Steroid-binding proteins and free steroids in birds

Within the comparative literature, corticosteroid-binding globulin (CBG) has recently emerged as a potential modulator of the glucocorticoids-driven stress response. Many avian field studies include the measurement of CBG with the goal of making behavioral and ecological inferences. However, the field of stress physiology is divided on how to interpret the biological importance of the different states of circulating hormones. Here we review evidence for the biological relevance of each fraction of glucocorticoid hormone; the CBG–glucocorticoid complex (the bound fraction) and the remainder which is either unbound or loosely attached to albumin (the free fraction). We suggest that the biological importance of free vs. bound hormone depends on the location of interest (plasma or tissues), and the time frame of interest (current or future need). While a large body of evidence suggests that free hormones are the biologically active fraction, evidence also suggests that the bound fraction is a biologically relevant reservoir of glucocorticoids. We review two salient topics from the avian stress literature; stress-induced decreases in CBG capacity and glucocorticoid influences in life history strategies. These topics are discussed with an emphasis on free vs. bound hormone concentration and how that compares to current vs. future glucocorticoid needs.