The stress response—increases in circulating glucocorticoids following a stressor—is typically considered adaptive, but few studies address the fitness consequences of individual variation in stress response. Generally, due to negative consequences of prolonged elevation of glucocorticoids, animals should have a transient stress response just sufficient to cope with the stressor. In rodents, stress responsiveness is affected by early developmental experience, and hyper-responsiveness to stress is linked to morbidity and mortality. We assessed individual variation in stress responses in free-living song sparrows, Melospiza melodia, in relation to fitness-related measures including song and overwinter survival. Birds with greater increases in corticosterone 30 min following restraint stress were less likely to return to breed the following year. Stress responsiveness was also correlated with song complexity: males with fewer syllables in their song repertoires had greater stress reactivity. Our findings support the hypothesis that developmental stressors both impair song development and affect the adult stress response. Thus, individual variation in the stress response may relate to variation in fitness.
We mist-netted 24 adult male song sparrows breeding near Newboro, Canada, in May and June 2007. We collected a blood sample (baseline) within 3 min of the bird entering the net, then placed the bird in a paper bag. After 30 min we collected a second blood sample (stressed; Clinchy et al. 2004), and released the bird. Blood was kept cool in the field, centrifuged later that day, and supernatant plasma kept frozen until assay. Plasma was assayed for total corticosterone using a specific and sensitive radioimmunoassay kit (ImmuChem 07-120103, MP Biomedicals, Orangeburg, NY, USA; Washburn et al.2007) previously validated for song sparrows (Newman et al. 2008). All samples were measured in a single assay. Sensitivity of the assay was 12.5 ng ml−1 and within-assay coefficients of variation were 9.6 and 3.9 per cent for low and high controls.
We recorded complete song repertoires of 18 of these males (as validated by Pfaff et al. 2007) using Marantz Professional Solid State PMD 671 recorders and Telinga Twin Science parabolic microphones (Uppsala, Sweden). We generated sound spectrograms in Syrinx 2.6h (J. Burt; www.syrinxpc.com) and counted the total number of song types (song repertoire size) following Pfaff et al. (2007) and total number of syllables (syllable repertoire size) following Stewart & MacDougall-Shackleton (2008). These two measures are highly correlated (r = 0.7), but nonetheless predict different aspects of male phenotype (MacDougall-Shackleton et al. 2009).
We monitored breeding success for the remainder of the 2007 season, and the subsequent year (2008). As an index of territory quality we monitored the number of offspring produced (see electronic supplementary material). Song sparrows are highly philopatric following their first breeding attempt. In our population, males typically nest within 30 m of their nest the previous year. Males who do not return from the wintering grounds are presumed to have died overwinter. We thus assessed whether the birds sampled above did, or did not return to breed the following year.