SEASONAL-CHANGES IN BODY-MASS AND USE OF TORPOR IN A MIGRATORY HUMMINGBIRD

Authors
Citation
S. Hiebert, SEASONAL-CHANGES IN BODY-MASS AND USE OF TORPOR IN A MIGRATORY HUMMINGBIRD, The Auk, 110(4), 1993, pp. 787-797
Citations number
44
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Ornithology
Journal title
ISSN journal
0004-8038
Volume
110
Issue
4
Year of publication
1993
Pages
787 - 797
Database
ISI
SICI code
0004-8038(1993)110:4<787:SIBAUO>2.0.ZU;2-B
Abstract
In a study designed to determine seasonal patterns of body mass and to rpor in Rufous Hummingbirds (Selasphorus rufus), birds were maintained for 12 months in the laboratory on a photoregime approximating that e xperienced by free-living birds. Ambient temperature cycled from 20-de grees-C during the day to 5-degrees-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 ar e 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 autum n, suggesting that torpor is not reserved for immediate energy crises at this time, but may be important in maximizing energy savings and th us 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 intermed iate and use of torpor was also significantly lower than in autumn. Ma ss-specific rates of oxygen consumption during both normothermia and t orpor were inversely related to body mass when data from all seasons w ere combined; large fat stores may contribute to lower metabolic rates by providing additional insulation, as well as by decreasing the prop ortion of highly metabolically active tissue in the body. Low fat stor es also coincide with the molt, which itself may result in higher meta bolic rates. Although the propensity for using torpor has a strong sea sonal component that appears to reflect different energetic circumstan ces during such activities as migration and molt, Rufous Hummingbirds retain the ability to enter nocturnal torpor at all times of year, thu s improving their chances of survival year-round.