Seasonal variation in thermogenic capacity of migratory warbling vireos

-Seasonal physiological adjustments are similar for migration and winter acclimatization in passerine birds, as both involve endurance muscular activity. Consequently, migratory disposition should elevate maximal thermogenic capacity (VO,,m) under cold stress, as winter acclimatization does for most temperate-wintering passerines. This hypothesis was tested with a Neotropical migrant, the Warbling Vireo (Vireo gilvus), during migration and summer residency in southeastern South Dakota. Vireos were subjected to cold stress in an atmosphere of 79% He/21% 02 (helox), with concurrent measurement of oxygen consumption during spring, summer and fall. In the spring, vireos had a significantly higher VO,,,m (17.7%) and tolerated colder temperatures in helox than either summer or fall birds. Standard metabolic rate was 0.643 ? SD of 0.133 ml 02 min-' (n = 6) in summer, but could not be measured in spring and fall as vireos were nocturnally active and failed to attain steady-state conditions, presumably as a result of Zugunruhe. The increment of VO,tum in spring, apparently, was not a function of higher body mass in spring, given that mass did not vary significantly between seasons. Elevated thermogenic capacity in spring may be adaptive as chances of encountering cold weather during spring migration in the northcentral United States are greater than at other seasons. Alternatively, enhanced thermogenic capacity in spring may be related to differential migratory speeds between spring and fall, which might select for increased endurance (with thermogenic capacity increasing as a byproduct) in spring birds. The ubiquity of this pattern among migratory passerines is unknown. Received 12 May 1994, accepted 27 January 1995. SHIVERING THERMOGENESIS in cold temperatures and long-distance migration are energetically expensive endeavors for small birds. Since both sustained shivering and long-distance flight represent forms of endurance muscular activity, adaptation for flight might be expected to influence thermogenic performance, or viceversa. Indeed, such coupled changes in maximal capacity for thermogenesis and exercise (locomotion) have been detected for some mammals (Hayes and Chappell 1986), although this apparently is not universal (Conley et al. 1985). Similar studies have not been performed on birds, probably because convincing measurements for maximal metabolism during flight are nonexistent (Marsh and Dawson 1989). An indirect test for such coupled changes in maximal metabolism associated with flight and thermogenesis is to measure thermogenic capacity in birds in migratory disposition. These birds are presumably adapted for sustained flight, but are not exposed to the cold-temperature extremes faced by Temperate Zone residents during winter. Nevertheless, a number of physiological and biochemical adjustments associated with improved cold tolerance and migratory disposition are similar. Fat is the major fuel for shivering (Marsh and Dawson 1989) and winter-acclimatized passerines often show increased levels of stored fat relative to summer-acclimatized birds (Blem 1990), especially in species exploiting unpredictable winter food resources (Rogers and Smith 1993). Stored fat also increases markedly before migration in most small migrants, especially in those migrating long distances (Blem 1990), and fat serves as the major fuel for migration (Dawson et al. 1983). Winter-acclimatized passerines exhibit elevated pectoralis-muscle activities of enzymes involved in lipid and carbohydrate catabolism, particularly the former (Marsh and Dawson 1982, Yacoe and Dawson 1983), suggesting an increased reliance on fat during shivering thermogenesis relative to summer birds. An increase in activities of enzymes involved in fatty acid oxidation is also a component of migratory disposition in several migrants (Marsh 1981, Lundgren and Kiessling 1985, 1986, Driedzic et al. 1993). In addition, plasma-free fatty acids and triglycerides, as well as pectoralis lipids, increase during premigratory fattening in some migrants (Driedzic et al. 1993); plasma-free fatty acids, glycerol and triglycerides, in addition to