Exploring the Movement Patterns of the Northern Saw-whet Owl

The Northern Saw-whet Owl, Aegolius acadicus, is an incredibly elusive, nocturnal bird of prey that lives throughout North America. The Northern Saw-whet Owl (NSWO) ranges between 18-21.5 cm long and predates small animals such as deer mice, voles, shrews, and other small mammals (Duncan et al. 2009). In most areas, NSWOs are migratory species and begin migrating south in autumn to follow resources and to escape harsh winter conditions (Beckett & Proudfoot 2011).

In order to gain a closer look into the lives of these enigmatic species, Project Ownlet was created by Dave Brinker and colleagues in 1994. Project Owlnet is a growing network that has reached over 125 banding sites. This network aims to study migratory owls, more specifically, the NSWO, by creating banding stations across North America. Using standardized netting protocols, researchers at banding sites can contribute to a continental effort of gaining a better understanding of NSWO ecology.

Although quite small, the NSWO possesses long, sharp talons and a sharp beak which allows them to predate on small mammals. They also have large eyes to aid in their nocturnal foraging.

Additionally, they are characterized by their whiteish facial disk and brown and white speckled head. Juveniles, however, possess a darker brown plumage and cinnamon underside (see image to the left).

NSWOs are often silent, except during breeding season. During this time of the year, male vocalizations can be described as a "too- too- too" call, in which females respond with a series of whistles. Explore the Cornell Lab of Ornithology "All About Birds" page below to listen to the different calls.

NSWOs breed in forested and some shrub-steppe environments throughout North America. Their roosting environment is typically in evergreen forest, while they often nest in deciduous forest.

Below, a map of North America illustrates forest cover and an estimated population range for the NSWO based on 2023 sightings from eBird (an online public program which allows individuals to track and document bird sightings) by state. Forest cover is divided into four categories, deciduous, evergreen, woody, and mixed forest.

Like many birds, the NSWO is primarily a migratory species, although some can be partial migrants, nomadic, or residents in some locations. For the majority that choose to migrate, they do so in order to follow resources and more tolerable winter weather conditions farther south. Departure times vary geographically, in which birds migrating from higher latitudes, will typically leave sooner than those from lower latitudes. Moreover, there is still plenty to learn about the movement patterns of NSWOs.

With over 125 banding stations across North America, the Project Owlnet network contributes to filling the knowledge gap associated with Northern Saw-whet Owl migration. Through the use of audio lures, mist nets, leg bands, and avid volunteers, Project Owl Net Stations provide valuable information about movement patterns as well as gaining a closer look into the species itself.

Indiana Dunes National Park and Indiana Dunes State Park are one of the 125+ Project Owlnet stations in North America and have been involved for over a decade now. Located on the southern tip of Lake Michigan, Indiana Dunes hosts a variety of different plants and animals, including NSWOs. The annual banding events typically begin in the first week of October and end in mid-late November.

Audio lures and mist nets are the primary method of the capture of this small bird. At the Indiana Dunes station, there are a total of 3 netting sites, Tremont, Paradise, and Moraine. At each site, there are between 4 and 6 mist net that are set up in a cross-like formation. Each mist net possesses 4 different pouches in which the owl can fly into. An audio lure loaded with NSWO calls is placed at each netting site. The goal of the audio lure is to perk the interest of any nearby owls and cause them to fly into the mist nets.

Mist nets are opened by volunteers nightly (pending weather conditions) around one-half hour before sunset. Once nets are open, audio lures are set to the NSWO calls and placed between 90-110 decibels, which is the equivalent to somewhere between the sound of a lawn mower (90) and the sound of a helicopter (110). After they are successfully set, the nets are checked for captured birds every 30-60 minutes.

If an owl is caught in a mist net, the exact net and pouch number as well as the direction of entry are documented. The owl is then safely removed from the net, placed in a bird bag, and transported to the banding site at the Indiana Dunes Visitor's Center.

At the banding site, numerous measurements are documented about the owl as well as the current environmental conditions.

The first step is to band the bird with a unique leg band provided by USGS and to record that band number (see image to the left). Next, bird specific measurements including weight, wing chord, flattened wing length, tail length, bill length, underwing, age, and sex are recorded.

Age is determined by observing wing feather color as well as using ultraviolet light. Sex is determined using a chart that applies weight and wing chord length.

Environmental conditions such as date and time, temperature, sky cover, wind speed, wind direction, humidity, and moon phase are also documented.

Once the required information is documented, the owl can be released, but not until after an "adoption photo" is taken to be posted on the Indiana Audubon website.

Across the 3 netting sites at Indiana Dunes, there were a total of 23 owls captured between October 20, 2023 and November 5, 2023. The site with the highest success was the Tremont site (10 captures), followed by Paradise (8 captures) and Moraine (5 captures). The map below illustrates the Indiana Dunes netting and banding sites with graduated symbols emphasizing the sites with higher capture success. The map also contains forest cover type for reference.

As illustrated in the map above, the Tremont site was the site with the highest capture success, while the Moraine site was the site with the lowest capture success. Also shown in the map above is forest cover. Evidently, there is denser forest cover at the Tremont and Paradise sites than at the Moraine site. Upon closer examination of the satellite imagery, there also appears to be higher fragmentation by roads and development at the Moraine site (see images below).

Northern Saw-whet Owl migratory behavior varies by location. Some studies have documented that there are three main corridors of migration east of the Mississippi in North America, one that ranges from central Ontario down the Ohio River Valley, another routing from Nova Scotia to North Carolina across the coast, as well as around the western side of Lake Superior through Wisconsin (Brittain et al. 2019). Owls that migrate east of the Mississippi have also been noted to have cyclical "eruption years" every four years, but this has not been documented west of the Mississippi (Beckett & Proudfoot 2011).

Northern Saw-whet Owls aren't as well documented on the west coast of North America simply because the majority of banding stations are on the east coast. Data collected from banded birds between the years of 2010 and 2022 on the west coast were retrieved from the United States Geological Survey. There was a total of 1,217 NSWOs from this data set, and 91 of those individuals were recaptured at a later date. Individuals that are recaptured can reveal a lot of information about their movement patterns such as distance and direction over time. Below is a map depicting distance and direction traveled by the recaptured owls.

Surprisingly, there were only two travel distances that were visible at the scale depicted on the map above. The longest travel distance by an owl from this data set was in 2014 which took place between October 28 and November 18 of that same year. In fact, this individual traveled 148 miles northwest, which contradicts typical migratory behavior.

Another surprise was how little most individuals traveled from their initial capture site. Regardless of whether they were recaptured months or years later, they were recaptured fairly close to their initial capture location. Ultimately, the owls banded and recaptured throughout western North America did not showcase any obvious migratory patterns. If anything, they demonstrated possible residency (little to no movement) or nomadism (irregular movement patterns as result of following resources).

Northern Saw-whet Owls are an incredible species that ornithologists still have a lot to learn about. By establishing banding sites across North America, they can do exactly that. Project Owlnet has given ornithologists across the continent the opportunity to learn more about these small, elusive, nocturnal predators.

At Indiana Dunes, banding stations are active annually every fall, where dedicated volunteers can work with licensed bird banders from the Indiana Audubon to capture and band NSWOs migrating near the Great Lakes between early October and mid-late November. This banding station is just one of 125+ across the continent. With active cooperation between banding stations, more information can be gathered about the NSWO, especially in regards to its movement patterns.

Want to learn more about Project Owlnet? Explore the link below!