When you’re hungry and far from home, you may find yourself being less picky about what you eat.

Not so for birds. The further they are from the core of their habitats, the pickier they become, according to a unique study by Conservation International, the University of Canterbury, São Paulo State University and others, which examined the feeding behavior of nearly 100 bird species across six continents.

It turns out that birds’ feeding preferences are closely linked to environmental stressors. Changes in their behavior could have major consequences for forests and other critical ecosystems that rely on birds to disperse seeds, said Camila Donatti, a climate expert at Conservation International and co-author of the study.

Researchers were surprised by the findings.

“In general, birds look for fruit that is a perfect fit for their beak size, so they can just grab it and go,” she said. “We thought that in the birds’ core habitat, where individuals typically spend most of their time, they would be more selective about the fruit they eat because there’s a greater variety of it.”

Not quite. Donatti and others have found that when birds are at the edge of their range, they face stressors such as new climates, other predators, or competition for resources.

“Because it’s stressful to be at the outer limits of their range, they have to be very careful to eat the exact plants that will give them the biggest return on their investment,” she said. “Essentially, they become more picky because they don’t have any energy to waste. That means they eat the biggest fruit that they can fit in their beaks with the least amount of effort.”

Although the findings seem hyperspecific, their significance extends far beyond birds’ dietary preferences.

That’s because birds play a crucial role in dispersing seeds to new locations. When they ingest fruit, they fly away and carry the seeds to other places to germinate. Birds disperse the seeds of about 90 percent of tropical plants.

“Birds are extremely important animals in maintaining the health of an ecosystem,” Donatti said. “When seeds pass through their gut, they undergo a chemical and physical process that actually facilitates germination and in turn new plants.”

But climate change and human activities such as agriculture and development threaten to disrupt the bird-plant cycle by destroying habitats and pushing species to the edges of their ranges — and beyond. This could create a mismatch between the types of birds in a given area and the food available to them.

“Birds are fast breeders compared to plants — and they can fly over large areas,” Donatti added. “Given climate change and habitat loss, their distribution could change much faster than the decades it would likely take for plants to adapt. And without birds to spread their seeds, the trees and plants in an area would die over the long term, which would have a dramatic cascading effect on entire ecosystems, such as forests that store climate-warming carbon.”

Researchers spent thousands of hours collecting data for the massive study, said Donatti, whose research on birds in Brazil’s Pantanal was used in the paper. She estimates her dataset alone included nearly 900 hours of birdwatching.

Collectively, these kinds of efforts help scientists get a better picture of how species function in nature, and how seemingly small details, like what birds eat, can affect the overall ecosystem.

“Biodiversity is the backbone of nature,” Donatti continued. “And if these systems are out of balance, they will not function as we expect them to, delivering the essential services we rely on, from food to clean water to climate regulation.”

“If we want to protect that, we have to understand how it works,” she added.

Mary Kate McCoy is a staff writer at Conservation International. Want to read more stories like this? Sign up for email updates. Also consider supporting our critical work.