santa Barbara: Independent and Combined Effects of Temperature and Altitude on Hover-feeding Metabolic Rate in Migratory Rufous Hummingbirds, Selasphorus rufus

Because of their small size and energetically expensive mode of locomotion, hummingbirds display high rates of energy metabolism. Thus, hummingbirds represent an ideal system in which to gain understanding of how animals achieve energy balance. Migratory rufous hummingbirds (Selasphorus rufus) arrive in flowering meadows throughout the Eastern Sierra in the summer and spend a few weeks gaining mass in preparation for the next period of migratory flight. During refueling, S. rufus achieve high rates of net energy intake and mass gain, despite the fact that they are often foraging at high altitudes and low ambient temperatures. In order to determine the combined as well as independent effects of increasing altitude and decreasing ambient temperature on hover-feeding metabolic rate, we measured oxygen consumption rate on captive S. rufus at several locations along an altitudinal gradient and at a variety of ambient temperatures. Among S. rufus in general, decreasing ambient temperature significantly increases hover-feeding metabolic rate. Additionally, increasing altitude also significantly increases hover-feeding metabolic rate. Data indicate that the effects of these two environmental variables are additive. Scaling of hover-feeding metabolic rate as a function of temperature or altitude varies between the sexes and between juveniles and adults. The possible effect of wing morphology on the differences in scaling of metabolic rate versus temperature or altitude is discussed. In addition, these differences are evaluated in light of potential differences in territorial dominance and foraging strategy.