On a cool, clear summer morning a few weeks ago in northern California, several members of the KBO team and I crouched behind a screen of willows next to our mist-net, swatting at the abundant mosquitoes and listening to the birds singing all around us. We were a bit nervous, as our research quarry was a Yellow-breasted Chat, a strikingly beautiful bird known for its garrulous song, but also a bird that can be furtive and shy, preferring to keep to the densest thickets of blackberry. Our goal was to capture, tag, and release 22 male chats… and we only had four days to do it!
We would attach a lightweight scientific device called a geolocator to the Yellow-breasted Chats we captured in order to track the birds throughout the year. We wished to learn their migratory routes and the location of their wintering grounds. Understanding migratory connectivity – the way a single bird population links geographic areas through its breeding, migratory, and wintering behaviors – has long been a significant scientific challenge. Many songbirds travel incredible distances over the course of a year, and in most cases they are too small to carry GPS satellite transmitters that would allow biologists to study them year-round. The resulting gap in our knowledge is a barrier to successful full life cycle conservation. Scientists and land managers need to know where birds are throughout the year in order to better understand habitat needs and identify threats.
Fortunately, advances in technology are helping us overcome the logistical challenges of monitoring small songbirds, and we’ve learned a tremendous amount about the movements of North American breeding birds during the past decade. Small light-level geolocators for tracking birds now weigh less than half a gram. A geolocator is attached to a bird via a tiny backpack with two leg loops, and it records ambient light levels throughout the day. The data it collects can later be used to estimate the bird’s prior locations within a few hundred kilometers. Day length gives an estimate of latitude, as days are longer in the north in summer, and longitude can be calculated from the timing of sunrise, as the sun rises earlier as you travel east across the globe. One limitation of geolocators is they are so small they cannot transmit information; therefore, these units must be retrieved—typically following a roundtrip migratory journey—in order to extract the data.
On this cool June morning, Klamath Bird Observatory was attempting to employ the new geolocator technology on Yellow-breasted Chats at our Trinity River field site in northern California. We were trying to lure the male Yellow-breasted Chats into our mist-nets by playing audio recordings of other males singing territorial songs. The hope was that male chats in our area would rush in to investigate the new “rival” and inadvertently fly into one of our soft mist-nets. We also had a painted wooden chat decoy to use as additional bait. We weren’t sure how strongly the males would respond to audio playback or the decoy, and thus we waited anxiously; the success of our mission hinged upon their behavioral response.
After some time passed and we hadn’t heard our target male singing, two of us broke off to set up a new net in a (hopefully) better location. Before long, we heard KBO Executive Director John Alexander’s voice over the walkie-talkie: “He’s in the net!” With excitement, we raced back to the banding station to attach our first geolocator in what would become a very busy and thrilling week.
Assisting in this endeavor were KBO’s Trinity River field interns, who had been mapping the territories of Yellow-breasted Chat pairs, and several other bird species, for the past six weeks. They guided us to each known chat territory, allowing us to quickly locate and capture the resident males. We also had experienced bird banders from the US Forest Service’s Redwood Sciences Lab, including CJ Ralph and Andrew Wiegardt, and volunteer David Price, as well as experienced KBO staff, such as John Alexander and myself, to do the job.
We captured four males on the first day alone. Having the male chat in hand, however, was only the first step of the process! Placing a small geolocator on a small bird whose dense feathers obscure your ability to see what you’re doing requires significant manual dexterity. Each geolocator has a harness threaded through it, consisting of two leg loops made of Stretch Magic, a common craft supply item. The night before, we measured out the moderately stretchy rubber threads and fused them into closed loops in the field house. We used a formula to calculate what size of bird would match up with each harness we created. The chats in northern California ranged from about 22-29 grams in mass, requiring harnesses with spans of 45-51 mm. The difference of a few millimeters may seem negligible, but a harness that fits correctly is vital for bird safety. A harness that is too big or too small could hinder the birds’ wings or legs, and it could fall off or create other problems during the long migratory journey. While our method required some preparatory work, it allowed us to quickly attach harnesses in the field, thereby saving valuable field time and reducing stress on chats during the handling period.
