Home / Blogs / Why So Blue? The Plight of the Cerulean Warbler
May 17, 2021
A male Cerulean Warbler. Photo courtesy of the U.S. Fish and Wildlife Service, via flickr creative commons.
Cerulean Warblers can be hard to find. If you can hear them singing, you may notice that the namesake bright blue plumage of males blends in surprisingly well among treetops and a clear sky. Searching for them high up through binoculars may lead to a sore neck that’s well worth it; these stunning songbirds are increasingly rare in our forests. Since 1966, when surveys began, the Cerulean Warbler population has decreased by over 70 percent. As specialists of healthy mature forests, their decline has bigger implications for our Appalachian landscape than just the loss of an interesting songbird.
Cerulean Warblers (Setophaga cerulea) are neotropical migrants, meaning they spend the winter in the tropics of Central and South America and the summer in temperate North America. As dramatic as it seems to fly thousands of miles a year (especially when you only weigh 0.3 ounces), it’s worth it. The tropics are lush and productive but crowded with other birds, mammals, herps, and invertebrates that are also competing for food and space. By flying north to seek out the explosion of insect biomass that spring brings to the northern hemisphere each year, neotropical migrants are able to find enough protein and space to raise their young. Like all impressive adaptations in the animal kingdom, the biannual journey of neotropical migrants is simply a result of evolution. Ancestors of these songbirds who migrated were able to raise more offspring than those who didn’t, despite the lengths (literally and figuratively) that the birds had to travel, so the migratory genes eventually dominated the population.
The world has changed substantially since the primordial warblers began their big migrations, however. Deforestation for agriculture and development has been rampant in both the summer and winter ranges of the Cerulean Warbler and other neotropical migrant songbirds. Ceruleans overwinter in the Andes Mountains of Peru, Ecuador, Colombia, and Venezuela in forests that range from 1,600 to 6,000 feet in elevation. While our knowledge of Cerulean Warbler winter habitat is limited, we know that they select forests with mature canopies and robust understories. According to the US Fish and Wildlife Service, over 60% of Cerulean Warbler winter habitat has been lost to clearing for agriculture. You may have heard that shade-grown coffee is better for wildlife, and the Cerulean Warbler is a species whose picture you may even see on a bag of ethically grown coffee beans. Ceruleans, and many other species, will overwinter in coffee plantations that have mature tree canopies, but are very rarely found in sun-grown plantations. They also have no habitat at all in pastures or in row crop fields. Habitat loss in the southern hemisphere has been a major driver of population decline for the Cerulean Warbler. The songbird has also hemorrhaged habitat along its migration corridor through North and Central America, and certainly breeding habitat here in forests of the eastern US.
A male Cerulean Warbler. In addition to his blue plumage, note his “necklace” which separates a white throat and belly, and the dark streaks along his belly (Photo courtesy of Shannon Curley).
A Cerulean Warbler migrating back to West Virginia in the spring is hoping for conditions that were common thousands of years ago: forests with large old canopy trees and an ample midstory and understory that produces abundant insects to eat and feed to growing chicks. They are increasingly unlikely to find that each April, however. We have of course lost substantial forest cover in the northern hemisphere due to clearing too. Across the Cerulean’s breeding range, we now have 50% less forest cover than we did before Europeans arrived in North America. On top of general forest loss, specific habitat that is suitable for the songbird has plummeted as our forest landscape has shifted. Cerulean Warblers are known as area-sensitive species, meaning they need the landscape to be intact forest or largely forested in order to hold substantial population sizes. In heavily agricultural areas of its range, the Cerulean Warbler needs around 4,000 acres of woods, while tracts can be as small as 60 acres in deeply forested areas. The forest cover that we have retained in Appalachia, where an estimated 80% of Cerulean Warbler breeding takes place, is increasingly fragmented by development and infrastructure for towns and fossil fuel extraction.
When a Cerulean Warbler does find an amply forested landscape in its breeding range, it may still not be suitable habitat. The songbirds are searching for woods that are akin to what its ancestors would have found our region to be brimming with: big old oaks and hickories that are well-spaced, leaving ample light for diverse, lush growth of smaller trees in the midstory and saplings and shrubs in the understory. These conditions are factories for insects which the warblers need (the genus Setophaga means “moth-eater” for a reason) to survive and raise their chicks, and create enough foliage density for the birds to feel safe building nests. They didn’t fly 4,000 miles just for their eggs to get gobbled up by nest predators!
