For six months out of every year, Ashlee Mikkelsen spends her days hiking for miles off-trail in the Ponderosa pine-filled forests of central Washington, hooting like an owl, and carefully listening for responses. These days, responses can be few and far between. You see, Ashlee isn’t just a wildlife enthusiast; she is a research assistant in a long-term US Forest Service monitoring program focused on the northern spotted owl.
Since being listed as threatened by the US Fish and Wildlife Service in 1990, populations of northern spotted owls have continued to decline. In some areas, the number of spotted owls has decreased by more than half in only 20 years (see (Dugger et al. (2016)). Northern spotted owls are inhabitants of old-growth forests. Although northern spotted owls historically could be found in almost every forest from northern California to British Columbia, as forests have shrunk in size through timber harvesting and through changing land use, the amount of suitable habitat has drastically decreased. A second major contributor to the decline of the northern spotted owl is arrival during the last century of the barred owl, which are native to northeastern North America. Barred owls competed with spotted owls for territory and resources, and have been observed fighting with spotted owls. Ashlee’s master’s research at Oregon State aims to quantify the stress experienced by spotted owls.
When birds experience stress, their bodies respond by releasing larger-than-usual quantities of the hormone corticosterone. Similar to cortisol in humans, corticosterone is always present, but having levels that are very high or that are very low is associated with poor health outcomes. It used to be that in order to measure the physical stress response of a bird, researchers had to take a blood sample. The problem with this is that the process of taking a blood sample itself is a source of stress for the bird. Recently, however, a new technique was introduced based on the fact that corticosterone is also present in feathers. Being able to use feathers is a distinct advantage: birds are constantly dropping feathers, so collecting feathers is fairly non-invasive, and importantly, similar to the benefits of measuring cortisol in hair, feather corticosterone measurements show the average level of the hormone over a long period, rather than just the instant that the feather is collected.
Working with professor Katie Dugger (who, incidentally, was Ashlee’s supervisor in the owl-monitoring field crew for the two years prior to beginning graduate school), Ashlee is analyzing a collection of feathers that spans over a 30-year time period. Measuring corticosterone levels in feathers is a high-tech process involving organic chemistry and radioactive isotopes. Although there are many complications that need to be accounted for, tracking the levels of corticosterone in these feathers gives Ashlee insight into the impact of stressors such as environmental degradation and competition with barred owls. Because the data spans so many years, she is able to examine the average stress in spotted owls over periods of change in the populations of barred owls. Ashlee’s data shows a strong response in corticosterone in spotted owls when the number of barred owls in the neighborhood goes up. This supports the view that spotted owls’ woes are not just due to habitat loss, but also due to competition with barred owls.