The Hypothalamus and Adrenal Regulate Modulation of Corticosterone Release in Redpolls ( Carduelis flammea—An Arctic-Breeding Song Bird) L. Michael Romero, 1 Kiran K. Soma, and John C. Wingfield Department of Zoology, Box 351800, University of Washington, Seattle, Washington 98195 Accepted December 5, 1997 Free-living redpolls ( Carduelis flammea—a species that breeds in the Alaskan Arctic), modulate corticosterone release in response to capture and restraint depending upon the breeding site. We extended these findings to adults undergoing a prebasic molt (the energetically costly replacement of feathers) and to juveniles. Results indicate not only that the stress response is dramatically reduced at one breeding site, but that the stress response during molt and in juveniles is lower still. In fact, juveniles failed to secrete any corticosterone in response to capture and handling, suggesting a stress hyporespon- sive period. We also examined possible mechanisms underlying stress modulation. Corticosterone binding protein capacity does not change, and the stress response is only correlated with the overall condition of the bird (assessed by fat storage) at one site, suggesting that neither can explain the different corticosterone re- sponses. Adrenal insensitivity also does not appear to fully explain reduced maximal output since exogenous ACTH enhanced corticosterone release. Exogenous ACTH, however, cannot stimulate corticosterone to stress- induced levels at the high-response site, implying re- duced adrenal capacity. Redpoll pituitaries responded to exogenous corticotrophin-releasing factor and arginine vasotocin, suggesting a mechanism upstream from the pituitary blunts corticosterone release. Taken together, these results indicate that corticosterone release in this species is modulated depending upon the ecological and physiological state of the animal, and that the maximal corticosterone response is controlled at multiple sites in the hypothalamic–pituitary–adrenal axis.  1998 Academic Press Several bird species have recently been shown to modulate the secretion of corticosterone (the primary glucocorticoid in birds (Holmes and Phillips, 1976) ) seasonally in response to an identical stressor (Wing- field et al., 1982, 1992, 1994a,b, 1995a,b; Astheimer et al., 1994, 1995; Romero et al., 1997). In Gambel’s white- crowned sparrows (Zonotrichia leucophrys gambelii ), for instance, peak corticosterone levels in response to the stressors of capture and restraint are several fold lower during winter (Romero et al., 1997) and during molt when the birds are replacing all their feathers (Asthei- mer et al., 1994) than during the breeding season. Other studies have indicated that different breeding sites (Wingfield et al., 1994a) and severe storms (Astheimer et al., 1995) can be associated with altered corticoste- rone responses. Thus birds apparently regulate their corticosterone response to stress depending upon the environmental and/or physiological context of the stimulus. Most species that have been demonstrated to season- ally modulate corticosterone release breed in either the desert (Wingfield et al., 1992) or the Arctic (Wingfield et al., 1982, 1994a). Both habitats are typified by a short effective breeding season and harsh weather condi- 1 To whom reprint requests should be addressed at Department of Biology, Tufts University, Medford, MA02155. Fax: (617) 627-3805. E-mail: mromero@tufts.edu. General and Comparative Endocrinology 109, 347–355 (1998) Article No. GC977048 347 0016-6480/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.