A golden-winged warbler caught in a mist net. Credit: David Edlund

By Lauren Brunk


Harsh wind nipped at my bare hand as I carefully unfolded the mist nets at Homer Lake, a forest preserve a dozen miles east of the U of I campus. We unfolded a collection of three nets around bird feeders. Tall trees surrounding us hosted a plethora of birds that watched from afar; what felt like an orchestra of bird calls filled my ears. The discomfort that comes with cold and rainy conditions was not unique to us; as fellow endotherms, conserving energy and internal body temperature for birds is key for survival, and cold temperatures make that difficult. For that reason, there was a larger audience than normal waiting for us to leave the easily accessible snacks of seeds and nuts.

The goal of the mist net was to create a nearly invisible barrier between the highly sought-after bird feeder and the safety of the treeline to capture birds. Mist nets are thin polyester mesh hung between two metal poles, resembling volleyball nets. When the shelves of mesh are connected together, they create pockets, so that when a bird hits the net, it falls into the pocket and entangles itself.

Although the idea may sound strange, mist nets are a very common research technique used by ornithologists, and studies show it is extremely safe for birds. This technique is an incredibly valuable tool for answering questions about bird ecology, such as how long birds live, where they migrate and when, and what areas they inhabit. More importantly however, this information is used for bird management and conservation, which is becoming increasingly vital as almost 50% of all bird species face drastic declines. These declines are not restricted to rare and threatened species, either: Common and widespread species are also diminishing. A recent comprehensive study estimates North America has lost 3 billion birds in 50 years, an unfathomable number.

Mike Ward is one of many passionate ornithologists seeking to understand these declines and find conservation solutions, particularly for migratory birds. He is a Principal Research Scientist at the Illinois Natural History Survey, the instructor of my Ornithology class at the University of Illinois, and leader of our mist-netting adventure.

Author Lauren Brunk holds a tufted titmouse in a bander’s grip. Credit: Izabelle Jaquet

Post set-up, Ward, his lab members, and we students filed back into the Educational Center at Homer Lake and shut the glass door behind us. “And now we wait,” Ward said with a smile. Gathered together, we watched the nets. For the first five minutes no movement occurred, an eerie stillness. Then, one brave male American goldfinch darted from the trees straight toward the feeders for food. The yellow blur bounced off the net and fell into the pocket, getting tangled. All it took was one — birds of many species now dove toward the feeders in waves. Some bounced off completely like a trampoline and retreated to the treescape, some avoided the nets and successfully reached the feeders, and the rest became stuck in the camouflaged mesh. For 15 minutes or so, we watched the nets until it was time to collect. Because I had the proper permits and experience, I followed Ward outside.

Birds were stuck everywhere in the net, suspended in air. As we approached a bird, the first step was “determining which side the bird had flown into the net” to allow for the easiest extraction. Once Ward determined the entry position, he traveled to that side of the net, wrapped his hand gently around the bird, and slowly slipped the net around its wings and legs. Within a minute the bird was net-free. Ward then handed me a tufted titmouse, an acrobatic gray songbird with a crest. I wrapped my fingers around its wings and torso to immobilize it, a technique known as bander’s grip. I then placed the titmouse into a breathable cloth bag to reduce its stress. All around me, experienced ornithologists removed birds from the nets with care and placed them in bags. We brought the bags back into the educational center where a station for measurements had been prepared.

Inside, I passed the cloth bag onto Mike Avara, Ward’s lab manager, who has years of experience with the science of bird banding. Avara pulled the surprised tufted titmouse from the bag, its showy blue crest flashing, and placed it in a firm bander’s grip. The sex and age were taken first, primarily determined using plumage. Avara then used a caliper, a specialized ruler, to measure the wing chord, tail, and tarsus length. The wing chord length represents the distance from the curve of the wing to the longest primary feather, while the tarsus length measures the lower leg of the bird from its “ankle” joint to the end of the leg. Birds have modified bone structures for flight, which causes their anatomy to look different from ours. Although the visible joint immediately below the torso may look like a knee bending backward, their knees are actually farther up their leg hidden in their feathers, while the ankle joint is the one exposed. The length of the wing chord, tail, and tarsus informs researchers about the condition of the birds and how it changes over time, especially given age, habitat changes, and food availability.

