Summary
Glucocorticoid (cort) hormones are increasingly applied in studies of free-ranging animals, with elevated baseline cort levels generally assumed to indicate individuals or populations in worse condition and with lower fitness (the Cort-Fitness Hypothesis). The relationship between cort and fitness is rarely validated and studies investigating the cort-fitness relationship often find results inconsistent with the Cort-Fitness Hypothesis. The inconsistency of these studies may result in part from variation in the cort-fitness relationship across life history stages. Here we address the following questions in a two-year study in free-ranging tree swallows (Tachycineta bicolor): (1) Do baseline cort levels correlate with fitness within a life history stage? (2) Does the cort-fitness relationship vary across different life history stages? (3) Does the cort-fitness relationship vary across life history stages within an individual? (4) Does reproductive effort influence cort levels, and do cort levels influence reproductive effort? We measured baseline cort and fitness components in female birds of known breeding stages. We find correlations between baseline cort levels and fitness within some life history stages, but the relationship shifts from negative during early breeding to positive during late breeding, even within the same individuals. A positive relationship between baseline cort and fitness components during the nestling period suggests that reproductive investment may elicit higher cort levels that feedback to reallocate more effort to reproduction during critical periods of nestling provisioning. Our findings provide reason to question the Cort-Fitness Hypothesis, and have implications for the application of cort measures in monitoring the condition of populations of conservation concern.
Methodology
Study system
All methods described herein conform to the regulatory standards of the Queen’s University Animal Care and Use Committee. We conducted this study in May and June of 2007 and 2008 in a free-ranging population of box-nesting tree swallows (Tachycineta bicolor, Fig. 1) at the Queen’s University Biological Station near Chaffey’s Lock, Ontario, Canada (44°34′ N, 76°19′ W, ∼135 m elevation). T. bicolor is a migratory swallow found breeding throughout much of North America (Robertson et al., 1992). Box-nesting species offer a unique opportunity for repeated sampling of individuals of known breeding stage, and thus are ideal for this study. The Queen’s University population of T. bicolor has been the subject of intense monitoring and study for more than three decades (cf. Dunn et al., 1994, Robertson and Rendell, 2001, Stapleton et al., 2007, Whittingham et al., 1992). All breeding individuals and nestlings in the study population are marked with numbered leg bands for individual identification. T. bicolor have high breeding site fidelity, with more than 85% of individuals in a nearby (New York, USA) box-nesting population returning to their initial breeding site (Winkler et al., 2004). The entire study site consists of eight grids of 6–35 nest boxes placed in hay fields and 24 solitary boxes distributed along a road connecting the study grids. For the present study, we focused on birds nesting on one grid of 35 boxes in 2007, and this grid plus two additional grids of 23 and 25 boxes in 2008. Cort levels did not differ among sampling grids. The 35- and 23-box grids are set up with identical inter-box spacing of 40 m along each row and 28 m diagonally among rows. The 25-box grid is set up with two rings of boxes, one evenly spaced around the perimeter of a swamp in the center of the field and one around the outer perimeter of the hay field.
Sample collection and field monitoring
We only include females in the present study. To control for diel variation in cort levels, we trapped birds between 9 and 11:45 a.m. inside their nest boxes either with trap-doors set on the entryways (set when the bird was out of the box and closing upon the bird’s return) or by placing our hands over the entryway when the bird was inside the box. Method of capture did not influence measured cort levels (Student’s t-test, d.f. = 55, t = 0.224, P = 0.824). Once captured, we collected a small (<150 μL) sample of blood into heparinized microcapillary tubes through puncture of the brachial vein. Blood was sampled within 3 min of the time when females were trapped in the boxes. After blood sampling, we collected morphometric measures from all birds and placed leg bands on any unbanded birds. We measured mass (to nearest 0.1 g with a Pesola spring scale), tarsus length (to nearest 0.1 mm with a caliper), and wing length (to nearest 0.1 mm with a wing ruler) of each bird. We then released the bird, usually within 10 min of capture. We stored all blood samples on ice until they could be transported to the lab for processing.
We recorded breeding activity of all focal birds through routine monitoring of nest box contents. We checked nest boxes once every three days until the appearance of the first egg in the nest box. We determined onset of incubation by monitoring egg-laying behavior daily – T. bicolor lay one egg every day, with an occasional skipped day, until clutch completion and then initiate incubation behavior (Robertson et al., 1992). Once incubation was initiated, we again monitored box contents every three days until all offspring fledged or the breeding effort failed.
During 2007, we captured 15 birds on day 3 or 4 after onset of incubation. We then monitored the outcome of those 15 nesting attempts and recorded the number of offspring that reached the age of departure from the nest box (fledging success). During 2008, we captured 14 birds 4–12 days prior to initiation of incubation (nest-building), 16 birds on day 2 or 3 of the incubation period (early incubation), 20 birds on day 7–9 of the incubation period (late incubation), and 16 birds on days 3–5 and 8 birds on days 10–12 of the period of nestling provisioning (nestling). Because of high rates of nest predation in 2008, we pooled samples collected early and late in the nestling period. Results do not differ if these data are considered separately. In total, we sampled 39 adult females during 2008, with 13 females sampled once, 17 sampled twice, and 9 sampled three times. No females were sampled more than once during a given breeding stage. In 2008 we recorded fledging success as described above and also collected measures of clutch mass and nestling growth rate. We measured clutch mass on day 2 of the incubation stage and nestling mass (to nearest 0.1 g using an electric balance) on days 4 and 12 of the nestling period to assess nestling growth (analyzed as change in mass of the entire brood over an 8-day period).