Competition limits the range of mountain birds

Vancouver, BC and Ithaca, NY—A new study helps uncover why tropical mountain birds occupy such a narrow elevation range, a mystery that has baffled scientists for centuries. While many temperature assumptions are responsible for this limited distribution, recent research suggests competition from other species plays a larger role in shaping bird ranges.

The study, conducted by researchers at the University of British Columbia and the Cornell Lab of Ornithology, included 4.4 million citizen science observations of 2,879 bird species worldwide. The findings were published in Science on July 21.

“You have incredible biodiversity in the mountains, especially in the tropics. From one scenic point in the Andes, you can see mountain slopes that are home to as many species as the rest of North America,” said lead author Benjamin Freeman, a postdoctoral fellow at the University of British Columbia. “We want to know, how does it work?”

Freeman and his collaborators obtained the range of data by analyzing records from eBird, a citizen science project run by Cornell Lab that contains sightings from hundreds of thousands of birdwatchers around the world. This allowed them to examine the ranges of more than a quarter of the world’s bird species spread across five continents—a scale previously unimaginable by researchers.

“Other than eBird, you only have very rough coverage maps, especially on a global scale,” said co-author Eliot Miller at Cornell Lab. “The eBird database is vast in both space and time, giving us more insight into the distribution of birds around the world than we have for any other organism.”

The researchers looked for a relationship between the size of the elevation range and two factors: temperature stability throughout the year, and range overlap with other species. If range size is correlated with consistent climate, this would suggest that birds are constrained by their own biology—they have become so accustomed to certain temperatures that they cannot survive anywhere else. If birds occupy smaller areas where they overlap with many other species, then increased competition for resources may limit them.

The results show that consistent climate does not predict the size of a smaller range. However, there is strong evidence to suggest that when there is more overlap among a large number of species ranges, the ranges become smaller.

Despite his suspicions that competition was a key factor, Freeman said he was still caught off guard by the clarity of the results.

“I’ve noticed some patterns that suggest competition may be important for bird distribution, but I was still surprised to find such a strong signal in this study,” he said.

In many cases, closely related species also limit each other’s distribution. For example, the Ornate Hawk-Eagle, a large predator in tropical forests, occupies a greater altitude range in areas that do not coincide with the similar Black-and-chestnut Eagle. When the two inhabited the same mountains, they avoided each other, with the Ornamental Eagles crossing the heights they normally favored anywhere else within their reach.

While this study highlights one aspect of the mountain’s species range, the authors suggest much remains to be learned. Many details are still unknown about how other aspects of the bird ecosystem affect their distribution.

This research was supported by funding from the Natural Sciences and Engineering Council and the National Science Foundation.

Reference:

Benjamin G. Freeman, Matthew Strimas-Mackey, and Eliot Miller. Interspecific competition limits the range of bird species in tropical mountains. Science. July 2022.

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