Two bird species that look the same, but have songs so different they can’t recognize each other, should be considered distinct species, according to a study by University of British Columbia postdoctoral researcher Benjamin Freeman and Cornell University researcher Graham Montgomery. Among the 72 related populations of Neotropical passerine birds the authors tested, they found evidence for 21 new species.
Top: an orange-billed sparrow (Arremon aurantiirostris). Bottom left: a golden-crowned warbler (Basileuterus culicivorus). Bottom right: a yellow-olive flycatcher (Tolmomyias sulphurescens). Image credit: Dominic Sherony / Dario Sanches / Gary L. Clark / CC BY-SA 2.0.
“Songs are important for birds and who they choose to mate with,” Dr. Freeman said.
“Birds evolve different songs and we wanted to find out which populations are so different in song that they should be considered different species.”
The researchers compared two methods (analysis in the lab and experiments in the field) for 72 pairs of related but geographically separated bird populations in Costa Rica, Panama, and Ecuador.
In addition to analyzing more than a thousand song recordings for seven variables, they used playback experiments to test birds’ real-world reactions to recordings of their relatives, observing whether or not they approached the speaker.
“In the tropical forests of Central and South America where the vegetation is dense, birds rely heavily on song to claim their territory and let other birds know where they are,” the authors explained.
“In the field experiments, we conducted playback experiments by hanging wireless speakers in the trees and broadcasting songs from related subspecies and then observing how the birds responded.”
“If the birds continued on with their natural behavior and ignored the speaker and sound, it indicated that they distinguished the songs. They did not feel like another bird was encroaching on their territory and trying to mate with their partner. If the bird got angry and started to try and kick the ‘intruder’ out, it indicated they recognized the song.”
Twenty-one species pairs currently classified as subspecies that show strong behavioral discrimination against allopatric song. Dr. Freeman and Dr. Montgomery suggest that these populations merit recognition as distinct biological species. Image credit: B.G. Freeman & G.A. Montgomery, doi: 10.1642/AUK-17-63.1.
Published in the Auk: Ornithological Advances, the results show that when the divergence between the characteristics of the recordings is high, birds consistently fail to recognize recordings of their relatives in the field, but when divergence is low, birds’ discrimination is much less consistent.
In other words, analyzing recordings can’t accurately predict how birds will act when presented with songs just slightly different from their own.
Many pairs that failed to recognize each other are currently categorized as members of the same species, suggesting that current taxonomy does not reflect actual bird behavior when it comes to song.
The authors propose that 21 such pairs should be recognized as separate species based on song discrimination and that playback experiments should be the standard for assessing whether song divergence between populations is a barrier to interbreeding.
“It’s interesting that with one study in one year, we came up with good evidence that there are 21 new species that authorities should recognize. We know so much about birds but this demonstrates that we still have a lot to learn,” Dr. Freeman said.
“Playback experiments between geographically isolated taxa provide key data on how populations might perceive each other in terms of ‘same’ or ‘different’ if they were in actual contact,” said Louisiana State University researcher Dr. James Vanderbeek ‘Van’ Remsen, Jr., an expert on Neotropical birds who was not involved in the study.
“Hopefully, this pioneering study will catalyze a wave of similar studies around the globe as a way to approach the always-thorny problem of species limits in these birds.”
_____
Benjamin G. Freeman & Graham A. Montgomery. 2017. Using song playback experiments to measure species recognition between geographically isolated populations: A comparison with acoustic trait analyses. The Auk 134 (4): 857-870; doi: 10.1642/AUK-17-63.1
