HOLBERTON,R.*; WILSON,C.M.; SIMS,C.; CASH, W.B.: The Role of Corticosterone and the Effects of its Inhibition During the Development of Migratory Condition in Dark-eyed Juncos.

To understand corticosterone’s role in the development of migratory feeding and fattening, dexamethasone (DXM) was used to inhibit an increase in baseline corticosterone often observed during migration. Dark-eyed juncos (Junco hyemalis) were transferred from short to long-days and monitored for food intake, body mass, fat deposition, and plasma corticosterone for 46 days. Every 48 h for the first 17 days, controls (C, n = 10) received saline while three other groups received injections of either 0.09 ug (LD, n = 10), 1.8 ug (MD, n = 10) or 9.0 ug (HD, n = 9) DXM/bird. Ten days after injections began, MD and HD corticosterone levels were similar to each other and significantly lower than those in C and LD (which were elevated above pre migratory levels). C and LD birds began to fatten and gain body mass soon after transfer to long days. Three weeks after treatments ended there was no difference in corticosterone or body mass between the four groups. However, MD and HD birds were significantly delayed in the onset of fattening; MD birds increased mass immediately after injections ceased and HD birds began to fatten 5-7 days later. Differences in patterns of mass gain could not be explained by food intake as all groups showed the same pattern of increased food intake. These results suggest that an increase in baseline corticosterone is needed to facilitate normal spring migratory fattening but, while corticosterone may increase food intake during the non-migratory period in a �stress� or �stress avoidance� response, it is dissociated from the hyperphagia that occurs as a normal component of migratory condition.