Variation in baseline glucocorticoid (cort) levels can be attributed, at least in part, to differences in energetic demands confronting individuals. Elevated baseline cort levels are routinely interpreted as indicating individuals in poor condition, with low relative fitness. However, when greater reproductive effort increases energetic demands, individuals with high cort might paradoxically be those with the highest fitness. Here, we experimentally test the hypothesis that increased reproductive demand causes increases in baseline cort (the Cort-Adaptation hypothesis). We measured maternal baseline cort before and after experimentally enlarging and reducing brood sizes in tree swallows (Tachycineta bicolor). Females with experimentally enlarged broods had greater increases in baseline cort and fledged more offspring than females with reduced broods. Additionally, females with greater increases in baseline cort had higher offspring-provisioning rates than females with lower changes in cort. These findings demonstrate that increased reproductive demand can cause increased baseline cort. As yet, we do not know if these increases in cort cause increased allocation of resources towards reproduction, but the positive relationship between parental behaviour and cort suggests that increased cort does not always interfere with reproductive investment, and might instead facilitate it.
Bird capture and sampling
We captured all adult females twice during breeding either with trap-doors set on the nest-box entryways or by placing our hands over the entryway when the bird was inside the box. Method of capture did not influence cort levels (t-test of all 2010 female cort levels, n = 33 by hand, 54 by trap, t = −1.06, p = 0.29). To control for diel variation in cort, all captures occurred between 08.55 and 11.50 h. We collected samples before brood size manipulation on day 3 or 4 of the incubation period, and collected post-manipulation samples on day 11 or 12 of the nestling period. At each time point, we collected a small (approx. 120 µl) blood sample within 3 min of capture to ensure that measured hormone levels do not reflect the stress of capture , and painted females with a thin line of white acrylic paint across the wing flight feathers to permit differentiation of males and females in videotaped observations. We stored all blood samples on ice until transport to the laboratory.
Brood size manipulation
We conducted the brood size manipulation with 4-day-old nestlings. We paired broods that hatched within 24 h of each other, and randomly assigned each nest to one of three experimental treatments: control (n = 10), where two nestlings were removed and replaced with two nestlings from a paired control nest; reduced (n = 14), where two nestlings were removed and translocated into a paired enlarged nest, and enlarged (n = 14), where two nestlings were introduced from reduced nests. Because of mortality, sample sizes for hormone and fledging success analyses were reduced to eight controls, 10 reduced and seven enlarged broods. Mean-manipulated brood size was 3.10 (reduced), 5.13 (control) and 7.29 (enlarged) nestlings.
We videotaped parental behaviour at 16 nests (n = 7 control, 4 reduced and 5 enlarged broods) for a minimum of 3.9 h (mean 4.4 h) on day 10 of the nestling period between 06.30 and 19.15 h using cameras set at a minimum of 20 m from the nest-box. We conducted recordings of paired broods at the same time on the same day when possible (i.e. when nestlings from both nests of the pair survived to the day of recording). We scored the hourly rate of parental visitation to the nest-box (a proxy of offspring food provisioning rate) for males and females. To calculate provisioning rate, we considered the duration of the recording from the time of the first parental visit to the nest-box to the time of the last visit, to minimize effects of the disturbance associated with set-up and removal of the cameras.
We centrifuged blood samples within 8 h of collection to separate plasma, which was then stored at −20°C until assay. We quantified plasma levels of total corticosterone (the primary cort in birds) in each sample in duplicate through direct radioimmunoassay, following extraction with re-distilled dichloromethane (see  for details). Within-assay variation among replicate known-concentration standard samples was 13 per cent (six standards).