The Cerulean Warbler Is In Decline but Help Is on the Way

Nestled within the canopy of mature forests across the eastern United States and southern Canada, the Cerulean Warbler is a songbird perfectly suited to treetop life. Males flaunt sky-blue and white plumage, while females display rare blue-green hues that blend into the forest's high reaches. Spotting this tiny warbler is challenging not only because of its lofty haunts but also because of a dramatic population decline of nearly 70 percent since the 1980s. Symbolic of older-growth deciduous forests in the breeding range, this species is increasingly difficult to find. 

Scientists and conservationists have organized initiatives such as the Cerulean Warbler Technical Group and the Cerulean Warbler Conservation Initiative to combat this decline, uniting government, academia, industry, and organizations like Audubon. These collaborations aim to develop comprehensive, science-based solutions across the Cerulean Warbler's annual migratory cycle. At the core of these efforts is the need for a deeper understanding of the species' movements and habitat preferences throughout the year. Enter Douglas Raybuck. 

Between 2014 and 2017, Raybuck and collaborators deployed light-level geolocator loggers on 257 male Cerulean Warblers at 13 U.S. study sites and 25 loggers at two sites in Colombia. Their objectives were clear: evaluating connectivity between breeding and non-breeding regions, identifying migration routes and stopover areas, and documenting migration timing—firsts for the species. While these data are now published, accessible, and incorporated into the Bird Migration Explorer, no previous tagging initiatives existed for Cerulean Warblers due to the lack of suitable tags for birds weighing nine to ten grams.  

Even with new technology, tracking Ceruleans presents challenges. Raybuck used light-level geolocators, which allow for calculating latitude and longitude based on the timing of sunrise and sunset, which forest canopies and mountains can obscure through “shading effects” making location estimates fairly imprecise. Therefore, these must be analyzed with caution. Researchers must also recapture tagged birds because geolocators only store data and do not transmit it to a secure server. Fortunately, many songbirds exhibit site fidelity and territoriality, aiding recapture efforts. However, females were less responsive to capture techniques and were not tagged, a conservation limitation when the need to understand female Cerulean Warblers' ecology is vital to securing their future. 

Despite retrieving data from only 28 individuals, Raybuck's findings revealed notable migratory patterns. Most Appalachian breeders—about 93 percent—wintered in the Colombian and Venezuelan Andes, with 80 percent spending the winter in Colombia, suggesting that protecting these birds may require targeted efforts in Colombian forests. In contrast, approximately 71 percent of breeders from the Ozarks spent their non-breeding season in the southwestern part of their non-breeding range, specifically in Peru and Ecuador. Consistent with stable isotope studies, these results tell us the places we need to protect and restore in order to secure a future for the species. 

Raybuck emphasizes that the Cerulean Warbler’s decline results from habitat loss and degradation across its global range. In North America, extensive forest clearing has historically reduced breeding grounds, and although some forest recovery has occurred in recent decades due to an increase in aging forests, long-term impacts from past clear-cutting and high-grading practices remain. On the non-breeding grounds, habitat loss remains severe in the Andes due to development and cattle ranching. Cerulean Warblers depend on a narrow elevation band (800 to 2,000 meters), which coincides with areas ideal for human settlement. It is a perfect recipe for species declines to occur. However, these recent develpments in tracking data now help pinpoint the habitats essential to the species. 

Conserving Cerulean Warblers during migration is equally essential, with Raybuck advocating for protecting stopover habitats as well: "Ceruleans face unique challenges across countries, even when threats stem from similar baseline elements like habitat loss and degradation." The bottom line is that these warblers, like all migratory birds, require conservation efforts beyond their breeding and non-breeding strongholds. Areas such as Central America, the southeastern U.S., and the Mississippi Alluvial Valley should not be overlooked—and thanks to Audubon, they’re not. 

Since 2011, Audubon staff in the St. Louis region at Audubon's Center at Riverlands have surveyed nine forest-breeding species, including Cerulean Warblers, to develop habitat models for critical forest management. Led by Tara Hohman, conservation science manager for Audubon's Upper Mississippi Flyway, this project, in partnership with the Army Corps of Engineers, highlights the scarcity of Cerulean Warblers along the Mississippi, where habitat quality is currently suboptimal. 

"They need a mix of habitats, particularly mature old-growth forests along the river’s floodplain," Hohman explains. "Bottomland forests struggle to regenerate due to repeated flooding, making long-term restoration efforts crucial for Cerulean Warblers." Hohman and the warblers wait for managed areas to develop. "It will be a long time before sizable chunks of restored habitat are available to help them." 

Despite these considerable challenges and thanks to Raybuck's tracking efforts, Hohman's habitat monitoring, and ongoing conservation work, the future for Cerulean Warblers is looking brighter as vital habitats are identified and protected across their full annual cycle.