Spring in Wyoming

Spring is slow to come at 7,000’ elevation. According to our on-site weather station observations, the average temperature for the month of April was 32 degrees F., and May has had its fair share of snow showers. But as the snow and ice melted we received our first spring arrivals (Figure 1).

Figure 1: Female (L) and male (R ) Barrow’s Goldeneye

Barrow’s Goldeneyes show up as soon as there’s any open water around. These charismatic ducks are more common further north, but like several other species, the southern edge of their breeding range lies within the Yellowstone Ecosystem.

Figure 2: Probable vole nest exposed after snowmelt. Wyoming Mountains in the background.

Other signs of life appear as the snow melts. The nests (Figure 2) and tunnels (Figure 3) that mice and voles created in the subnivean (under snow) zone during the winter are exposed. Surprisingly, the deep snows of winter create a safe and relatively warm habitat for small mammals.

Figure 3: grass-lined tunnels created by mice or voles exposed by melting snow.

Once snow sticks to the ground, the layer in contact with the ground melts or sublimates, and the resulting water vapor rises into the snowpack, where it freezes and forms an icy layer (Figure 4).

Figure 4: The subnivean zone http://archive.audubonmagazine.org/truenature/images/ice_Castle-marcocibola-lg.jpg

The snow above the ice layer provides great insulation, so the resulting space between the ground and snow remains near 32 degrees throughout the winter. Protected from most predators by this blanket of snow, rodents and shrews are free to move around at will. Some species even breed during winters as it can be safer to do so then than during summer. However, life is never risk-free for small mammals. Short-tailed weasels hunt them in their tunnels, while red fox and Great Gray Owls use their hearing to locate and then pounce on them. But on balance, life is safer for rodents in winter, as evidenced by the southward migration of most of the raptor species that prey on them from spring through fall.

Give a Hoot

Several species of owls breed near or migrate through the Upper Hoback River Valley. Most hunt small rodents that they capture using their keen vision and hearing. Each fall, two of the smaller species, Northern Saw-whet Owl and Boreal Owl (Figure 1), migrate to lower elevations to avoid the deep snow that provides a protective blanket under which mice, gophers, and voles thrive in the resulting subnivean layer during our long winters (link to next blogpost).

We can capture these owl species during fall migration by placing mist nets around a loudspeaker and playing male owl territorial calls on it (Figure 2). In 2021, Ricketts Conservation Foundation biologists decided to find out if Northern Saw-whet Owls migrated past the ranch in fall. Sixty-three birds later, we had our answer. Since then, we’ve continued our efforts and captured 773 Northern Saw-whet Owls through the end of 2024 (Figure 3). This past fall, we opened a second banding site for Boreal Owls and captured 68 individuals. This was probably the highest number of Boreal Owls captured at a single banding site in the lower 48 states.

Our reason for conducting this work is better to understand the movement patterns of these two species. Organized banding of Northern Saw-whet owls has been done in eastern and central North America for over thirty years, and we follow a capture protocol established by Project Owlnet. We also have state and federal permits to conduct this work. Before we started our banding effort, there was no sense of how many owls migrated across western Wyoming. We now know that significant numbers of birds move past us every fall. The question is to find out where they’ve come from and where they’re going.

To answer the first question, we’ve collected feathers from captured birds and sent them to Colorado State University. We’re partnering with other banding sites to provide the Bird Genoscape Project with samples that can be used to identify different populations of owls and determine where they breed. This has significant conservation implications as we can then identify populations that may face challenges to their survival.

It's harder to find out where they’re going. Back East, there are multiple banding stations, so it’s common to capture birds banded at other sites and thereby gain an understanding of regional movements. We don’t have this opportunity because there are few owl banding stations in the Intermountain West. Instead, beginning in fall 2025, we plan to place transmitters on up to 15 birds that will provide us with GPS locations for up to two years. We can follow both owl species to their wintering and breeding grounds if successful. Stay tuned!