Stress modulation over the annual cycle: Effects of molt on the stress response (#24)
The rapid rise in circulating glucocorticoid (GC) levels above baseline values is a hallmark of the vertebrate response to acute stress. There is substantial evidence that both the magnitude of GC secretion following exposure to a given stressor and pre-stress baseline levels vary in a consistent manner at specific periods of an individual’s annual cycle. In most birds, the pre-basic moult period is coincident with a significant decrease in baseline and stress-induced GC levels. Because feathers comprise up to 25% of a bird’s total body protein and GCs are known to inhibit protein synthesis, downregulation of the HPA axis is suggested to be an adaptive response to ensure replaced feathers are of acceptable quality. This is corroborated by studies showing that moult rate and feather quality are adversely affected when exogenous corticosterone is administered to moulting birds. Exceptions to this pattern of GC modulation during moult have been identified in several species, which raises doubt regarding the functional significance of GC downregulation during moult. We have characterised the pattern of GC secretion in free-living White-plumed Honeyeaters (Lichenostomus penicillatus) using a standardised capture-stress protocol and found no evidence of seasonal modulation of HPA sensitivity. Furthermore, these birds often show moult/breeding overlap and moult is not suppressed by reproductive hormones as found in strongly seasonal species. These results, along with emerging evidence from studies of other species with flexible breeding schedules, suggest that GC downregulation is not a prerequisite for ensuring the quality of replacement feathers. The physiological processes potentially offsetting GC-induced compromises in feather synthesis will be discussed.