These uneven-aged woods are not as common anymore though. After “the big cut”, a period of wholesale forest clearing for industry and railroads in the late 1800s and early 1900s, our landscape was left with forests that regenerated as a single age. Oaks and hickories happily grew in the open sun, but formed dense, homogenous woods without the structural and species diversity that Cerulean Warblers and many other species require.
Our modern iteration of unsustainable timber harvests have further reduced suitable Cerulean Warbler breeding habitat. They may look much more gentle, but are also costly to our forest ecosystems. “Select cuts”, “diameter-limit cuts”, and “high-grades” are all terms for a practice that is still very common in our region, where large, commercially valuable trees are felled and smaller ones are left. Rather than giving room for desirable species like oaks to grow, what this actually results in is a shift in species away from oaks and hickories, which can’t regenerate in dense shade, and towards species like red maple, beech, and black birch, which are tolerant of shade and are left by loggers who are selecting more valuable timber. Many nature enthusiasts revile clearcuts, but if properly prepared those harvests can be immensely more beneficial for wildlife and the forest’s floral community than select cuts, which can shift species composition towards one that is less diverse and less accommodating of most other species.
A complete lack of management can also result in forest degradation, as counter-intuitive as that may sound. Taking no action to recover from select cut harvests will result in woods that will continue to decline in diversity and utility for wildlife and humans alike. Additionally, invasive plants will strangle a forest floor if left unchecked, hampering regeneration completely. When canopy trees eventually die, there will be nothing to replace them and over time all that is left of a formerly valuable forest is a tangle of invasive plants, which do not contribute much to the ecosystem.
Luckily, humans can manage their woods to increase their suitability for the lovely Cerulean Warblers and many other wildlife species. And in doing so, can create more healthy, resilient, productive forests for trees too. This typically entails thinning the forest just as you would in a garden: removing undesired plants to give more space for everything else. By removing invasive plants and light-intercepting birch, beech, and red maple and retaining large oaks and hickories in the canopy and younger members of those genera in the mid and understory, we can create conditions that are quite conducive to Cerulean Warblers in the near future.
This management technique also happens to be the recipe to regenerate the oaks and hickories that are valuable for commercial use. A sustainable way to harvest oaks is what’s called the “shelterwood system”, where less-desired canopy trees are thinned out and oaks are retained. This increases light to the forest floor, allowing a carpet of oak seedlings to grow tall enough to avoid undignified deaths by deer browse. Then, typically 10-20 years after the thinning, the oak overstory can be removed and the saplings can begin their journey towards becoming the canopy. During the period between harvests, the open, sun-drenched woods are excellent for Cerulean Warblers and many other species. A true win-win, the landowner will make good money for their logged trees, which compensates for the decades of investment in hard management work to get to the point where the oak saplings were ready to be released. Ideally, patches of shelterwood systems are interspersed throughout the forested landscape, so that at any time there are stands of older forest, young forest, and everything in between. Diversity is the foundation of healthy ecosystems, and given how homogenous our landscape has become, humans must intervene to set things on a more healthy and resilient course.
A shelterwood cut for Cerulean Warblers in Somerset County, PA. Retaining around 70 square feet of tree basal area per acre creates the open conditions that are needed for both Cerulean Warblers and desirable seedlings (Photo by Ryan Davis).
I have worked in Appalachia as an ecologist for around a decade now. To me, the sky-blue Cerulean Warbler is an emblem of what these mountains can be. The old, complex forests that these gorgeous little birds call home all summer long are rich in plant diversity and loud with diverse birdsong. These ancient ridges have been home to healthy forests ever since they’ve been free from glacier cover and they can be again. Careful, thoughtful, ecologically sound management can support this stunning songbird and thousands of other species, while still accommodating the human economy. We can’t control habitat loss or degradation in the Cerulean Warbler’s winter range or migration routes, but we sure can improve our own mountain forests to ensure that they have somewhere to breed.
Pennsylvania Forests Projects Manager
(717) 517 8698
Conserving Chesapeake Forests