In 2019, scientist Crinan Jarrett was working a mistnet on a cocoa farm in Cameroon when she let out an excited yell. Her African colleagues working nearby rushed over to see what the fuss was about—and they were both amused and confused. The bird in the net didn’t look particularly spectacular to them. It was a Wood Warbler, a common migratory songbird that breeds in Europe and Asia and winters in Central and West Africa. “Why is it such a big deal, Crinan? It’s just a little green bird, ” she recalls them saying.
Jarrett wasn’t just excited by the species, but also by its feathers. It had brand new feathers growing alongside the old ones—a sign the bird was molting nearby. The minute observation points to a larger finding about the ability of songbirds to find productive habitat on cocoa farms in western Africa.
The mist-netting survey was originally intended to study how cocoa farming affects biodiversity, especially birds like the Wood Warbler whose populations have steadily declined over the last three decades. Experts believe that they may be affected by land use change in Africa, particularly conversion of forests into farmlands, since studies have found that Wood Warblers prefer areas with many tall trees. But thanks to the accidental mist-netted bird, the team has also published a new study establishing that the birds molt successfully in cocoa farms at their wintering grounds in Cameroon—suggesting that cocoa farms can be supportive bird habitat when nearby forests are preserved.
Molting, the process of replacing old and worn feathers with fresh ones, requires energy. It takes a lot of protein to grow new feathers, which means a molting bird needs a lot of food. Due to this, birds are adapted to go through the molt at times and places where they have easy access to food, water, and shelter.
Ideally, birds would find these resources in their natural habitat. But due to the growing global demand for products like chocolate, agriculture is a major driver of deforestation. Cocoa agriculture in particular has contributed to the loss of up to 80 percent rainforest cover in Africa. Because so much bird habitat has been and continues to be lost to farming, it is important to use these farms in a way that they still enable birds to survive and thrive.
Led by Jarrett, a PhD student at University of Glasgow, a research team examined the flight feathers of 12 Wood Warblers living on cocoa farms in Cameroon over a period of two winters by catching them with mist nets in different farms. Some of the farms were less intensely managed, with more tall trees growing on the property, and others had more intense management and fewer trees. Jarrett found that the birds weren’t picky, but were using all the farms and managing to find enough resources to grow new feathers in them.
"I suspect the habitats were sufficient for them," says John Mallord, a conservation scientist at the Royal Society for the Protection of Birds who was not involved with the study. “Wood Warblers like open woodland and well-wooded farmlands, which are often very similar looking in terms of the number of trees.” Although the surveyed farms were managed with varying intensities, most were shady because the farmers retained nearby blocks of forests. These forest blocks likely offset the negative effects of the tree removal from the farms by providing refuge and resources for the birds.
Jose Carlos Morante-Filho, an ecologist at Universidade Estadual de Santa Cruz, Brazil, who did not participate in the study, agreed. “Although there are several studies reporting the importance of agroforestry for bird diversity in anthropic landscapes, this study highlights the importance of this human environment for this vital activity [molt].”
However, both Mallord and Morante-Filho caution drawing firm conclusions about Wood Warblers from a sample size of just 12 birds. Still, Mallord says he's not too worried about the study's scope knowing how elusive the species can be.
For her part, Jarrett believes that her study adds at least “a few grains of sand” to the argument that agroforestry can support biodiversity. The fact that Wood Warblers are molting on a cocoa farm suggests that these places can be great bird habitat if nearby trees are left standing. Although no farm can replace a forest, agriculture does not need to be a threat to biodiversity anymore, especially in the tropics, if we can find what Jarrett calls, “a win-win scenario”—the degree of shade that is necessary to maintain good biodiversity as well as a good crop for local people.