Forest Fragments Act Like ‘Convenience Stores’ for Migrating Birds, Study Finds

Fengyi Guo’s fascination with bird migration began in 2017 after reading No Way Home, a book by Princeton University ecology professor David Wilcove that highlights the decline of migratory animals. Intrigued by the idea that so many species travel such great distances every year, along with the challenges that protecting them involves, Guo joined Wilcove’s lab a year later as a PhD candidate studying conservation ecology.

Saving any species is hard work, but conserving migratory animals adds an extra layer of complexity: They live in two different places depending on the season, and when migrating, they can cross state or country borders. Migratory birds present even more of a challenge, adjusting their routes in flight as needed and landing wherever they want. “They can be everywhere because they don't really have a fixed migration route,” Guo says. “In order to conserve them, we need to figure out [their] most important habitat.”

In a study published in PNAS in January, Guo and her colleagues set out to do just that. Using weather radar, the team identified the most popular stopover sites for migratory songbirds—areas where they can rest and refuel before resuming their energy-intensive journeys—throughout the eastern United States. The researchers chose to focus on eastern migratory landbirds because those populations see the largest declines among North American migrants during their travels.

For the study, Guo and her team reviewed data from fall migrations from 2014 through 2019 from 60 radar stations covering 244,016 square miles, or around 24 percent of the eastern United States. Intended to mostly monitor weather conditions, the radars can also detect movements of insects, bats, and birds. In the case of migratory songbirds like warblers and thrushes, the radars can capture when they take off from stopover sites to resume their journeys. Using supercomputers to analyze 12 terabytes of radar data—roughly equivalent to more than 1 million of phone books—the researchers mapped areas where the birds consistently return to each year.

Among the team’s key findings was that, in addition to large tracts of primary forest, fragments of deciduous forest are some of the most important stopover sites for migrants across the east. These tiny patches of forest become “convenience stores” for migratory birds, the authors write—especially when flying over inhospitable landscapes like urban areas or the Midwest farmlands that cover what was once prairie.

However, the team also found high concentrations of migratory birds in the large tracts of forest that make up the “prairie-forest boundary” to the east of the Great Plains. According to the researchers, these two findings suggest that some birds may use habitat patches to hopscotch across the farmlands of the Midwest, while others use the border areas as rest stops after winging across the expanse in a single flight—similar to how birds gather on coasts after crossing large lakes or the ocean. “The agricultural landscape might be a migration barrier for those birds,” Guo says.

Though scientists already knew that migrants depend on forest fragments while migrating, the study is important because it confirms previous research while also putting the value of these strongholds into a larger context, says Scott Robinson, a bird ecologist at University of Florida who wrote a comment paper on the study’s results. He finds the popularity of the patches within agricultural areas especially notable because, while they are not the first choice for birds to nest and breed, they are still important to conserve. “Just because nesting success is low, it doesn’t mean they don’t have tremendous value. Let’s not just let them disappear,” he says. “They are really good for migration.” 

Understanding how these small areas connect and fit together on a broader scale was the biggest benefit of using radar data. Earlier studies of stopover sites have mostly only focused on singular patches, hindering researchers from capturing the full picture of bird migration—how many stop where and when.“It’s very hard,” says Adriaan Dokter, migration ecologist at Cornell Lab of Ornithology, who was not involved in the study. “You always have to survey in many forest patches at the same time to make that comparison."

Using radar data is also more efficient to monitor large bird populations compared to other methods, such as using GPS trackers on individual birds. Aside from the cost it would require to track large numbers of birds, researchers would need to retrieve the trackers to get the data. “But birds die during migration, so people often have very limited data,” Guo says. “Whereas using radar, we monitor the entire crowd of birds whenever they are up in the atmosphere. It can be millions to billions of birds.”

Dokter notes, however, that radar data currently has limitations in the Midwest, where coverage is not as robust and may prevent researchers from painting a complete picture of migration through the region. He cites the high concentration of birds that use the boundary areas as a finding that needs more research. “The closer you are to that forest-prairie boundary, the densities are really quite high,” Dokter says. “But it might also be that there’s some great forest there.” Essentially, without more data, it’s hard to know if some migrants are skipping the farmlands entirely and landing in these areas for reprieve, or whether birds simply like them because they have good habitat and plenty of food. “I think that’s something we have to find out in more detail."

Guo also points out that radar might miss stopover sites for endangered or threatened species with lower populations, such as the Cerulean Warbler. “The abundance is not high enough to be identified as an hotspot,” she says. Additionally, while the radar is valuable for following large groups of birds, the data are limited when it comes to identifying which species are on the move and using certain areas. 

But researchers can fill in those holes using other sources of data, suggests Princeton’s David Wilcove, now Guo’s adviser and co-author of the study. Field data can confirm which species rest in certain patches, at what times, and why. Birders can also do their part: Community science platforms like eBird, which allows birders to record and share their observations with researchers, can provide valuable supplementary information.

While Guo recognizes that further research needs to be done for an even more comprehensive understanding of stopover sites, she hopes the findings of this study will help guide bird conservationists when trying to save or restore land most used during migration. “I think most people in the birding world know the importance of stopover habitat, but we just don’t know where they are,” Guo says. By identifying and highlighting the hotspots, Guo and her team hope “people can start conservation efforts accordingly.”

That work won't always be easy, though, with so many of the stopover sites identified being located on farms and other privately held land. But Wilcove remains optimistic that, when presented with the proper information, farmers and other landowners can become partners in conservation. “We all benefit from having healthy populations of wild birds,” he says. “So we all have a stake in ensuring that their populations remain strong.”