A researcher measures the tarsus of a black turnstone. Credit: Prince William Sound Science Center

Mass was the last measurement needed, and the technique for weighing birds in the field is peculiar and fascinating. Depending on the size of the bird, ornithologists will use various mundane containers to hold it. For the smaller songbirds we were dealing with, old Walgreen’s pill bottles fit perfectly. The pill bottle was first tared, or zeroed out, on the scale. Then, the titmouse was slid gently into the bottle upside down and placed on the scale.

After all measurements were complete, the final step was to attach a small, metal band on the bird’s leg: bird banding. This band contained a unique number that connected all the measurements taken to that individual bird. Different birds require different band sizes and types to ensure minimal discomfort for the bird and maximum longevity of the band.

“All of this information we took right now will be uploaded to the BBL (Bird Banding Laboratory) once we’re done, and then ornithologists all over the country will have access,” Avara said. “That way, when birds migrate north to their breeding grounds or south for wintering, we’ll know where they’ve been and how their physical condition changes.”

The BBL is a collaborative scientific organization that supports the collection, curation, and archiving of information from banded birds in North America. When this bird is caught again, researchers will upload the new data to the BBL, and species records can be kept and analyzed across space and time.

With the band attached and all measurements taken, the entire process taking less than five minutes, Avara handed the tufted titmouse back to me. I walked outside, placed the songbird on my palm, and released my grip. With a second to gather its wits, the titmouse flew off my hand and into the nearest tree, with a few angry squawks for good measure. Although a little disgruntled, the tufted titmouse went about its day unscathed. This fast process provides ornithologists with a wealth of information regarding the physiology and behavior of birds.

But how did such a peculiar technique come about and become so effective for studying birds?

During the Punic Wars in 218 BCE, Roman officers tied long, trailing threads on bird legs as messages to their soldiers. The 1600s saw the first recorded instances of bird banding for identification purposes in the practice of falconry. Describing falconry in Asia, Marco Polo wrote, “Each falcon belonging to the sovereign and the barons has a tablet of silver on its feet with its name and that of its owner inscribed so that wherever it is caught it may be returned to him.” Early falconers were also known to band herons captured by falcons as a testament to their strength and stamina. By banding and recapturing the herons, bird hobbyists established the lifespans and migratory routes of these birds, helping develop early understandings of bird ecology.

Brunk attaches a small metal band to a gray catbird’s leg. Credit: David Edlund

Bird banding in the Western hemisphere was inaugurated by John James Audubon in the early 1800s. Audubon is revered as a significant contributor to the understanding of bird physiology and behavior through his field notes in North America, although he is controversial for his association with academic fraud and racism. Audubon’s banding of Eastern phoebes, a small songbird, marked a historical turning point in ornithological research. Near his home in Philadelphia, Audubon wrote that “when (the eastern Phoebes) were about to leave the nest, I fixed a light silver thread to the leg of each, loose enough not to hurt the part, but so fastened that no exertions of theirs could remove it.” Audubon was testing whether the same Eastern phoebes were coming back to the same place to nest, otherwise known as site fidelity. The following year he noticed the same banded phoebes did in fact return to the nesting site. This represented the first known understanding of site fidelity in a migrating songbird through bird bands, and reflected basic scientific principles used in bird banding today (for example, being careful with the attachment of bands).

Today, in the spirit of John James Audubon and others, citizen science is a huge contributor to the successful research system run by the BBL. Scientists rely on contributions from birders and others to track bird movement and answer questions about survival and reproduction. 


50-year Study

Like many other avian ecologists, Henry Pollock, a postdoctoral researcher and instructor at the U of I, has worked on a diversity of projects where mist netting and bird banding were crucial. As an undergraduate he studied abroad in Costa Rica, where he was first introduced to mist netting, and from there developed a strong passion for tropical avian ecology. After his Ph.D. studying bird seed dispersal in Guam, Pollock joined a long-term data collection project on tropical birds alongside his Ph.D. advisor, Natural Resources and Environmental Sciences Professor Jeffrey Brawn.

Started in 1977, this project sought to untangle hypotheses about the life history traits of tropical birds. As Pollock explains, many people assumed “tropical birds were more K-selected and had slower life-history traits, but there were few studies demonstrating this on a long-term scale.” In other words, people hypothesized that tropical birds generally lived longer, matured at slower rates, and had lower reproductive output because they lived closer to the equator.

