Connecting Dots: The Science of Tracking Wildlife

The first known bird-banding study in North America took place more than 200 years ago when naturalist John James Audubon tied lightweight threads around the legs of songbirds nesting near his home before they migrated for winter. The following spring, two birds came back with the threads still attached—answering his question about whether the same birds returned year after year.

Banding (or tagging) is a routine technique for studying the migration patterns of birds and other migratory species—whether they travel through the air, on land or in water. The process involves attaching a non-invasive identifier to the animal with information about reporting a sighting if someone sees, catches or finds the tagged animal. These identifiers can range from a small sticker on a butterfly's wing to a lightweight metal band around a bird's leg to a plastic tag attached to a seal's hind flipper. Scientists determine the type of tag to use and where it's placed to avoid causing the animal pain or impeding it in any way.

While simple tagging remains a useful and economical tracking technique, in recent years, new technologies have given scientists more options—and more information. Through the use of radio and satellite tags, researchers can see precisely when individual animals depart from one location and arrive at another, the exact routes they travel, how far and how fast they go, and more. These insights not only reveal an individual animal's seasonal movements, feeding and breeding locations, survivorship and longevity, but also shed light on trends and patterns within and across species.

Why is this information valuable? The more we know about animals' seasonal usage of habitats, the more we can protect the areas they need to survive. Tracking data lets scientists prioritize habitat conservation and restoration efforts, and justify protective measures like seasonal public-use closures of critical areas.

Tracking data also gives scientists information about how climate change and other human-driven environmental impacts affect animal behavior—like a study tracking sperm whales in the Gulf of Mexico following the Deepwater Horizon oil spill.

As part of the National Aquarium's conservation mission and focus on protecting wildlife and the places they live, Aquarium experts collaborate with scientists to help tag, track and study wildlife. Recent work in this area has included local species such as ospreys, monarch butterflies, diamondback terrapins, grey seals and sand tiger sharks.

Ospreys

Naturalist Greg Kearns banded his first osprey in 1983 as part of a long-term population study at Patuxent River Park, part of the Maryland-National Capital Park and Planning Commission in Prince George's County. Because of their high-level role in the Chesapeake Bay food web, ospreys are a sentinel species of environmental health in the region. Their behavior and diet can offer clues about the overall health of the Bay ecosystem.

The Patuxent River osprey study is still underway today. Greg is now a master banding permit holder—one of only 2,000 in the country—and has taken the reins from Steve Cardano, the researcher he worked with 40 years ago.

Over the past several years, groups of staff and volunteers from the National Aquarium have helped Greg band osprey chicks at Patuxent River Park. "I've had more groups from the Aquarium than any other organization," he says. Volunteers record basic information about each chick, such as its sex and weight, and then work with Greg to attach a lightweight aluminum alloy band issued by the Bird Banding Lab of the U.S. Geological Survey to the chick's leg before placing it back in the nest. The band has a nine-digit number and a web address to report sightings.

"We get a surprising number of them back," Greg says. "Ospreys banded here have been recovered in places as diverse as Brazil, Bolivia and the Dominican Republic—and we've had some that have reached over 22 years in age."

He continues, "The metal bands are reliable and cost almost nothing, but they have limits. They can tell us where a banded bird went and how old it is, but not where it came from, when it arrived or how long it took to get there."

Greg also has a project tracking sora rails in the middle Patuxent marshes at Jug Bay. He started out using metal bands on those birds as well but has since switched to radio tags—a more expensive method, but one that offers some significant advantages.

"Ospreys are big birds. They stay out in the open and are highly visible, which explains why we receive information back about so many banded ospreys," Greg says. "Rails are secretive; people rarely see them. And they're small, so the bands are tiny. The birds and the bands don't often get recovered and reported back to us."

Because of these challenges, Greg started using radio technology to track soras, which has proven worthwhile.

"We've learned more about sora migration in the past five years since I started using radio tags than the past 100 years of bird banding had revealed," he said.

The tags provide a wealth of data about each banded bird, including exact flight paths and speeds. Greg uploads data from his rail study to a site called Motus; it's also shared on Movebank, a worldwide database with tracking information about a wide array of species—from insects to whales—across the planet.

Looking back on his decades of experience banding ospreys, soras, hawks and other birds, Greg says, "It's pretty remarkable how far this technology has come."

Monarch Butterflies

This fall, at an Urban Wildlife Conservation Day celebration at Masonville Cove in South Baltimore, U.S. Fish and Wildlife Service Biologist Robbie Callahan was among those working with volunteers to tag monarch butterflies.

