We found a significant effect of sampling time point on Cort levels (F = 21.97, p < 0.001, nobservations = 50, nindividuals = 17; Fig. 1, Table 1). There was no effect of body mass, fat, muscle score, or the calculated BMI on Cort levels at T0, T15, and T30, the AUCg or the AUCi of birds. We report descriptives of these variables in Table 2. The best models for single Cort levels, AUCg and AUCi, included only bleeding time point as a significant effect. There was no relationship between capture date or time of day and individual Cort levels at T0 (r = 0.167, p = 0.52), the AUCg (r = − 0.02, p = 0.93) or the AUCi (r = − 0.168, p = 0.53). Further, there was no significant correlation between single Cort and LCC peak levels (slope ± se = − 0.01 ± 0.01, t = − 1.54, p = 0.134, marginal R2 = 0.05; nobs = 50, nind = 17).
LCC peak levels decreased linearly with sampling time (slope ± se = − 0.34 ± 0.087, p < 0.001; nobs = 51, nind = 17) (Fig. 2). We did not find a significant effect of body mass, fat score, muscle score, or BMI on LCC peak levels in our individuals (all p values > 0.15). LCC peak levels and LCC AUC were highly significantly correlated (slope ± se = 0.02 ± 0.0004, t = 60.65, p = < 0.0001, marginal R2 = 0.98; nobs = 51, nind = 17; suppl Fig. 1), revealing that LCC peak levels are a good proxy for the entire LCC response over 80 min.