Baseline corticosterone is positively related to juvenile survival in a migrant passerine bird

Abstract

Summary1. Glucocorticoid hormones are an integral part of the vertebrate stress response, and theoretical models argue for a link between glucocorticoid levels and individual fitness. The cort-fitness hypothesis posits that elevated levels of baseline glucocorticoids are reflective of an individual in poor condition and with a reduced likelihood of survival. Surprisingly, this hypothesis remains virtually untested for the juvenile life-history stage, a period that is often characterized by high mortality rates. 2. To address this issue, we explored whether glucocorticoid levels just prior to fledging were related to survival during the juvenile period in the Swainson's thrush (Catharus ustulatus), a short-lived, temperate-breeding passerine bird. Over 2 years, we blood-sampled nestling thrushes to quantify glucocorticoid levels and then used radio telemetry to assess whether individuals died or survived to emigrate from the study area. Finally, we measured vegetation characteristics at the nest and at locations used by individuals during the juvenile period to quantify the relative importance of habitat features and glucocorticoid levels on survival. 3. Predation was the leading cause of death, and overall juvenile survival rate was 34·6%. We found that survival was positively associated with baseline corticosterone and, to a lesser extent, size-corrected body mass and date of fledging. Contrary to expectations, we found no evidence that the amount of vegetative cover at the nest site or at locations used during the juvenile period was associated with survival. 5. Although we cannot completely rule out the cort-fitness hypothesis, our data appear to support the cort-activity hypothesis and suggest that elevated baseline corticosterone levels in juvenile thrushes may be linked to enhanced post-fledging survival via increased locomotor activity that promotes foraging, more effective escape from predators or both.

Experimental effects of early‐life corticosterone on the hypothalamic–pituitary–adrenal axis and pre‐migratory behaviour in a wild songbird

“…An increased degree of movement could also serve to enhance survival prospects by increasing anti-predator and locomotory functions (Astheimer, Buttemer & Wingfield 1992;Belthoff & Dufty Jr. 1998;Breuner & Hahn 2003). This was recently supported in a wild population of migratory Swainson's thrush (Catharus ustulatus), where juveniles with higher baseline corticosterone as nestlings had greater subsequent survivorship throughout the postfledging period (Rivers et al 2012).…”

“…Dixon 1972;Anders et al 1997;Cohen & Lindell 2004;King et al 2006;Rivers et al 2012), I did not find evidence that corticosterone-treated birds had higher survival rates during the pre-migratory period compared to sham or controls. One explanation may be the lack of power due to the low number of recaptures.…”

“…In general, the effects associated with acute elevations of CORT are considered beneficial for organisms to cope with stressors , Rivers et al 2012. However, chronic elevations of CORT are known to cause deleterious effects, such as immune suppression (Martin 2009), protein depletion (Johnson et al 1992, Sapolsky et al 2000, disrupted glucose transport (Johnson et al 1992, Sapolsky et al 2000, and neurobiological effects on behaviour and memory (Siegel 1980, McEwen & Sapolsky 1995, Sapolsky et al 2000.…”

“…The first, known as the 'CORT-fitness hypothesis' (see Bonier et al 2009a,b), proposes that individuals with elevated baseline and stress-induced corticosterone levels will have lower survival due to the deleterious effects associated with chronic HPA axis activation Blas et al 2007) and song sparrows in some years (Melospiza melodia; MacDougallShackleton et al 2009MacDougallShackleton et al , 2013. Another hypothesis is that elevated baseline corticosterone levels could increase movement due to the physiological and behavioural changes associated with HPA axis activation, known as the 'CORT-activity hypothesis' (Breuner & Hahn 2003;Rivers et al 2012). An increased degree of movement could also serve to enhance survival prospects by increasing anti-predator and locomotory functions (Astheimer, Buttemer & Wingfield 1992;Belthoff & Dufty Jr. 1998;Breuner & Hahn 2003).…”

Repeated exposure to stressful conditions can have beneficial effects on survival

“…There may be U-shaped relationships between glucocorticoids and survival, with a threshold below which HPA axis stimulation can promote health and longevity, and above which detrimental effects may arise (Pravosudov et al 2001;Brown et al 2005). For example, in the Swainson's thrush (Catharus ustulatus), individuals with naturally high baseline corticosterone levels during the nestling stages showed increased survival rate post-fledging compared to birds showing naturally low basal corticosterone concentrations (Rivers et al 2012). On the other hand, during severe naturally occurring stressful events, such as during the El Niño, corticosterone levels have been observed to increase disproportionately in Galapagos marine iguanas (Amblyrhynchus cristatus) and were negatively correlated with likelihood of survival (Romero & Wikelski 2001).…”