Determining whether wildlife populations are health or not is challenging. Animals can range over huge areas, it is generally hard to monitor their reproduction, and usually we don’t know how long most individuals live. Therefore, conservation biologists are interested in using physiological markers as proxy measures for the health of a population. One such proxy measure that has been widely used is levels of corticosterone, the primary stress hormone in birds. If stress hormone levels are high in most individuals, then we might assume that the population is experiencing challenging conditions and could be in decline. But is this really the case? What are stress hormone levels really indicative of? In a new paper in Oecologia, we reviewed the literature and combined data from 31 published studies that looked at corticosterone levels and measures of fitness in seabirds. Using meta-analytic techniques, we found that corticosterone levels are generally related to food availability, and reproductive success, but were not consistently related to other metrics of fitness such as body condition, foraging effort or breeding effort.