"This is similar to banding a bird," Robbie explains, "except we use small stickers that are applied to the monarch's wing."

For each tag, Robbie and the other USFWS biologists record where and when the tag was applied, if the butterfly is a male or female, and whether it was wild-caught or reared. USFWS biologists caught nine butterflies at Masonville Cove the morning of the event. They placed them in collapsible mesh cylinders until volunteers arrived and could help tag and release them. (In addition to the monarchs, the team caught one viceroy butterfly, a small monarch lookalike, which was not tagged.)

Masonville Cove has a lot of goldenrod, asters and other fall-flowering plants that attract monarchs. They feed on the nectar to fuel their migration south to central Mexico, where they overwinter in high-elevation fir forests.

Because a monarch's wings are so delicate, Robbie says it's important to use clean, dry hands without any lotion, bug spray or sunscreen when applying a tag. "Monarchs actually have four individual wings, so you have to catch them when their wings are closed so you can gently pinch all four together."

The next step is to apply the small, lightweight sticker tag using a toothpick to roll it onto the outside of the upper wing on the mitten-shaped discal cell.

USFWS buys tags from Monarch Watch, a nonprofit education, conservation and research program based at the University of Kansas that's been tracking monarch butterflies since 1992. Community scientists can submit and view the tagging data on the Monarch Watch website. Because monarchs migrate to Mexico, the organization pays residents who live near monarchs' wintering grounds to find tagged butterflies there and report the data.

Robbie estimates that if USFWS tags 500 monarchs in a season, one or two might be reported back to Monarch Watch. This data is still valuable—particularly while tiny, lightweight trackers are developed and refined so scientists can better study butterflies, bees, beetles and other insects.

Earlier this year, monarchs were added to the International Union for Conservation of Nature (IUCN) Red List of Threatened Species as endangered. Their populations are threatened by habitat loss in the U.S. and Mexico, climate change, and the widespread use of herbicides and insecticides.

Diamondback Terrapins

Terrapins in the Classroom is a two-pronged program. The Aquarium's educational component places turtle hatchlings in Maryland schools each fall so students can care for, observe and learn about the state reptile throughout the school year before releasing them back on Poplar Island in the spring.

There's also an ongoing research component conducted by Willem Roosenburg, director of the Center for Ecological and Evolutionary Studies at Ohio University.

"It's a controlled experiment to understand the conservation benefits of giving a head start to turtles in general and terrapins specifically," he explains.

Terrapin hatchlings housed in warm classrooms are given a head start because they're active throughout the winter months, feeding and growing. At the same time, their wild counterparts are hibernating and inactive, not feeding or growing.

Each fall, Willem and his assistants go to Poplar Island to collect hatchlings and put them into one of two groups. He gives turtles from the first group—the experimental group—to the National Aquarium and other educational programs collectively known as the Terrapin Education and Research Partnership (TERP) so they can distribute them to classrooms throughout Maryland. The turtles in the second group are marked with a wire tag and immediately released, serving as the control group.

Before the turtles from the experimental group are released back at Poplar Island each spring, they are marked with another type of tag called a passive integrated transponder, or PIT tag. These tags have a bar code and are injected under the skin of the turtle's right rear leg. "It's the same as what's used in cats and dogs," Willem says. "It's about the size of a grain of rice. It allows us to have unique identification information for each animal."

Willem visits Poplar Island frequently to track turtles from both the control and experimental groups to compare how they fare over time. He started this research in 2005 and began collaborating with the National Aquarium about four years later. Since then, his research has shown that terrapins in the experimental group grow substantially bigger than their counterparts during their first year and continue that growth trajectory throughout their lives. Females in the experimental group also reach reproductive maturity about two years earlier than their wild counterparts, beginning to nest at approximately 6 years old rather than 8.

"This shows a very significant effect," he says. "There are two ways to affect population growth. If you can affect survivorship, populations will grow faster, but more dramatic is changing generation time. The age when individuals start to reproduce greatly affects population growth rate. Head-started terrapins have similar nest or clutch sizes, but they start nesting two to three years earlier, which means shorter generation time and faster population growth."

Diamondback terrapin populations have declined considerably in many parts of their geographic range and are listed as vulnerable on the IUCN Red List of Threatened Species. Terrapin populations fell to dangerously low levels in the 20th century after a prolonged period of large-scale harvesting for their meat. Commercial harvest of terrapins ended in Maryland in 2007. Other factors causing declines in terrapin populations include the loss of salt marsh habitat and destruction of nesting beaches due to waterfront development, road mortalities of nesting females, boat strikes, bycatch and continued legal commercial harvesting in Louisiana.

