Seasonal and diel transitions in physiology and behavior in the migratory dark-eyed junco

Body mass, fat stores, activities of lipogenic and lipolytic enzymes, and p lasma corticosterone were measured throughout seasonal and diel transitions from fall through spring encompassing the non-migratory stages of early an d mid winter, the prealternate molt, and the spring migratory stage in capt ive dark-eyed juncos to determine the physiological mechanisms underlying a daptations for migration. On a seasonal basis, lipid enzymes and corticoste rone varied little throughout the stages even though the birds underwent dr amatic alterations in mass, fat deposition, behavior, and activation of the reproductive axis. By contrast, diel changes were found in lipogenesis, li polysis, muscle lipoprotein lipase, and plasma corticosterone when comparin g birds in the two phases of spring migration-active flight and resting, as during times of stopover. In these two phases of migration, coordination o f the lipogenic and lipolytic systems appear to maximize storage of fatty a cids during rest and delivery/utilization during flight. Diel patterns of c orticosterone revealed fairly consistent peaks during the night time (23:00 ) throughout the nonmigratory period. The profile of this pattern altered d uring the migratory period with variation between the flight and resting ph ases. In sum, the results from these captive studies offer a new approach f or studying the regulation of migratory physiology in free-living birds. (C ) 1999 Elsevier Science Inc. All rights reserved.