Glucocorticoid hormones facilitate responses to environmental challenges by mediating diverse physiological and behavioral changes, including resource mobilization and altered reproductive effort. Elevated glucocorticoids might indicate that an individual is facing high levels of environmental challenges and thus, elevated concentrations might be associated with reduced fitness (CORT-fitness hypothesis). Alternatively, the energetic demands of reproduction might be a challenge that requires elevated glucocorticoids to mobilize resources to support reproductive effort, ultimately increasing reproductive investment and fitness (CORT-adaptation hypothesis). Investigations of glucocorticoid-fitness relationships have yielded mixed results. Variation in the direction of this relationship could be caused in part by differences in the contexts in which the relationship was assessed. Incorporating context, such as life history stage, could be key to understanding the role of glucocorticoids in influencing fitness outcomes. We investigated the relationship between corticosterone and reproductive effort and success within a single life history stage: incubation of eggs. In an observational study, we measured baseline corticosterone in incubating female red-winged blackbirds (Agelaius phoeniceus), monitored incubation behavior, and determined hatching success for each nest. Incubating birds with higher baseline corticosterone concentrations had more frequent, shorter incubation bouts and spent less time overall incubating their clutches of eggs than birds with lower corticosterone concentrations. Elevated corticosterone was also associated with lower clutch mass, but neither corticosterone nor incubation effort were correlated with hatching success. Although experimental tests are needed to establish causation, these results suggest that during the incubation period, corticosterone might shift resource investment towards self-maintenance, and away from current reproductive effort.
Study species and population
Red-winged blackbirds are polygynous, and multiple females will nest on the territory of a single male (Searcy and Yasukawa, 1995). Females are the sole incubator of eggs and hatchlings (Holcomb, 1974) and are not fed by their mate (Beletsky, 1996). As a result, the relationships between a female's corticosterone, incubation behavior, and nest success might be more tightly linked than in species with bi-parental incubation or mate feeding. In red-winged blackbirds, incubation effort decreases across the incubation period (Holcomb, 1974), therefore we included day of incubation in all analyses of incubation effort. We studied red-winged blackbirds nesting across six marshes at the Queen's University Biological Station (44°34′02.3″ N, 76°19′28.4″ W) in southeastern Ontario during May–June 2014 and 2015. All marshes provided similar nesting habitat and we found no difference in the birds' corticosterone, scaled mass index, fat score, incubation effort, clutch size or mass, or hatching success across marshes (all P > 0.25), and thus, we do not distinguish among marshes for our analyses. All procedures were approved by the Virginia Tech Institutional Animal Care and Use Committee and the Queen's University Animal Care Committee and were in compliance with US and Canadian national standards for the use of animals in research.
Trapping and blood collection
In 2014 and 2015, we captured 59 incubating female red-winged blackbirds in 70 capture events. We captured females between 5:30–10:30 AM on days 3–11 of the 12-day incubation period by flushing them into mist nets placed 1–3 m from the nest or by placing a single-celled walk-in (Potter) trap directly on top of the nest. Both traps and nets were set-up when the female was away from the nest. We estimated the day of incubation by counting the number of days after clutch completion or before the hatching date. Four individuals were excluded from any analyses including the day of incubation because we were unable to determine either the clutch completion date or hatch date.
Rapidly after capture, we punctured the brachial vein with a 26 gauge ½ inch needle and collected 300 μL of blood into heparinized capillary tubes. We recorded the exact time of sample collection after the bird entered the net or trap and used only samples collected within three minutes for analyses including corticosterone (Romero and Reed, 2005). We found no relationship between the log of baseline corticosterone and time from capture among the samples collected within 3 min (β = 0.0004, 95% CI = − 0.002–0.003, P = 0.72, N = 40) or between log corticosterone and the time from net or trap set-up and capture (β = 0.004, 95% CI = − 0.01–0.003, P = 0.28, N = 28). Therefore, we concluded that these samples were reasonable representations of baseline concentrations of corticosterone.