Migratory birds are experiencing precipitous population declines due to land-use change in Central and South America.

These birds rely on forested areas in their southern overwintering grounds for sustenance, but these have been widely replaced by less hospitable agricultural landscapes.

Some vulnerable migratory birds use tropical hardwood plantations at the same rate as forests, making these for-profit agricultural lands an attractive prospect for conservation, especially in contrast with poorer habitats like cattle pasture.

Agroforestry solutions, such as the retention of tall trees, can also provide habitat for at-risk species like the golden-winged warbler while providing ecosystem services to farmers.

To many in North America, the sound of birds among the early spring trees is the sound of renewal: after a long, cold winter, the colorful warblers and vocal thrushes have finally returned, bringing with them the promise of warming weather. It’s a seasonal phenomenon as old as time — but one that’s in peril as the birds contend with increasingly desolate wintering habitats that leave them ill-prepared for the journey north, a recent study finds.

“Tropical forests and complex agroforests … are both declining in coverage, and many species of migratory bird are declining along with them,” says Ruth Bennet from the Cornell Laboratory of Ornithology in New York and a co-author of the study published in the journal PLOS One.

Much of what researchers know about the conservation of migratory birds in the tropics comes from studies done in shade-grown coffee ecosystems and adjacent forests. But land-use changes on a large scale throughout South and Central America mean these kinds of habitats are growing scarcer.

This recent study, conducted in Guatemala, focuses on how 42 species of migratory birds spend their time in other types of agricultural landscapes. The researchers classified five of these birds as conservation priority species: the golden-winged warbler (Vermivora chrysoptera), blue-winged warbler (Vermivora cyanoptera), Kentucky warbler (Geothlypis formosa), worm-eating warbler (Helmitheros vermivorum), and wood thrush (Hylocichla mustelina). All are currently listed as “red” or “yellow” under the Partners in Flight watch lists, which designate bird species by vulnerability factors such as population size. These conservation priority migratory species rely more heavily than other birds on forests for their wintering grounds, and may already be experiencing population declines most likely caused by the loss of those forests.

The waning popularity of shade-grown coffee plantations is hurting migratory birds, according to Bennett.

“Most coffee is now grown in full-sun monocultures that provide little value to migratory birds, while other types of tree crops, like teak, rubber, oil palm, and native hardwoods are expanding in tropical landscapes,” she tells Mongabay.

Governments and various organizations offer incentives for farmers to cultivate coffee under a canopy of assorted trees, in a form of agroforestry that promotes biodiversity and ecological services. But that hasn’t stopped agricultural intensification, where the coffee plants are cultivated on wide swaths of open farmland. Many growers believe that the perceived higher yields for this full-sun coffee — a perception not necessarily supported by current research — far outweigh financial incentives like subsidies or sustainability certifications for shade-grown coffee.

According to Bennett’s research group, mixed-native hardwood timber and teak plantations can provide alternative habitats for migratory bird species that require forest or shaded ecosystems to survive. Some species, though not conservation priority migratory birds, also use oil palm and rubber plantations. Still, researchers say better plantation management strategies are needed across the board to ensure that the agricultural landscapes that these birds winter in provide sufficient resources to ensure they can build up the required fat reserves to survive a migration of potentially thousands of miles.

Plummeting populations

“It is very worrying. You’ve seen all the numbers,” says Paul Salaman, chief executive of the Rainforest Trust and an Oxford-trained ornithologist. “Every year, there seem to be less migrants singing.”

Salaman, like Bennett, cites land-use changes in South America as the driving force behind declines in migratory species.

“There has [actually] been a massive net gain in forest across the United States over the last hundred years,” he tells Mongabay. “The decline in migratory birds started back in the 1960s and ’70s in a period when deforestation was really ratcheting up” in South and Central America.

According to Salaman and other conservationists, increasing forest cover in the continental United States doesn’t address the problem facing at-risk species like the golden-winged warbler and the cerulean warbler (Setophaga cerulea).

“In a year, they’re actually only really [spending] a third of the time — less than half — on the breeding ground [in North America]. So the majority of their time is actually spent at stopover sites, or wintering sites, in the Neotropics,” or tropical Americas, Salaman says. “And unfortunately, that’s where the greatest levels of deforestation have occurred.”

The Rainforest Trust is working to protect a strategic critical habitat near the northern tip of Colombia. The area, described by Salaman as a gateway to South America for migratory birds, is currently threatened by the cattle industry, which he says is responsible for a full three-quarters of all deforestation in the Neotropics. But cattle pastures don’t provide adequate food or shelter for most birds (or many other species), forcing them to seek resources elsewhere.

Migratory birds rely on the ubiquity of food in the tropics to survive the strenuous return journey in the spring; migration is the main cause of mortality for many of the birds on the migration routes that span South America to North America, according to experts.

A Risky Journey

Even for healthy birds, a successful return to northern breeding grounds is by no means guaranteed.