To answer these questions, 20 nets were set up at two sites to capture swaths of different sizes and types of birds in Central Panama. For the most part they caught “small passerines less than 150g, including sparrows, wrens, and manakins,” but birds as small as hummingbirds and as large as toucans and hawks got caught as well. Passerines are small songbirds that make up about 50% of all bird species; Midwestern examples include American robins and Northern cardinals.

Once captured and banded, many different measurements are taken. Pollock takes data “on molting patterns and what time of year they occurred, on body fat condition particularly for migratory birds, and on stress corticosterone levels through blood samples.” Researchers also take morphological data such as “bill dimensions, including bill depth, length, and width, alongside tail and wing length, tarsus, and body mass.” With this information compiled over their 43-year study, they can understand how physical conditions of birds and their populations change over space and time.

Consistent, long-term studies of this kind allow researchers to accurately answer questions on the life history traits of tropical birds while also documenting population fluctuations in response to dramatic habitat changes like deforestation for farming and urbanization. Specifically, Pollock has worked with Brawn to document species declines in response to variables related to climate change, such as rainfall. Only with standardized collection protocols over long periods can we understand these important life history traits and the complex relationship of birds to their environment.


Color Coordination

A grasshopper sparrow extracted from a mist net. Credit: Fish & Wildlife Foundation of Florida

While Pollock’s research contributed to a community-level study of bird species, that is not the case for Sarah Winnicki, a current Ph.D. candidate at Illinois. This project focused on an elusive yet common migratory songbird named the grasshopper sparrow in the Kansas prairie. Grasshopper sparrows are small brown-tan birds with a striped back and orange-yellow strips in front of the eye. Unlike the community-level study from Panama, Alice Boyle of Kansas State University, Winnicki’s advisor, started a long-term study focused on a single, understudied grassland species facing steep declines in 2013. The overall project is focused on capturing male grasshopper sparrows to understand site fidelity. As an undergrad, Winnicki studied where males were relative to each other, and determined if spatial clumping occurred between related or unrelated individuals and why, alongside other questions. To do this, researchers needed to understand which birds were which — that’s where color bands come in.

As Winnicki explained, “the (U.S. Geological Survey) issues aluminum or steel metal bands, each engraved with individual ID in that band size.” These are the bands used at Homer Lake and throughout the Panama tropical bird study. When these birds are recaptured, the unique number combo is reported to the BBL, which is important for long-term population studies. However, if all birds only have metal bands, from far away “all the sparrows look the same,” and there’s no way to know if related males nest close together through observations. To study these species’ post-banding behavior, researchers also attach “bright colored bands on them, which are colorful loops of plastic in the shape of aluminum band that you can gently squeeze around bird’s leg,” Winnicki said. Each bird is given a unique color combo, and with that information Winnicki and other researchers used spotting scopes and cameras with high-zoom-capabilities to identify each individual sparrow and its behavior.

Banding would start in early May when the grasshopper sparrows arrived. Winnicki emphasizes that “every bander has a different protocol,” but generally setting up mist nets in Kansas prairies requires different steps than in forested habitats. In particular, Winnicki and her fellow researchers “use rocks on the bottom trammel of the net to weigh it down because grasshopper sparrows, like other grassland birds, run along the ground.” Furthermore, the nets are not always set up in the same place, like the standardized Panama study. Instead, GPS points are taken where males without bands are spotted, and nets are set up there soon after. The study used “targeted mist-netting,” which is a method of luring and capturing a single species. To attract specifically grasshopper sparrows, they use audio lures, which are playback audios of a bird’s calls to target and capture the sparrow. Winnicki said the team would “place a speaker at the base of the net, connect it to an old iPod bought off eBay, and play grasshopper sparrow territorial songs.” The males often react instantly and “freak out” thinking another male is in their territory, then fly straight into the net. If it doesn’t happen instantly, Winnicki watches the nets closely and immediately extracts the bird once caught.

Winnicki said that males were the center of the study “because that was the sex our grant funding was targeting, and because females are especially cryptic and don’t respond to audio lures.” In other words, the males are easier to catch because they are territorial and more vocal relative to females, which is common in birds.

Everything from morphological data (like wing length and weight) to blood samples were collected for hormone and genetic analyses to understand relatedness of individuals. Compiling the data over many study years painted a clearer picture of grasshopper sparrows’ ecology, their site selection and nesting behaviors, and patterns in their population numbers. After testing a variety of hypotheses, Winnicki and her group found that grasshopper sparrows didn’t aggregate their nests near relatives, and there is still uncertainty regarding if and why spatial clumping occurs.


