HORTON, B.M.*; LONG, J.A.; HOLBERTON, R.L.; University of Maine, Orono: A possible role of corticosterone in mediating transitions from migration to breeding in male White-throated Sparrows (Zonotrichia albicollis).

Corticosterone plays an important role in both migratory preparation and refueling during stopover. Little is known, however, about how birds balance the competing energetic demands of recovering from migration and initiating breeding once they reach the breeding grounds. Here, we investigate how adjustments in baseline corticosterone may affect physiology and behavior during the transition from migration to breeding. Males were captured upon arrival on breeding territories, brought into captivity, and implanted with intra-peritoneal Alzet osmotic pumps filled with either a moderate dose of corticosterone (MD), a low dose of corticosterone (LD), RU-486 (RU), or vehicle only (VO). Plasma corticosterone, body mass, and furcular fat score were measured at 1, 4, and 7 days following implantation, while locomotor activity and food intake were measured daily. Testes and pectoral muscles were harvested on the 7th day. MD and LD males showed significant increases in food intake and furcular fat deposition, but had reduced pectoral muscle mass compared to RU and VO males. MD males, despite having higher food intake than LD males, also had significantly greater muscle loss. Changes in total body mass, in conjunction with muscle and fat data, indicate that muscle loss occurs rapidly in MD males and is compensated only by increased fat deposition. There was no effect of treatments on locomotor activity or testis size. These data suggest that increased baseline corticosterone may facilitate an increase in food intake to replenish fat stores used during migration, but at the expense of skeletal muscle. Testis size was not affected, however, suggesting that moderate increases in corticosterone may not compromise breeding readiness.