Patterns of corticosterone secretion in migrating Semipalmated Sandpipers at a major spring stopover site

Seasonal changes in corticosterone metabolism may be in part responsible fo r mediating the changes in nutrient reserves before and during migration. T o investigate the role of glucocorticoids in the migratory physiology of Se mipalmated Sandpipers (Calidris pusilla), we compared plasma levels of cort icosterone in response to capture and handling stress during prolonged stop overs in Delaware Bay during spring 1996 and 1997. Although Semipalmated Sa ndpipers showed significant stress responses to handling in both years, the magnitude of the response was significantly lower in 1997. We found no cor relation between energetic condition and initial corticosterone concentrati on in either year, or between energetic condition and corticosterone concen tration in samples collected 30 min after capture in 1996. However, energet ic condition was positively correlated with corticosterone concentration in 30 min sample collected in 1997. Between-year differences in regression fu nctions of energetic condition by Julian date suggested that birds were acc umulating fat mass more rapidly, or arriving in better energetic condition during 1996 compared to 1997. Those differences were especially evident dur ing the "early" stages (before 26 May) of stopover periods. In 1997, we fou nd that birds sampled early in the season did not show a significant increa se in corticosterone 30 min after capture, while birds sampled in early 199 6 did. In both years, birds sampled during the "late" stages of stopover pe riods (after and including 26 May) showed significant stress response. Bird s in relatively poor energetic condition, or with low rates of fat-mass gai n, were less likely to secrete additional corticosterone in response to acu te stress, possibly to protect skeletal muscle needed for migratory flight. Those results suggest that patterns of corticosterone secretion in sandpip ers can be modified in response to changes in energy demand during the migr atory period.