How the Breeding Season Teaches Us More About Migratory Birds

During the brief windows when migratory birds are busy breeding and relatively stationary, migration researchers leap into action to unravel the mysteries of their journeys.
Black Tern. Photo: Diana Whiting/Audubon Photography Awards

Amidst the verdant embrace of North American summers, the landscape comes alive with a symphony of songs and a flurry of activity as migratory birds settle down and engage in the timeless ritual of establishing territories and raising young. Summer is also a time of year pivotal for many migration scientists who eagerly look to unlock the enigmatic secrets these feathered voyagers hold. As the sun warms the land and birds remain relatively stationary while engaging in parenting duties, intrepid researchers embark on their own migrations, driven by fascination and purpose. The decisions scientists make about where and when to study birds are critical to understanding avian movements, behaviors, and the pressing need for hemispheric conservation.    

Each spring, scientists equipped with an array of tracking devices (e.g., nanotags, light-level geolocators, GPS, and satellite tags), optics, and other essential field gear, traverse vast distances, diligently following the invisible trails of winged wanderers. Their research takes them to a kaleidoscope of breathtaking habitats, each a unique piece in nature's mosaic. The dedicated researchers venture from the untamed grandeur and wilds of the Arctic tundra, bustling with nesting waterbirds, shorebirds, waterfowl, raptors, and high-latitude passerines, to serene temperate forests, the haunts of secretive wrens, thrushes, and warblers. They explore the vast expanse of aridlands and grasslands, where winds carry birdsongs of thrashers, sparrows, buntings, and longspurs great distances, and journey to the lively and often deafening seabird colonies in temperate and sub-tropical regions. However, despite their efforts to reach far-flung research sites, researchers can never be everywhere at once, nor is it practical. Instead, they journey to critical habitats at crucial times, including the breeding season, strategically deploying transmitters and gathering comprehensive information about the entire expanse of the region. 

One of the first significant benefits of studying birds on their breeding grounds during late spring and summer is the comparative ease of capturing birds at this time of year compared to other seasons. According to Bill DeLuca, senior manager of migration ecology for Audubon’s Migratory Bird Initiative, “Birds on the breeding grounds, especially males, are very territorial and defend their area to the death, which we use to our advantage by using decoys and song playbacks to make sure we deploy all the trackers we want to get out.”  

While territoriality during the breeding season assists with the deployment of tags, another factor in bird biology helps in their retrieval: site fidelity, i.e. the tendency of individual birds to return to and use the same locations year after year. Taking advantage of site fidelity behavior during the breeding season, researchers can deploy archival tags such as light-level geolocators or GPS loggers that require being physically retrieved after a designated period, usually the following year, to obtain the data. These tags cannot transmit data due to their smaller size and, thus, limited battery capacity. Thankfully, in the case of many smaller species that receive these types of tags, site fidelity during the breeding season can sometimes be exact, facilitating the recovery of these devices. “These birds come back year after year, after traveling thousands of miles, to the same patch of four or five trees,” exclaims DeLuca. “And because we have to get these transmitters off, that’s our best opportunity.”  

Another advantage of deploying tracking devices on the breeding grounds is that you can ensure what populations of breeding birds are being tracked, in contrast to capturing birds during migration, where populations often mix, sometimes unknowingly to the researcher. Studying the migrations of known populations is crucial for understanding patterns of migratory connectivity—a vital aspect with significant implications for conservation. Migratory connectivity measures the extent to which populations of animals are linked through full-lifecycle biology. High connectivity indicates limited mixing between populations on breeding and wintering grounds, where animals from one part of the species range move to another with little overlap. Conversely, low connectivity suggests more widespread mixing during migration and on the wintering grounds. Detailed knowledge of migratory routes, timing, and wintering locations is vital for effective conservation and management strategies. Knowledge gaps in migratory connectivity contribute to challenges scientists face in identifying, quantifying, and forecasting factors like climate change, which significantly impact the biology of migratory species. 

Deploying tags during the breeding season offers another valuable benefit: increased accessibility to certain species, even in remote or challenging locations. The Black Tern provides a fascinating example of a species living a dual terrestrial-marine life whose researchers take advantage during the nesting season to deploy tracking technology. Nesting in vast freshwater wetlands across much of the northern tier of North America, these elegant seabirds embark on an impressive migration journey, flying far offshore over the open ocean before wintering along tropical coasts. For researchers like Jenni Fuller, a project coordinator for Audubon’s Science Division, deploying tracking devices on these elusive birds is most effectively accomplished during the breeding season.  

Fuller explains that capturing Black Terns during winter is exceptionally challenging, making the breeding season the optimal and preferred option for capturing and tagging birds. “We time it right where they're most aggressive and really want to sit on their nest. We wait in the reeds and hide. Otherwise, they'll focus their attention on us,” says Fuller. By skillfully leveraging the terns’ nesting behaviors and aggressiveness during the breeding season, researchers trained and permitted to successfully deploy tracking devices, help unravel the mysteries of their migration and contribute to our increased understanding of avian movements.  

As you savor the beauty of summer, relishing in the sight of fledgling birds taking flight, spare a thought for the intrepid researchers embarking on their own migratory journeys. With dedication and purpose, they deploy tracking tags on these winged wonders during the breeding season, unraveling the mysteries of their annual migrations. Through their tireless efforts, a richer understanding of avian species and their conservation needs unfolds, illuminating the remarkable journeys these birds undertake across the hemisphere where they spend a significant portion of their year.