Ambient Temperature and the Daily Energetics of Two Species of Hummingbirds, Calypte anna and Selasphorus rufus

Abstract

Anna (Calypte anna) and rufous (Selasphorus rufus) hummingbirds exhibited regular diurnal changes in body mass, locomotor activity (flight time and frequency), and feeding activity (frequency and consumption) at ambient temperatures (Ta) from 38 to −1 C. Body mass of both species increased gradually throughout the day and amounted to increases of as much as 16.4% in C. anna (7 C) and 10.8% in S. rufus (22 C). These changes in body mass were related to marked decreases in flight expenditure in the afternoon: birds of both species flew only 15%–47% as frequently after midday as they did in the morning at all temperatures. Of these afternoon flights, a higher percentage were directed toward feeding. Calypte anna increased daily mass gain (DMG) with exposure to cold (DMG = 0.01 Ta + 0.49) by simultaneously increasing energy intake and reducing flight expenditures. Selasphorus rufus, which did not significantly increase in energy intake at Ta below 21 C, had approximately constant DMG at Ta from 28 to −1 C. However, overnight mass loss (OML) of S. rufus decreased when birds were exposed to cold (OML = −0.004 Ta + 0.36), while OML of C. anna increased at low Ta (OML = 0.006 Ta + 0.09). Reduction of overnight expenditures at low Ta by S. rufus may be attributed to longer torpor periods than were exhibited by C. anna. These two hummingbird species employed different strategies for maintenance of positive energy balance in this study: C. anna behaviorally regulated energy stores, maximizing daily net energy gain by manipulating both energy gains and expenditures; S. rufus relied more on physiological means (i.e., torpor) to conserve energy overnight. The latter species either behaviorally does not or physically cannot (under the conditions of this experiment) increase its daily energy stores at low Ta.