Every step you take, every flight you make, we’ll be watching you

Migratory birds—not subject to citizenship requirements or FAA regulations—cover the globe as they follow the seasons. How do scientists know where an individual bird has gone and where it’s going? It’s not as if they have Instagram accounts.

Ben Marcus introduces us to the Migratory Bird Center and biologist Brian Evans, who explains how we keep track of these peripatetic aviators.

Natural boundaries such as mountains and rivers mark the Earth’s surface, while human-made political and national borders divide it further. While we humans may base our laws and traditions on these demarcations, fostering national pride and regulating passage across international borders, migratory birds decidedly do not. They may travel back and forth for thousands of miles, from continent to continent, in search of a years’ worth of warmth, nutrients, and suitable nesting grounds, without filing their flight plans with the FAA.

We know as much as we do about the behavior of migratory birds because biologists at the Smithsonian’s National Zoo and Conservation Biology Institute (NZCBI) and other organizations follow birds’ movements across the globe using tracking devices. Way back in 1902—a year before the Wright brothers took to the sky in Kitty Hawk—the Smithsonian’s Paul Bartsch became the first person to tag birds with leg bands for scientific research. More than a century later, NZCBI’s tracking methods have become a lot more sophisticated, involving the likes of radio tags, GPS satellites, and more.

To identify and keep track of individual birds, scientists put aluminum or colored bands on birds’ legs. Similar to the license plate on a car, each aluminum band is engraved with a unique set of numbers. (Image courtesy NZCBI)

As part of the Smithsonian’s Movement of Life program, scientists at NZCBI’s Migratory Bird Center (SMBC) track birds’ migration patterns in hopes of informing strategies that protect their habitats. Each tracking device is a unique identifier: placed on a bird’s leg or back, the tracker can inform our scientists where an individual bird was and when. In fact, you can use this handy tool produced by NZCBI to see where individual birds like Parker the black-crowned night heron and Plimpy the brown pelican has been (spoiler: Parker spends his summers in Washington, D.C., and his winters in Zapata Swamp National Park in Cuba.)

Tracking devices are not one-size fits all, SMBC’s quantitative ecologist Brian Evans told The Torch. The types of trackers the team at SMBC use is dictated by the bird species (specifically, its weight and size), and what scientific question SMBC is asking. For example, the simplest approach is to apply bands on a bird’s leg that have a combination of colors that is unique to that bird. “The big winner here is in being able to identify an individual bird over time,” Brian said. Using bands and a pair of binoculars, SMBC scientists can identify individual birds as they fly overhead. By observing banded birds, participants of SMBCs’ Neighborhood Nestwatch have provided crucial information on the survival of birds in rural, suburban, and urban environments.

Not all heroes wear capes, some wear GPS-enabled backpacks and have feathers. (Photo courtesy of NZPCBI)

One level up, you find light-level geolocators, which predict a bird’s location based on sunrise and sunset times. The timing of light levels can indicate the bird’s location. (Think time zones as you travel east to west and daylight hours as you travel toward the poles and through the seasons.) Recording light levels does not require a lot of electricity, so batteries for these devices can be very small and can be placed on any bird that weighs more than 7 grams (the weight of about three pennies).

These trackers have their limitations though: “If our question is related to habitat use, a light-level geolocator is out as an option [because of the low spatial accuracy],” Brian explained. “Those tags are much more appropriate for addressing questions related to broad-scale movements,” like those of a Blackpoll warbler, who was tracked using a light-level geolocator and was found to travel from Long Island to Venezuela in a single, remarkable journey over the Atlantic Ocean.

Meanwhile, satellite tags can detect finer movements, just like how your phone can track you as you walk down a city block. “Depending on the type of satellite tag and when the tag is programmed to turn on or off, we can use this technology to assess local movements during the breeding season, tracks taken between the breeding and wintering grounds, local movements during the non-breeding season, or some combination of the above,” Brian explained.

A male Kirtland’s warbler perches on a young jack pine tree in Michigan. (Image courtesy NZCBI)

Smithsonian intern Patty Rodriques helps position a radio tag on the back of a male Kirtland’s warbler. (Image courtesy NZCBI)

For the smaller birds like warblers, SMBC scientists can use radio tags, which they communicate with using radio receivers. At large spatial scales, SMBC scientists have used radio receivers to track the migration of birds using the Motus Wildlife Tracking System, an international, coordinated bird tracking network. SMBC scientists have also used this technology to study the movement of birds within their territories and even the survival of young birds after they leave the nest. These tags are perfect for small birds because the trackers can weigh as little as 60 milligrams. (That’s approximately the weight of two grains of rice).

A 0.35-gram coded radio tag used to track Kirtland’s warblers and other species. (Photo courtesy oof NZCBI)

These are just a few of the many tracking devices used at SMBC. Bird tags range from simple aluminum bands – not that different than the originals from 1902 – to the complex technology designed and built by tracking tech companies (talk about a niche market). 

With all this fancy technology, you might wonder how SMBC can trust the trackers to survive a trip around the world. Fortunately, as Brian explained, the tags are wrapped in a plastic/silicone sheath to protect them from the elements. “One of the biggest things to protect them from, however, is the birds themselves,” Brian said. “During my Ph.D. research, I did a study with radio tags and northern cardinals. Cardinals have super strong bills — they would regularly pull at the antenna. Sometimes they would pull it out. Other times they would ‘spiralize’ it (like wrapping a present with curled ribbons). An annoying experience for a data-hungry Ph.D. student!”

Visit the Smithsonian Migratory Bird Center to learn about their scientific projects and their education and outreach programs, and to learn how you can live more Bird Friendly. Then, go visit the Bird House at the Zoo, where you can meet local migratory birds up close and view a bird banding demonstration by Brian and his team.