When unpredictable and life-threatening events occur, animals often mount a behavioural and physiological response to these stressors and this, in turn, activates an emergency life-history stage that redirects resources from activities nonessential to immediate survival towards processes that promote immediate survival (combating or escaping the stressor, Wingfield et al., 1998). In vertebrates, endocrine processes are well known to be involved in the stress response and to activate this emergency life-history stage (Ricklefs & Wikelski, 2002; Wingfield et al., 2008a; Angelier & Chastel, 2009). Thus, stressors induce a rapid elevation of circulating glucocorticosteroid hormones, i.e. corticosterone or cortisol (the glucocorticosteroid stress response, Wingfield, 1994, 2003; Wingfield et al., 1998). This endocrine response promotes the occurrence of behavioural and physiological forms that enhance the immediate survival, but also redirects resources away from growth, reproduction, growth and moulting processes (Sapolsky et al., 2000; Wingfield & Sapolsky, 2003; Romero et al., 2005; Wada, 2008; Wada & Breuner, 2008; Müller et al., 2009). This stress response is particularly relevant when studying life-history decisions because it is now well referenced that individuals can modulate their glucocorticoid stress response according to the fitness costs and benefits that the activation of an emergency life-history stage provides (Holberton et al., 1996; Wingfield & Sapolsky, 2003; Heidinger et al., 2006; Lendvai et al., 2007; Angelier et al., 2009a). For instance, the stress response is usually attenuated during the reproduction, the moult or the chick’s growth, and this has been interpreted as a way for individuals to reduce the important negative effects of elevated corticosterone levels on chick’s development, feather growth and reproduction (Sims & Holberton, 2000; Wingfield & Sapolsky, 2003; Romero et al., 2005; Lendvai & Chastel, 2008; Quillfeldt et al., 2009).
Interestingly, differences in the corticosterone stress response have frequently been reported between bird populations or closely related bird subspecies (Astheimer et al., 1994; Marra et al., 1995; Wingfield et al., 1995, 2008b; Holberton & Able, 2000; Breuner et al., 2003; Wilson & Holberton, 2004; Lindstrom et al., 2005; Wada et al., 2006; Müller et al., 2007; Horton & Holberton, 2010). For instance, Silverin et al. (1997) found that the corticosterone stress response varies between two populations of willow warblers (Phylloscopus trochilus) that cope with contrasting environmental situations. Similarly, tropical, temperate and arctic subspecies of the well-studied white-crowned sparrows (Zonotrichia sp.) show contrasted corticosterone stress responses, and this can be related to different environmental contexts (Astheimer et al., 1994; Breuner et al., 2003; Wada et al., 2006; Wingfield et al., 2008b). Although environmental conditions and life-history strategies can be related to these differences in the corticosterone stress response between populations and subspecies (Wingfield et al., 1995; Breuner et al., 2003; Holberton & Wingfield, 2003; Clinchy et al., 2004; Wilson & Holberton, 2004; Wada et al., 2006; Müller et al., 2007; Almasi et al., 2009; Bokony et al., 2009; Hau et al., 2010; Horton & Holberton, 2010), the origin of these differences in the corticosterone stress response between populations and subspecies remains unclear. The corticosterone stress response may differ between subspecies or populations because they live in different areas and, thus, adjust their corticosterone stress response to the contrasted environmental conditions that they experience. On the other hand, populations and subspecies may differ in the corticosterone stress response because of selection pressure that has favored individuals with specific corticosterone stress response (Satterlee & Johnson, 1988; Evans et al., 2006; Wada et al., 2009; Almasi et al., 2010) or because of environmental factors that have affected the development of the HPA axis during early life and, thus, the corticosterone stress response that adults can mount (Love & Williams, 2008a,b;Wada et al., 2009). Unfortunately, it is very challenging to disentangle these hypotheses because populations are sampled in contrasting environments and, at the same time, may have been confronted to different selection pressure and developmental conditions (Müller et al., 2007).
Here, we studied two closely related subspecies of passerine (swamp sparrows, Melospiza georgiana) that live in contrasted environments (Olsen et al., 2008, 2010; Ballentine & Greenberg, 2010) and we experimentally tested whether the intensity of their corticosterone stress response differs when they are held in similar environmental conditions for an extended period of time. Specifically, we experimentally controlled for the influence of environment by conducting a common garden experiment: chicks from these two subspecies were reared in captivity and maintained under similar environmental conditions for more than 1 year (Ballentine & Greenberg, 2010). We then asked whether the intensity of the corticosterone stress response differs despite these two groups being reared in the same environmental condition. By doing so, we are able to better understand what drives variation in the corticosterone stress response and its phenotypic plasticity, between closely related groups such as subspecies: (1) if the corticosterone stress response of individuals of the two subspecies of swamp sparrows has been shaped by selection and/or developmental processes, we predict that swamp sparrows subspecies will differ in their corticosterone stress response and, this, despite the similar environmental conditions of this ‘common garden’ experiment; (2) if individuals of these two subspecies adjust their corticosterone stress response to the environmental conditions that they are experiencing, we predict that the two subspecies of swamp sparrows will show a similar corticosterone stress response because of the similar environmental conditions of the common garden experiment.