Remember Spring 2020, when grocery store shelves were empty of essential items and ingredients. For birds living in the forests of Central America, replacing woodland with coffee plantations is essentially “clearing” the shelves of their preferred food, causing them to change their diet and habitat to survive.
A new study led by University of Utah researchers examines records of birds’ diets stored in their feathers and radio-tracks their movements to find that birds in coffee plantations eat far fewer invertebrates than those in forests, suggesting this that this leads to a disturbance of their ecosystem significantly influences the feeding possibilities of the birds.
“Human increasing ecological impacts on the planet, particularly through habitat loss and destruction and climate change, often have a negative impact on bird nutrition as well,” said Çağan H. Şekercioğlu, lead author of the study and professor of ecology and ornithology at the US School of Biological Sciences. “These adverse changes, including declines in key food resources such as insects and other invertebrates, can lead to reduced survival, particularly of fast-growing young, often resulting in population declines and losses of these malnourished birds.”
The study appeared in Frontiers of Ecology and Evolution. The full study can be found here.
The forests of Costa Rica
Across the world, forests are being reduced from once verdant oases of life to much smaller remnants scattered among the agricultural lands that replaced them. Only about one percent of bird species prefer habitats dominated by humans and human activities, but the rapid disappearance of natural forest habitats means that about a third of bird species now have to work to survive in human-dominated environments.
In Costa Rica, the land around the Las Cruces Biological Station near the Panama border has evolved from fully forested to now 50% coffee plantations, 20% cattle grazing and 10% other human habitats—only 20% of the land remains forested. Agricultural land is littered with pesticides, fertilizers and fungicides, drastically affecting the invertebrate communities on which native birds feed.
These native birds include four species that the researchers focused on in the study: orange-billed nightingale, silver-throated thrush, white-throated thrush, and ocher-bellied flycatcher. All four species are found in both the forests and the open countryside, where they feed on both fruit and invertebrates. But the invertebrates (including insects) are an important part of their diet as they provide important nutrients like protein and nitrogen.
Şekercioğlu and his colleagues, including researchers from the United States, Costa Rica and Singapore, wanted to understand how the bird species they studied get their nutrients between the agricultural and forest environments, particularly during the crucial breeding season when proper nutrition is key is conservation of the species.
An isotopic food diary
To learn more about the birds’ diet, the researchers analyzed isotopes in their feathers. We are what we eat, and the chemical signatures of the foods we eat, in the form of isotopic ratios, are built into our tissues.
Isotopes are different versions of the same element that differ only in the number of neutrons in their nucleus — a vanishingly small difference in mass between a carbon atom with, say, six neutrons and a carbon atom with seven. But biological and physical processes can favor either light or heavy isotopes, altering the resulting ratio in ways that can be measured and provide valuable information.
In humans, for example, a record of our diet is kept in the isotopes in our hair. In a previous study, co-author Thure Cerling, a distinguished professor in the US Department of Geology and Geophysics, and colleagues analyzed hair clippings from barbershops and salons around the Salt Lake Valley and learned the relative proportions of corn-fed meat and plants – based protein in the diet of local residents.
In Costa Rica, researchers hoped to do the same, but with the stable isotopes of carbon and nitrogen in the birds’ feathers. They collected 170 feathers from the four bird species to analyze diets and used radio tracking to track the movements of 49 birds to see where they were spending their time.
“It’s definitely not the first time that isotopic analysis of feathers has been used to study the diet of birds,” said co-author Seth Newsome of the University of New Mexico, “but it could be the first time, particularly in the tropics that it has been used in conjunction with radiotelemetry to study dietary composition and the relative use of agricultural versus natural habitats.”
The results showed that the birds’ preferred habitat had a significant impact on their diet. The isotope data indicated that three of the four species studied ate significantly fewer invertebrates on coffee plantations than in forests. For silver-throated tanagers and white-throated thrushes, data suggest that they eat twice as much invertebrate biomass in forests than in coffee plantations.
“Our results suggest that coffee plantations lack invertebrates favored by forest generalbirds that forage in both native forest remnants and coffee plantations,” Şekercioğlu said.
Consequences of habitat shifts
The coffee plantations were established decades ago, and researchers have no data to know how the birds behaved when the forest was intact. But from what we now know about bird behavior, we can deduce what the results mean for the birds’ way of life.
In order to eat enough invertebrates, the birds often have to forage in the small patches of forest of about 7 to 12 acres (about the size of the parking lot at U’s Rice-Eccles Stadium) and narrow forest corridors along rivers, Şekercioğlu said about 30-60 feet wide.
“We think that the more agile birds like the silver-throated tanagers and the white-throated thrush are constantly moving to get enough food, especially protein-rich invertebrates,” said Şekercioğlu, a hypothesis supported by a 2007 radio-tracking study. “Less mobile species like the Orange-billed Nightingale Thrush, which can have a lifetime range of as little as half a hectare, will either have to adapt to coffee plantations and eat fewer invertebrates, or they will disappear.” The Orange-billed Nightingale Thrush is not alone—a study from 2019 showed that more bird species in the region were declining than were stable.
Thus, for the birds of Costa Rica, and for birds in other similar tropical regions, forest reserves can represent critical resources for birds that have migrated their habitats into the remaining forest and travel through coffee plantations to reach other forest fragments.
“Shifting these birds’ foraging activities to other locations may lead to new ecological interactions, which can themselves have negative consequences,” Şekercioğlu said. “For example, increased competition with birds in these new locations, or overpredation of a prey species that was not previously consumed as much.”
how can you help
If you are a coffee drinker, you can help by choosing to buy one Bird friendly coffee. According to Şekercioğlu, bird-friendly coffee is grown in plantations with more tree cover and forest residues that are beneficial to native birds. He recommends buying shade-grown, certified coffee Bird friendly by the Smithsonian Migratory Bird Centeror coffee from Ethiopia, which, as he said, is among the most bird-friendly.
And local governments in tropical regions can help by prioritizing the conservation of intact forests, secondary forests, and forest strips along rivers to improve forest remnant connectivity.
“There is an urgency,” Şekercioğlu said, “to prioritize the conservation and regeneration of forest remnants in increasingly human-dominated agricultural areas that continue to replace the world’s most biodiverse tropical forests.”
The full study can be found here.