We continued to move through our study plots, setting up nets in the dense streamside vegetation and eagerly watching the male chats respond to our audio “intruders” and fly into our nets. We eventually captured 22 males, which allowed us to deploy all of our geolocators! Now, we must hope that a substantial number of our tagged males survive the roundtrip migratory journey and the long winter to return again next spring so we have a chance of recapturing them and retrieving their data. Due to this challenge, most geolocator studies have relatively small sample sizes; nevertheless, these studies have revolutionized our understanding of migratory connectivity.
We are partnering with Christine Bishop and her research team from Environment Canada and Simon Fraser University. Together, we will examine our data and compare the migratory routes and wintering grounds of our northern California population of Yellow-breasted Chats with those of an endangered population of chats that breed in British Columbia. This project will eventually form a tri-national partnership, including our San Pancho Bird Observatory partners working in overwintering areas in Mexico. We are excited to see the results, but for now we must wait. The Yellow-breasted Chats in northern California are finishing nesting for the season and will soon wing their way back to their southern homes. They will remain there for several months, until the lengthening days of spring urge them to return to us once more.
During the recent International Partners in Flight Conference in Snowbird, Utah, the emphasis was on protecting birds throughout their annual cycle. Yet, it is really difficult to set conservation priorities when there are uncertainties concerning the threats that birds face throughout the year. And in order to identify threats, we need to know exactly where populations of our northern breeding birds go during migration and winter.
Everyone at the meeting was talking about migratory connectivity. This refers to the way regional populations of breeding birds create linkages among geographic regions through their migratory behavior. Understanding connectivity is vital to the identification of factors that harm specific bird populations, and unfortunately there is a significant gap in scientific knowledge on this topic. The problem stems from the fact that it is extremely difficult to track such small animals as songbirds over the incredible distances they migrate.
GPS transmitters—which have been successfully used on raptors and shorebirds—are simply too heavy to place on most songbirds, which often weigh less than a few quarters. Radio transmitters are small enough, but these are limited by short signal ranges that would require a biologist to be within several miles of a migrating bird in order to detect it. Fortunately, recent advances in technology are helping us overcome these logistical challenges and are generating valuable knowledge about the movements of North American breeding birds.
Small tracking devices called light-level geolocators now weigh only 0.4 grams and have permitted some amazing advances in our knowledge of migratory connectivity. These geolocators are attached to a bird via a tiny backpack with two leg loops, and they record ambient light levels throughout the day. The light-level data they collect can later be used to estimate the bird’s location within about one hundred kilometers. Day length can give an indication of latitude, as days are longer in the north in summer, and longitude can be calculated from the timing of sunrise, as the sun rises earlier (relative to Greenwich Mean Time) in the east.
The latest advance has been the development of archival GPS geolocators. These weigh a bit more (~1g), but they are far more accurate – within a few meters! To fit this technology into such a small package, they can only record ten location points. However, you can program the geolocator to record these location points whenever you want—say, three fixes during spring migration, four during winter, and three during fall migration. A single bird with a geolocator backpack can depart its breeding grounds in the summer and return in spring with a wealth of information on migratory routes and wintering grounds.
The disadvantage of both types of geolocators is that they are only archival—which means you have to find and recapture your bird the following breeding season to retrieve the data. Due to this challenge, most geolocator studies have small sample sizes, but even so have produced amazing results. For instance, in a 2012 study by Franz Bairlein and colleagues, they discovered that Northern Wheatears in Alaska migrate 14,500 km across Asia to winter in eastern Africa—a unique and incredible journey that was previously undocumented.
In another study, Kira Delmore and colleagues discovered in 2012 that neighboring populations of Swainson’s Thrush in British Columbia exhibited dramatically different migration routes. Coastal birds traveled down the west coast to winter in western Mexico, whereas inland birds traveled overland across the Rockies and crossed the Gulf of Mexico to winter farther south in Central America. Clearly, the conservation of these two populations would require very different strategies.
I picked up brochures on both types of geolocators from the Lotek vendor booth at the conference. The challenge and opportunity now for KBO will be to determine how best to employ this technology to advance bird conservation.