Studying Endangered Species

These same methods used by Winnicki can be translated to studying federally endangered species. Gabby Jukkala, a master’s student at Illinois, does just that; she researches how nest success varies with age in the golden-cheeked warbler, a federally endangered migratory songbird. Golden cheeks, as she refers to them lovingly, are small, round, black and yellow songbirds with a lot of personality.

Jukkala explains how “in a lot of bird species, birds in their first breeding season, referred to as Second Years (SY), have lower reproductive success than older birds, referred to as After Second Years (ASY). This may be due to older birds breeding at a time when conditions are better, being more experienced parents, or just outcompeting younger birds for higher quality territories. My goal was to figure out if age-related differences in nest success occur in Golden-cheeks and, if so, what factors may be driving them.”

Jukkala sought to answer many questions, including if breeding timing, parental behavior, and nesting habitat varied with age, and if differences in these factors influenced nest success. She says that bird banding played an integral role in answering these questions because “capturing birds and looking at their plumage in hand is the only way to accurately age the species.” Once you know the age of a bird and band them with a unique color band combination, “you are able to follow that individual throughout the breeding season to determine their success, as well as identify them if they return in future breeding seasons. Over time this creates a large database of known-age birds and their reproductive success in the study area, which was critical for my study.” 

A golden-cheeked warbler. Credit: Steve Maslowski via Wikimedia Commons

Her research took place in Fort Hood, Texas, an active military installation where a long-term monitoring program has existed for golden-cheeked warblers since 2000. Fort Hood is unique in that “golden-cheeked warblers only breed in central Texas, and Fort Hood has the largest monitored population anywhere in its breeding range. This is because military bases have to comply with the Endangered Species Act, and the base is not subject to urban development pressures like surrounding private lands. This results in large tracts of protected, high-quality habitat that are actively managed for the warbler and other wildlife.”

To capture the birds on their arrival, she used target mist-netting, which required audio lures. Instead of old iPods, Jukkala employed two Bluetooth speakers to attract the warblers. The study used a free sound-mixing app, marketed toward DJs, to choose and mix a variety of golden-cheek audio so that the birds wouldn’t get accustomed to any one call. Both territorial songs and female vocalizations were used at different times of the season. Once the birds were released, Jukkala tracked them via color band resightings and took note of their nesting location. Nest success is determined by the number of  baby birds that survive and leave the nest. In addition to newly banded birds, she monitored any returning birds in the study area.

Her results showed that younger males experienced higher nest predation because they began breeding later in the season and nested in lower quality habitat than older males, explaining their reduced reproductive success. What she found especially exciting was how her results can be used in statistical modeling of golden-cheeked warblers. By incorporating the reproductive rates of males at different ages, the population models of these endangered species can become more accurate and aid in conservation efforts.

Without the use of mist netting and bird banding, Pollock and his colleagues wouldn’t have been able to track tropical species’ life spans and life history traits over time, Winnicki wouldn’t know if the same males were nesting closely to each other, and Jukkala wouldn’t know for certain if younger male birds had lower reproductive success than older birds and why. Mist netting was essential to analyze morphology, collect demographic data (age and sex), and track behavior in all three studies.

Setting up nets on that cold spring morning at Homer Lake was designed for much more than simply catching birds. It was about collecting data that’s added to a larger dataset, the Bird Banding Laboratory, encompassing the North American continent. It was about understanding where birds move and migrate, and how their condition changes. Each bird banded makes our understanding of that bird’s lifecycle and ecology more accurate — and increases our chances of conserving these cherished winged creatures for generations to come.

About the Author …

Lauren Brunk is a senior from Grayslake, Ill., majoring in Natural Resources & Environmental Sciences (NRES) with a concentration in Fish, Wildlife, and Conservation Biology, minoring in Integrative Biology, and pursuing the Certificate in Environmental Writing. She is conducting research with NRES Adjunct Professor Jinelle Sperry, using eDNA and camera traps to study endangered and cryptic species, and NRES Professor Mike Ward, using radio transmitters to study owl migration. She is considering graduate school programs in wildlife biology.

This piece was written for ESE 498, the CEW capstone course, in Spring 2022.