Global climate change and the transformation of habitats due to the unstoppable human invasion of pristine territories are bad for life on Earth. However, recent research has shown that many organisms can tweak their physiology in order to cope well with change. A current challenge for wildlife ecologists and managers is to connect physiological measurements with subsequent changes in the health status of populations and species. This is exactly what interested the team led by Ashlee Mikkelsen, Oregon State University and USDA Forest Service (Oregon, USA). Focusing on a vulnerable species that is currently considered at risk, the northern spotted owl (Strix occidentalis caurina), they explored the connections between physiological stress responses with environmental factors that might predict the population declines of this species.
Corticosterone is a main hormonal regulator of energy allocation in birds and generally increases in response to life challenges. Over 15 consecutive years (2001–2017), Mikkelsen and her team quantified the concentration of the hormone in 4720 feathers from 1056 juvenile spotted owls across 7 different study areas, in order to measure the stress responses of the youngsters as they developed. To minimise harm to the owls, most of the feathers were collected when the owls were banded and weighed as part of the annual historical monitoring of the population. Additionally, the team knew how big each owl territory was at the time of feather collection, how much cover – for breeding and foraging – each patch of forest provided and how good the parents were at caring for their chicks. They also gathered information on the local weather when the feather collections took place, to find out the conditions that the youngsters were facing.
When the team crunched all the data together to assess the connection between feather corticosterone levels and individual and landscape conditions, the analysis revealed three interesting patterns. First, feather corticosterone decreased as the body mass of the owl increased; the largest chicks were the least stressed. As heavier juvenile spotted owls are fitter and have greater chances of survival, this result confirms feather corticosterone as a reliable indicator of the overall health of free-living birds. Second, the levels of corticosterone in the spotted owl feathers were higher when it was warm and rainy, rather than when it was cold and dry, suggesting that the youngsters may have experienced higher energy demands possibly to avoid, or endure, higher temperatures or getting drenched. And third, the smallest owls had the highest corticosterone levels when there was heavy rain, so the owls’ size had an impact on how stressful the youngsters found wet conditions. In addition, the owls experienced the most stress during the fledging period, when they were gradually leaving their nests to begin facing the daily demands and dangers of an independent life.
Mikkelsen's study is one of the first to provide long-term data in a wild bird, showing that feather corticosterone can be a reliable indicator of individual stress responses to life challenges that they experienced in the past. But most importantly, these results show that feather corticosterone can help wildlife ecologists to predict how stress responses related to climate changes and global warming might affect bird populations, marking a significant step forward in the field.