Repeatability of baseline corticosterone concentrations.

One major assumption for endocrine studies is that hormone titers are consistent within an individual so that if hormone titers are low relative to the cohort on one day, they are relatively low compared to the cohort on other days. This is an especially important assumption for most field studies where researchers may have access to an individual animal only once. We used a laboratory study with captive house sparrows (Passer domesticus) to test this assumption using glucocorticoid titers. Baseline corticosterone titers were measured five different times for each bird under six different experimental conditions: during both day and night while birds were held on a short day photoperiod (11L, 13D), a long day photoperiod (19L, 5D), and while birds were undergoing a prebasic molt. Although the variation within an individual was often larger than the variation between individuals, the relative ranks of birds compared to their cohort were consistent during the night in all three conditions. In contrast, during the day the relative ranks of birds compared to their cohorts were only consistent on short days; on long days and during molt there was no significantly consistent ranking among individuals. Furthermore, the overall rank of an individual in its cohort was often different during the day and night. These data indicate that it is not always a good assumption that birds can be categorized as individuals with higher and lower titers, which will complicate analyses of the causes of interindividual variation.