Abstract
The effects of acute environmental stressors on reproduction in wildlife are often difficult to measure because of the labour and disturbance involved in collecting accurate reproductive data. Stress hormones represent a promising option for assessing the effects of environmental perturbations on altricial young; however, it is necessary first to establish how stress levels are affected by environmental conditions during development and whether elevated stress results in reduced survival and recruitment rates. In birds, the stress hormone corticosterone is deposited in feathers during the entire period of feather growth, making it an integrated measure of background stress levels during development. We tested the utility of feather corticosterone levels in 3- to 4-week-old nestling brown pelicans (Pelecanus occidentalis) for predicting survival rates at both the individual and colony levels. We also assessed the relationship of feather corticosterone to nestling body condition and rates of energy delivery to nestlings. Chicks with higher body condition and lower corticosterone levels were more likely to fledge and to be resighted after fledging, whereas those with lower body condition and higher corticosterone levels were less likely to fledge or be resighted after fledging. Feather corticosterone was also associated with intracolony differences in survival between ground and elevated nest sites. Colony-wide, mean feather corticosterone predicted nest productivity, chick survival and post-fledging dispersal more effectively than did body condition, although these relationships were strongest before fledglings dispersed away from the colony. Both reproductive success and nestling corticosterone were strongly related to nutritional conditions, particularly meal delivery rates. We conclude that feather corticosterone is a powerful predictor of reproductive success and could provide a useful metric for rapidly assessing the effects of changes in environmental conditions, provided pre-existing baseline variation is monitored and understood.
Highlights
Impacts of acute or chronic environmental stressors on,vildlife are typically quantified directly using mortality rates derived from carcass counts (Piatt et al, 1990; Burger, 1993 ) or 1nultiyear census data (Wiens et al, 1996; Yaukey, 2012), "vhich are incorporated into de1nographic models to esti1nate the population-level effects of stressors (Haney et al, 2014)
Feather CORT concentrations,vere significantly negatively correlated to body condition index (BCI)
Chicks tl1at died before fl edging had lo,ver BCI (F1,239 = 6.1, P = 0.01.) and higher CORT deposited in feathers (F1,239 = 24.7, P < 0.001) at 3-4 \veeks of age than chicks that
Summary
Impacts of acute or chronic environmental stressors on ,vildlife are typically quantified directly using mortality rates derived from carcass counts (Piatt et al, 1990; Burger, 1993 ) or 1nultiyear census data (Wiens et al, 1996; Yaukey, 2012), "vhich are incorporated into de1nographic models to esti1nate the population-level effects of stressors (Haney et al, 2014). In addition to causing im1nediate 1nortality, ho,vever, stressors can act sublethally through secondary path,\rays including reduced habitat quality (Cheng et al, 2009; Williams et al, 2010), con1promised physica l condition (Ro1nero and Wikelski, 2001 ), physiological and genetic modifications (M.0ller and 1vfousseau, 201 1) or increased susceptibility to existing threats, such as disease or environmental fluctuation (Balseiro et al, 2005; Whitehead, 2013) Many of these indirect and sublethal stressors subsequently i1npact de1nographic processes by reducing longtenn survival or reproductive fitness in surviving individuals (Krebs and Burns, 1977; Peterson, 2001) but often are not explicitly or adequately addressed in de1nographic calculations and projections. Ho"vever, these 1neasures have high short-term variability, and their relationship to demographic para1neters of interest (e.g. reproductive success) 111ust be evaluated in order to select appropriate 1netrics
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