Regulation of protein breakdown and adrenocortical response to stress in birds during migratory flight

Abstract

During long-term fasting at rest, protein utilization is maintained at low levels until it increases at a threshold adiposity. This study examines 1) whether such a shift in energy substrate use also occurs during endurance exercise while fasting, 2) the role of corticosterone, and 3) the adrenocortical response to an acute stressor. Ten species of migrating birds caught after an endurance flight over at least 500 km were examined. Plasma uric acid and corticosterone levels were low in birds with fat stores >5% of body mass and high in birds with smaller fat stores. Corticosterone levels were very high in birds with no visible fat stores and emaciated breast muscles. Corticosterone levels increased with handling time only in birds with large fat stores. These findings suggest that 1) migrating birds with appreciable fat stores are not stressed by endurance flight, 2) a metabolic shift (increased protein breakdown), regulated by an endocrine shift (medium corticosterone levels), occurs at a threshold adiposity, as observed in birds at rest, 3) adrenocortical response to an acute stressor is inhibited after this shift, and 4) an adrenocortical response typical for an emergency situation (high corticosterone levels) is only reached when muscle protein is dangerously low.

Seasonal stress physiology and body condition differ among co-occurring tropical finch species

“…For example, Gouldian finches with lower muscle measures had higher residual CORT concentrations. This equates to higher plasma CORT in birds with lower condition measures, a state commonly observed in underweight migrating birds (Schwabl et al 1991;Holberton 1999;Jenni et al 2000). Small increases in CORT are likely to drive these individuals to forage more and expend more energy, and this could explain their relatively high haematocrit concentrations (mean > 60).…”

“…Individuals with poor body condition that are exposed to acute stressors often have higher baseline or stress-induced CORT concentrations than birds in good condition. This is particularly apparent among birds exposed to high energetic demands, inclement weather, or food limitations (Rees et al 1985;Astheimer et al 1995;Marra and Holberton 1998;Kitaysky et al 1999;Jenni et al 2000). Birds exposed to long-term stress (over several days or longer) had reduced baseline and stress-induced CORT concentrations, suggesting that chronically stressed birds suppress the stress response, possibly to avoid corollary effects on breeding and other behaviors (Cyr and Romero 2006).…”

Are birds stressed during long-term flights? A wind-tunnel study on circulating corticosterone in the red knot

“…Whether this increase in baseline corticosterone concentration happens in anticipation to prepare the organism to state B (Landys-Ciannelli et al, 2002), or as a response to the increased demands, is not clear. In contrast to waders, low baseline levels of circulating corticosterone (<10 ng/ml) were found in several passerine species during migratory episodes (Schwabl et al, 1991;Gwinner et al, 1992;Jenni et al, 2000) suggesting a lack of activation of the hypothalamo-pituary-adrenocortical axis (HPA). However, a direct comparison with nonmigrating birds was only presented in one species, the garden warbler Sylvia borin (Schwabl et al, 1991).…”

“…State B should not be confounded with a state of stress, an ''emergency life-history stage", during which in reaction to lifethreatening stimuli corticosterone is released extensively to trigger physiological and behavioural changes which help the individual to survive . Such high stress levels of corticosterone were only observed in migrating birds with completely depleted fat stores and emaciated breast muscles (Gwinner et al, 1992;Jenni et al, 2000). In fact, for state B it was hypothesized that, analogous to inactively fasting birds (e.g.…”

“…Cherel et al, 1988;Lynn et al, 2003), corticosterone levels increase when fat stores are depleted, in order to increase protein catabolism and to trigger a change in behaviour, i.e. a switch to searching for a stopover site and to landing (Jenni et al, 2000).…”