ses, we decided to include all birds in the statistical analysis to increase statistical power.

At day 0 (starting the hormone treatment, see below) and day 14 we took measurements of color and size of the eye ring as well as of body mass (see below) of all birds. At day 20 we took blood samples to determine the effectiveness of the hormone implants and blood levels of lipoproteins and carotenoids. Since half of the birds of each hormone treatment participated in an additional experiment where birds were immuno-challenged on day 14, which turned out to influence their hormone levels (Casagrande and Groothuis, submitted), hormone levels on day 20 could only be used from the other half of the birds for the current experiment. Since birds allocated to the immunization experiment were randomly chosen, the cohort we used should give a reliable indication for the whole batch. Due to the volume of blood required for the analyses (450 μL) we could only collect blood once over the experimental period and decided to do this on day 20 since blood sampling was required on that day for the additional experiment. Therefore, we correlate eye ring measurements at day 14 with blood plasma parameters at day 20. We are confident that the blood levels of hormones at day 20 closely resemble those at day 14, since the implants release stable levels of hormones during that time (Wingfield et al. [@CR83]; Ketterson et al. [@CR32]). In addition, eye ring parameters of those birds that could be measured both on day 14 and day 20, did not change significantly (paired *t* test: size: *t*~(1,46)~ = 0.61, *p* = 0.54; hue, *t*~(1,46)~ = 0.79, *p* = 0.43, UV-hue, *t*~(1,46)~ = 0.49, *p* = 0.49; chroma, *t*~(1,46)~ = 0.77, *p* = 0.45; UV-chroma, *t*~(1,46)~ = 0.79, *p* = 0.43).

We also took small biopsies from the eye ring of two birds for characterization of carotenoids by HPLC. These pilot measurements were carried out on birds not included in the experimental sample. In addition, we performed this characterization on s