A study led by the University of Utah has compared the diet of birds in coffee plantations to those living in forests.
The research was conducted by performing isotope analysis of 170 feathers from four different bird species in Costa Rica. Isotopes are variations of the same element that are different in the number of neutrons in the nucleus of the atom.
Food consumed by an animal leaves a 'signature' in the ratios of different isotopes in its tissue. By performing isotope analysis, scientists can understand the diet of animals just by collecting a sample, and in birds this means gathering feathers.
The diet of Silver-throated Tanager was studied through isotope analysis, alongside that of three other generalist species (Stuart Reeds).
About 50% of the once-forested land around Las Cruces Biological Station in Costa Rica, near the border with Panama, has given way to coffee plantations. Other human uses have left only 10% of the forest standing.
Çağan H Şekercioğlu, lead author of the study, and his colleagues focussed on four generalist birds that inhabit forested and open landscapes, and feed on fruits as well as invertebrates. The latter forms an important part of the birds' diet as a source of protein and nitrogen.
The team wanted to learn how Orange-billed Nightingale-Thrush, Silver-throated Tanager, White-throated Thrush and Ochre-bellied Flycatcher obtained nutrients in their diet in both forested and agricultural areas. They focused on the breeding season when good nutrition is especially important.
In addition to the isotope analysis of the feathers, the movements of 49 individual birds were followed using radio-tracking equipment to see which areas the birds were feeding in.
The results of the analysis showed that, in three of the four species, individuals that fed in coffee plantantions ate fewer insects than birds in forested areas. The data for Silver-throated Tanagers and White-throated Thrushes, the data indicated that birds consumed half as many invertebrates in coffee plantations than in forests.
Şekercioğlu said: "Our results suggest that coffee plantations are deficient in invertebrates preferred by forest generalist birds that forage in both native forest remnants and coffee plantations."
Because the coffee plantations were created long before anyone considered gathering baseline data, scientists are unable to compare the behaviour of birds in the current patchwork of plantations and forest fragments to how they would have fed in the original untouched forest. However, the results of the study advance our understanding of the differences in diet in certain habitats.
Şekercioğlu said that the birds studied need to forage frequently in forest fragments of at least 3 ha in order to gather enough invertebrate food during the breeding season. Forest reserves are therefore critical resources for birds in the vicinity of coffee plantations. He explained that Silver-throated Tanager and White-throated Thrush are mobile species capable of gathering enough invertebrates, but that Orange-billed Nightingale-Thrush is an example of a highly sedentary species that is forced to survive on fewer invertebrates if its home range is a coffee plantation.
Şekercioğlu said: "These birds shifting their feeding to other places may result in new ecological interactions that can themselves have negative consequences. For example, increased competition with birds in these new places or overpredation on a prey species that was formerly not consumed as much."
Coffee drinkers who want to play their part to protect birds should look for shade-grown coffee certified by the Smithsonian Migratory Bird Center, or coffee from Ethiopia, which Şekercioğlu said is among the most bird-friendly in the world. He emphasised that governments need to play their part by preserving intact forest and strips of forest along rivers in order to boost connectivity of the habitat that remains.
Şekercioğlu said: "It is urgent to prioritise the conservation and regeneration of forest remnants in increasingly human-dominated agricultural areas that continue to replace the world's most biodiverse tropical forests."
Şekercioğlu, Ç H, Fullwood, M J, Cerling, T, Oviedo Brenes, F, Daily, G C, Ehrlich, P R, Chamberlain, P, & Newsome, S D. 2023. Using stable isotopes to measure the dietary responses of Costa Rican forest birds to agricultural countryside. Frontiers in Ecology and Evolution, 2023; 11 DOI: 10.3389/fevo.2023.1086616