Corticosterone and growth hormone levels in shorebirds during spring and fall migration stopover

Large numbers of shorebirds stop over at Delaware Bay during spring migration and undergo major mass increases within a two- to three-week period. We studied plasma levels of corticosterone and growth hormone in three species of migrants that use this site, sanderlings, Calidris alba, semipalmated plovers, Charadrius semipalmatus, and semipalmated sandpipers, Calidris pusilla. Semipalmated sandpipers were also studied at a fall migration stopover in Manomet, Massachusetts. These two hormones were chosen because they modulate the physiological processes of lipogenesis/lipolysis and promote increased feeding in birds. The stress response was not suppressed in the shorebirds studied, and plasma levels of corticosterone were elevated compared to other studies. We believe that the high levels of corticosterone relate to the rapid fat deposition that takes place at this stop-over site. There was a significant negative correlation between plasma growth hormone and body mass, indicating the lipolytic effects of the growth hormone. Because the lighter birds are recent arrivals to Delaware Bay they may have elevated plasma growth because of fat breakdown during flight to this stop-over site. High levels of growth hormone may also result in protein synthesis, replenishing tissues broken down during the previous migratory bout.