Seasonal Changes in Body Mass and Use of Torpor in a Migratory Hummingbird

-In a study designed to determine seasonal patterns of body mass and torpor in Rufous Hummingbirds (Selasphorus rufus), birds were maintained for 12 months in the laboratory on a photoregime approximating that experienced by free-living birds. Ambient temperature cycled from 20?C during the day to 5?C at night. Body mass, torpor, and rates of nighttime oxygen consumption were measured under conditions of ad libitum feeding in LD 12:12 in autumn (when free-living birds are normally migrating south), LD 12:12 in spring (during molt), and LD 16:8 in summer. Both body mass and use of torpor were highest in autumn, suggesting that torpor is not reserved for immediate energy crises at this time, but may be important in maximizing energy savings and thus minimizing the time required for premigratory fattening. In spring, body mass was lowest; use of torpor, however, was significantly lower than in autumn, suggesting that torpor is used primarily for "energy emergencies" at this time of year. In summer, body mass was intermediate and use of torpor was also significantly lower than in autumn. Mass-specific rates of oxygen consumption during both normothermia and torpor were inversely related to body mass when data from all seasons were combined; large fat stores may contribute to lower metabolic rates by providing additional insulation, as well as by decreasing the proportion of highly metabolically active tissue in the body. Low fat stores also coincide with the molt, which itself may result in higher metabolic rates. Although the propensity for using torpor has a strong seasonal component that appears to reflect different energetic circumstances during such activities as migration and molt, Rufous Hummingbirds retain the ability to enter nocturnal torpor at all times of year, thus improving their chances of survival year-round. Submitted 14 May 1992, accepted 12 November 1992. MIGRATORY HUMMINGBIRDS living in cool climates face potentially extreme threats to energy balance. Like other small homeotherms, they have high thermoregulatory costs resulting from high surface-to-volume ratios that increase rates of heat loss to the environment. Like other hummingbirds, they incur the additional cost of hovering, the most energetically demanding mode of flight. Unlike the many species of hummingbirds that are year-round residents in lowlatitude habitats, however, the relatively few migratory species have an additional energetic requirement-they need to accumulate large fat stores to fuel long-distance flight. Nocturnal torpor, during which energy expenditure is greatly reduced, is an important means by which hummingbirds offset large daytime energy expenditures and maintain energy balance. All species of hummingbirds that have been studied show this physiological ad' Present address: Department of Biology, Swarthmore College, Swarthmore, Pennsylvania 19081, USA. aptation. Most studies, however, have focused on instantaneous features of torpor (e.g. comparisons between rates of metabolism during steady-state torpor and steady-state normothermia), or on the effects of torpor on total nighttime energy balance (Hiebert 1990). Little consideration has been given to the fact that hummingbirds, particularly species residing at least part of the year in temperate zones, engage in activities that place very different energetic demands on the animal at different times of year. Therefore, one might expect there to be corresponding seasonal changes in the use of torpor. Yet, with the exception of Carpenter's (1974) study of the Andean Hillstar (Oreotrochilus estella), there have been no systematic comparisons of seasonal changes in the use of torpor. The seasonality of torpor has never been studied in a migratory species. The Rufous Hummingbird (Selasphorus rufus) is one of the most highly migratory species of hummingbirds. It breeds at high latitudes, as far north as Alaska, but winters in Mexico. Yearly round-trip migratory flights may exceed 6,000 km (Phillips 1975, Calder 1987). Because of their