Owls in Arizona – Delving into Screech Owl Microclimates
By Kassandra Townsend
Kassandra Townsend holding a Northern Saw-whet owl she banded as part of Hawkwatch International's Following Forest Owl project. The first NSWO banded in Arizona in twenty years!
I was very excited to start my new PhD position with HawkWatch International’s Following Forest owl project during the summer of 2021 in Arizona. Dr. Dave Oleyar is HawkWatch International’s director of Long-term Monitoring and Community Science. He started studying the dynamics of tree cavities and owl populations in Utah and Arizona in 2016 with the goal of understanding the dynamics and distributions of tree cavities and their importance to multiple owl species. I was grateful to be given the opportunity to work on this project, but I had never really been to Arizona. I knew about the Chiricahua mountains and the significance of the range to the Apache tribe, but I was unsure what to expect, other than there are owls. Lots of them!
When I first arrived, I landed in Tucson, then we jumped in a rental vehicle and started a three-hour journey to Cave Creek canyon. My first initial thoughts while traveling to the study site were “how are there owls out here?”. As we continue our journey, we cross over into New Mexico and then cut back into Arizona where we approach the town of Portal. So far, the landscape has been just dry, arid desert with some ocotillo, grasses, multiple species of agave, yucca, and many variations of cacti - short, tall, skinny, and fat. Once we reach the town of Portal, we travel past Portal Peak Lodge Store and Café which is the hotel, bar, restaurant, stage for concerts, and convenience store and begin to enter the canyon. As you travel along the road, the landscape begins to change. As we climb in elevation, the desert scrub and grassland we have been in for hours gradually changes into an oak woodland, Arizona White oaks and Emory oaks are the most dominant with Arizona juniper and cypress trees mixed in as well. We start to see small creeks appear, adding riparian areas to the landscape and large Arizona Sycamore trees and Cottonwoods start to bridge across the road. We arrive at the American Museum of Natural History Southwestern Research Station which is about 1,600 meters in elevation. I would soon come to learn that the entrance of the canyon in Portal sits around 1,400 meters in elevation and the highest elevation in the Chiricahua mountains is 2,900 meters at Chiricahua peak. These higher elevations are where the pine forest occurs, and I was shocked the first time I experienced this drastic change in forest type compared to the base of the canyon. It looked like where I am from in Idaho, there were Ponderosa pine, Arizona White Pine, Douglas fir, Apache pine, Chihuahua pine, and Grand fir. More than just the trees were familiar to me, there are even wildflowers I recognize such as Lupine, Arrowleaf balsamroot, Penstemon, Indian paintbrush, and yarrow. It felt like home.
Coronado National Forest. Lupine in the foreground and Ponderosa Pine in the background at a higher elevation site
This range is home to many sky islands, which are high elevation mountains surrounded by an “ocean” of desert. The Chiricahua mountains are part of the Madrean Sky Island Archipelago which stretches from the Sierra Madre Occidental in Mexico to the Rocky Mountains in the United States. The Madrean Sky Island is an important biodiversity hotspot due to its elevational and ecological gradient, providing refugia against climatic variation, and its North and South reach. This allows the area to support multiple different habitat types from low desert to high alpine conifer forests. Since sky islands support a large amount of biodiversity, it is considered a global resource of high conservation value. Cave Creek canyon, where I work, has 1,100 meters of elevational change which ranges from desert grasslands to pine forests.
Cave Creek canyon with an oak/juniper mix woodland.
I experienced this biodiversity on my first night of night surveys. We conduct night surveys using a standardized protocol of silent listening paired with acoustic playbacks of owl calls. On a good night, we can hear the three dominant owl species. First, the world’s smallest owl, the Elf Owl, Micrathene whitneyi, whose call is indicative of its name because they sound as though there is an elf cackling in the forest. The next most common owl is the Whiskered Screech owl, Megascops trichopsis, which is a series of quick 6 or 7 toots that slow down at the end. Lastly, our biggest little owl we study, the Western Screech-owl, Megascops kennicottii, has a call opposite of the Whiskered. Their call is a series of toots that speed up at the end, much like the cadence of a ping-pong ball being dropped on the ground.
On a great night we can hear those three owls plus the Northern Saw-whet Owl, Aegolius acadicus and Flammulated Owl, Psiloscops flammeolus. To top off a great day, we would have heard a Northern Pygmy Owl, Glaucidium gnoma earlier when the sun was up because this is one of the diurnal owls in this area. These are only our study species, there are many other owl species that utilize this area as well, such as the elusive Mexican Spotted Owl, Strix occidentalis lucida.