Grey Seals

Before National Aquarium Animal Rescue released rehabilitated grey seal Louis Armstrong back into the Atlantic Ocean in June 2022, we attached a GPS satellite tag to his back.

Thanks to the tag, we know that he left Assateague Island National Seashore and began swimming north. As we tracked Louis's journey over the first 15 days, he stopped in Lewes, Delaware; Atlantic City, New Jersey; Long Island, New York; and Nantucket, Massachusetts, home to a grey seal rookery and abundant food sources. He soon moved into deeper waters off the coast in an area known as the Nantucket Shelf, most likely to forage, and stayed there until the tag stopped transmitting in late July, 51 days after he was released. The tag probably fell off when he molted, which the team expected.

"Louis Armstrong's case was special because of the type of tag we were able to outfit him with," says National Aquarium Senior Rehabilitation Biologist Margot Madden.

The Aquarium releases all rescued seals with a roto tag, a plastic tag that's the same as what's used on the ears of cattle. These tags are attached to the webbing on the seals' hind flippers. Margot explains that all organizations that respond to injured and stranded seals on the East Coast pick a unique roto tag color so that when a seal is observed in the wild, it's easy to tell that the animal has been in human care and with what organization.

National Aquarium Animal Rescue began using an upgraded roto tag during the 2022 season. Louis was the first seal to be released with one of the new tags, which allow the team to take a genetic sample simultaneously while applying the tag.

"We wanted to utilize these genetic samples in a meaningful way," says Margot, "so we connected with Dr. Kristina Cammen at the University of Maine, who's leading research to link disease susceptibility between seal species."

The team also occasionally uses satellite tags. The type of tag used on Louis was a SPLASH tag, which transmits location and diving depth data. Another standard satellite tag used on seals is a SPOT tag, which gives location data only.

"Our team will deploy a SPOT tag if we have concerns about an animal's trajectory post-release," Margot says. "Our most recent case deployed with a SPOT tag was Edwin Hubble in 2019. He was a young grey seal who bounced from Virginia to South Carolina to Georgia before being transported to the National Aquarium for rehab. The team was interested in tracking him to ensure he wasn't going to head south again, which he did not. The last ping we got from his tracker showed that he was off the coast of New Jersey."

The National Oceanic and Atmospheric Administration (NOAA) estimates that there are roughly 450,000 grey seals combined in Canadian and U.S. waters. Like all marine mammals, they are protected under the Marine Mammal Protection Act.

Sand Tiger Sharks

The IUCN lists sand tiger sharks as vulnerable globally and critically endangered in certain parts of the world. The sand tiger shark has one of the lowest known reproductive rates among sharks, giving birth to only one or two pups every two to three years. As a result, their population growth is slow.

Because no one knows where sand tiger sharks pup their young on the East Coast of the U.S., that's a primary question that researchers are looking to answer. The Aquarium's research vessel, Nani, was custom-built to shark research specifications. It provides researchers with a stable platform and a pool where sharks can be safely placed for procedures.

The Aquarium has collaborated with the Association of Zoos and Aquariums (AZA) and university partners to study sand tiger shark behavior, genetics, diet, reproductive biology and the impact of conservation efforts both in the field and with the sharks in the Aquarium's care.

Because these sharks are at risk and reproduce so slowly, it's important to advance research of these sharks in their natural habitat and in the Aquarium to understand how they successfully reproduce.

Tracking using acoustic tags is the most effective technique for monitoring sharks and their movements along the Atlantic coast, but it's not easy or inexpensive. This type of tracking requires that researchers not only catch sharks to implant acoustic tags but also place and maintain multiple receivers in the water—called an array—to pick up signals from tagged sharks as they swim by, pinging their location. Scientists must pull these receivers from the water every few months to record the stored data to be analyzed for interpretation. Storms pose a risk to the equipment and can make it difficult to access. Catching and tagging sand tiger sharks, which can be up to 9 feet long, requires the coordination of a boat with a trained crew, research biologists and veterinary staff. Implanting the tag requires a procedure that can take about 30 minutes. Using a local anesthetic, researchers slip the device under the shark's skin through a 2-inch slit near their fin and suture it before releasing the shark.

Despite the challenges, this type of tagging brings quick results and offers a wealth of migration and habitat-use preference information that standard tags cannot. Researchers can start getting location pings right away, whereas data from a non-acoustic dart tag might not come back for years—if it comes back at all.