“Let’s start with factors that absolutely prevent a migrant from returning,” says David Flaspohler of Michigan Tech, who studies the body condition of migratory birds that overwinter in Mexico’s oil palm plantations. “Those that [survive] until about March must begin a trip northward of hundreds or thousands of miles, mostly through unfamiliar land and water, facing storms, potentially unpredictable food availability, and a host of relatively new hazards like window glass and outdoor cats that kill hundreds of millions of birds each year in the U.S. alone.”

Birds forced to overwinter in poor habitats may not build up the necessary reserves of fat — and the consequences, say researchers, can be deadly.

“Imagine setting out on a solo kayak crossing of some large body of water with a full store of food for the three day trip and a good breakfast [versus] leaving with a few crackers and cheese for lunch and a bowl of oatmeal that morning,” Flaspohler tells Mongabay. “You might make it in the latter case, but if a storm blew up or a headwind suddenly made paddling harder, you might not reach the other side.”

A win-win solution

Not all agricultural lands are the same when it comes to their ability to host migratory birds.

“Agroforestry, at the smaller scale, is certainly one [strategy] that is important,” Salaman says.

Agroforestry systems, which employ a diverse species of plants of mixed heights to form a pseudo-natural habitat structure, benefit biodiversity while still providing a landowner with income.

“The key to successful conservation efforts … is finding the win-win scenarios for landowners and biodiversity,” Bennett says. She adds she believes her group’s study can help provide insight into mutually beneficial land practices.

“No agroforestry system can replace primary forest in terms of biodiversity conservation,” Bennet adds. However, she says, tree plantations can provide a bird-friendly alternative to harsh habitats like cattle pastures and soy monocultures.

Scientists like Bennett and Flaspohler are looking for solutions that complement traditional protected areas, which, while undoubtedly effective, can only make up part of a landscape-scale strategy.

Bennett’s research group focused on the suitability of types of plantation other than coffee. Specifically, they compared mixed-native hardwood for timber, oil palm, teak and rubber. Privately owned sustainable hardwood plantations showed especially promising results. Teak plantations, though managed as monocultures, were also attractive to migratory birds.

“Even our conservation priority species, which are generally found in forest, are using mixed-native hardwood plantations and teak plantations at the same level as forest,” Bennett says.

She says locals should invest in these types of hardwood plantations rather than more environmentally harmful commodities like cattle or annual crops.

While the study found oil palm and rubber plantations less attractive to conservation priority birds, landowners can still incorporate strategies to help maintain suitable bird habitat. Something as simple as retaining tall trees, known as remnant trees, within a plantation can provide huge benefits to both humans and wildlife.

“Tall remnant trees often have cultural value, timber value, shade value, and help to stabilize slopes and protect water sources,” Bennett says.

The trees also help the birds: the study found that in all plantation types, whether tropical hardwood, rubber or even oil palm, the retention of tall trees was associated with a higher abundance of birds and a higher diversity of bird species. Although the birds use the plantations, including oil palm, it’s unknown how successfully they gain weight in these man-made environments.

Bennett says further research is needed: “[I]t’s important to stress that our study only evaluated use, and not survival rates or body condition in these plantation types.”

There likely won’t be a simple solution to the dwindling bird populations. Land-use change in the tropical Americas is unlikely to stop anytime soon, but a shift to more responsible practices on agricultural lands may give wildlife a fighting chance while conservationists and policymakers work to end deforestation.

“As [the] human population grows and economies expand, this process continues,” Flaspohler says.

An increased emphasis on sustainable production strategies in commodities like hardwoods, rubber and oil palm, and not just coffee, could help the landscape work for both humans and wildlife, according to Bennett.

Conservationists also say strategic protected areas, such as those championed by the Rainforest Trust, as well as a reduction in shade-free lands like cattle pastures, will be essential for future efforts to protect both migratory and endemic birds.

Many of the migratory birds have already begun their biannual journey, streaming up through the Gulf of Mexico, pulled toward their northern breeding grounds by the primal urge to survive. Many will succeed, but some, weakened by low energy stores, will not. Within the next month, as birders strap on their binoculars to witness one of the greatest migrations on the planet, they should take a moment to listen to the songs of the birds, and to appreciate them. The successful return of these indomitable creatures, inevitable though it may seem, is by no means a guarantee.

Citations

Bennett, R. E., Leuenberger, W., Leja, B. B. B., Cáceres, A. S., Johnson, K., & Larkin, J. (2018). Conservation of Neotropical migratory birds in tropical hardwood and oil palm plantations. PLOS One, 13(12). doi:10.1371/journal.pone.0210293.

Jha, S., Bacon, C. M., Philpott, S. M., Méndez, V. E., Läderach, P., & Rice, R. A. (2014). Shade coffee: Update on a disappearing refuge for biodiversity. BioScience, 64(5), 416-428. doi:10.1093/biosci/biu038