I have now worked in these mountains for three years, and one of my questions for my dissertation is how all of these different environmental characteristics are impacting what the owl experiences on a microclimate level within the tree cavities that owls use. Microclimatic refugia are spatially restricted habitats that provide favorable microclimates that allow individuals to cope with harsh environments. At the ecosystem level, microclimates are important because they act as a regulator of many ecosystem functions. Understanding climate refugia that species select for can allow more understanding of the distribution of species, traits, and local adaptations. Forest vegetation structures have a strong influence on microclimate conditions, many plant and animal species rely on their temporal consistency to buffer against climactic extremes. Within a forest, the understory is protected against macroclimate temperatures. According to De Frenne et al. (2021), the understory has about a 4.1 °C temperature difference compared to an open field during high heat, and 1 °C warmer during cold weather. The vegetation within a forest controls the vertical temperature gradients that occur within multiple microclimatic layers, there is a 0.15 to 0.25 °C per meter temperature difference between the ground and canopy. Individuals and whole populations rely on these aspects of a forest to reduce the pressures of responding to the increasing effects of climate change.
Western Screech owl in an Arizona Sycamore tree cavity
This is also important because individual tree microclimates have a significant influence on the reproductive success of multiple bird species (like owls that nest in tree cavities). Temperatures within a cavity are important for the development of eggs, nestling growth rate, and energy consumption of nestlings and adults. Poor insulation increases the risk of hypothermia and can increase parental energy expenditure of warming nestlings in some habitats. Among birds, extreme air temperatures that exceed physiological limits can lead to direct mortality due to dehydration, especially among embryos and nestlings which are more sensitive to thermal and hydric conditions. In adults, microclimate conditions affect physiology, activity patterns, behaviors, and fitness. It has been found in other studies that greater tree size and certain decay classes of trees should buffer against ambient temperature fluctuations and cavity aspects or the direction a cavity is facing can limit solar radiation. Clement and Castleberry (2013) found that tree type, size of tree, and the number of entrances affected cavity temperature and humidity was affected by canopy cover.
So, what is important to consider for an Arizona landscape? Well during the breeding season, temperatures can fluctuate from high 32 degrees Celsius (90 degrees Fahrenheit) in the daytime to 10 degrees Celsius (50 degrees Fahrenheit), or lower, in the nighttime. I can go from wearing shorts and a tank top during the day to conducting night surveys in a sweatshirt and a puffer jacket (this is subjective, of course, I tend to run cold anyway). How do tree cavities mitigate these temperature fluctuations? …and what kind of buffer do they provide for owls in Arizona when there are extreme temperatures? Other studies have found that internal daily extreme temperatures were reduced and lagged the ambient temperature. Using iButtonLink temperature loggers both inside and outside the cavity, I hope to understand what is occurring within these trees, and what an owl is experiencing. How do these temperatures vary based on what tree species the cavity is in? How does elevation and landscape impact temperature in the cavity? What characteristics of a cavity (height, type of cavity, direction the cavity is facing) are influencing temperature fluctuations?
Understanding thermal properties of cavities is the first step to predicting how climate change will impact the distribution and success of species that rely on them. My research is important for better understanding how cavity adopting species rely on tree cavity microclimates rather than just the abundance of cavities within an area, and how climate change may alter these thermal refugia that will affect individual species. This will allow managers and conservationists to successfully collaborate on protecting an important habitat element that many species rely on. This will be especially important in outdoor recreation areas (camping, hiking, and birding specifically for Cave Creek canyon) where wildlife and humans both use space, and for preservation and recovery of a species by increasing available suitable habitat. Understanding the effects of climate change on microclimates will allow conservationists and managers to predict the negative effects on species and possibly mitigate these impacts.
Overlooking Cave Creek canyon above the tree canopy
Three years later, it is just as exciting as my very first field season to hear the owl diversity within the canyon. Every time we monitor tree cavities; I am excited for what we find. It could be a nesting pair, eggs, nestlings, or even the occasional woodpecker. It is even more fulfilling to know that my research can be shared with citizen scientists. Through the Earthwatch Institute, we have the privilege to bring out citizen scientists with us in the field as part of the Following Forest Owls project. I have met people from all over the world who are passionate about science and conservation efforts being made. They learn and conduct the fieldwork with us, and it is such a great experience seeing how these owls can touch people’s lives like they have mine.
