ITHACA, N.Y. — America’s Corn Belt is a barrier for migrating landbirds, causing them to adjust their flight behaviors, research from the Cornell Lab of Ornithology shows.

“Our definition of barrier is any kind of landscape with a lack of stopover sites, which are basically the resting and refueling points for birds when they’re migrating,” Fengyi (Freda) Guo, a postdoctoral scholar in the Cornell Lab of Ornithology, said. “Birds then need to negotiate [the barrier] through changes in their behaviors.”

For example, parts of the ocean and deserts have few or no stopover sites, making them natural barriers. The Corn Belt is an anthropogenic, or human-made, barrier; 76.4 percent of precolonial vegetation in the Midwestern US has been replaced by agricultural and urban areas and the landscape is primarily composed of corn fields. 

Guo and her colleagues used five years of data from 47 weather radar stations in the United States to compare flight patterns of migrating landbirds crossing the Corn Belt and the forested landscapes south and north of the Corn Belt in spring and fall. Compared to forested landscapes, birds crossing the Corn Belt fly faster and are more selective about flying with favorable tailwinds. Guo added that birds carefully adjust their flight time and height to take advantage of tailwinds.

This changed flight pattern is similar to the flight pattern of birds crossing the Gulf of Mexico, a natural barrier that birds must make a nonstop flight across. However, radar data shows that birds that do not make a nonstop flight over the Corn Belt are more likely to make stops in the scattered forested areas still present in the landscape. Guo emphasized that these forest fragments still present within the Corn Belt are extremely important because birds are making heavy use of them during migration.

“These forest fragments are the only available habitats for the billions of birds migrating through, as a lot of the forest species cannot make use of cornfields,” she said. “It would be great if all of these forest fragments could be protected or restored. Or even restoring more habitat for migrating birds to create what we call stepping stones throughout this area.”

While the Corn Belt is still a weaker barrier than other natural barriers, if the remaining stepping stones or forest areas continue to disappear, the landscape will become significantly more challenging for birds to cross. The forests surrounding and bordering the Corn Belt are also critical, Guo added, because birds tend to pile up in those areas after crossing barriers to rest and refuel.

According to the 2025 State of the Birds report, published by Cornell University, birds across most habitats have suffered major losses since 1970; airland and grassland species have both lost over 40 percent of their total populations over the past 55 years. The report also determined that around one-third of US bird species are of high or moderate conservation concern, meaning the species are at a greater risk of extinction or degradation. The North American bird population is down nearly three billion birds since 1970, the vast majority being migratory birds.

Habitat loss and degradation are two of the leading causes of bird declines. Climate change has also impacted the timing of migration, sometimes leading to a lack of food sources for birds as they migrate. Migratory barriers could lead to some species becoming locally extinct or a species “reshuffle” as some species disappear and others become the majority, Guo added. Declining bird populations also impact human society, as birds provide pest control and seed dispersal.

Currently, Guo and her research group are collaborating with the USDA Natural Resources Conservation Service on an incentive-based conservation program, centering on habitat protection and restoration on private working lands.

“Specifically, we are helping to design a new conservation framework that includes part of the Midwest Corn Belt,” Guo said. “We are working on providing maps to help identify priority areas for this new conservation framework for land that is identified as stopover hotspots.”

The group is hopeful that the framework will be implemented in the next few years to provide financial and technical assistance to farmers. Guo and her colleagues are also continuing to investigate the factors driving declining bird populations, examining stopover habitats, and quantifying the impact of climate change.