Birds Sing To Avoid Confusion.

Cinnamon Becard (Pachyramphus cinnamomeus).

Credit: Matt Brady/Macaulay Library

Using song playback experiments to measure species recognition between geographically isolated populations: A comparison with acoustic trait analyses | The Auk

An interesting piece of research published yesterday shows how birds use song to recognise members of their own species, so avoiding hybridisation between closely related species, especially when there is little or no difference in appearance.

The researchers believe they have identified 21 new species of bird from Central and South America in the 72 related populations examined where different songs effectively produce isolated breeding populations.

A while ago I wrote about how barriers to hybridisation arise as one species diverges into two or more. As each adapts to it's particular niche, any hybrids might be maladapted to either, so anything which reduces the tendency to hybridise will be favoured over anything which facilitates it. These barriers fall into two main types: pre-zygotic and post-zygotic barriers. Simply stated, pre-zygotic barriers are those things that prevent sperm reaching the egg and fertilising it; post-zygotic barriers are those things that prevent a fertilised egg developing into a viable adult.

This paper, by researchers from the University of British Columbia, Vancouver, BC Canada, and published by Auk (the journal of the American Ornithological Society, shows how birds can use different songs to set up pre-zygotic barriers.

Abstract
Geographically isolated populations of birds often differ in song. Because birds often choose mates on the basis of their song, song differentiation between isolated populations constitutes a behavioral barrier to reproduction. If this barrier is judged to be sufficiently strong, then isolated populations with divergent songs may merit classification as distinct species under the biological species concept. We used a dataset of 72 pairs of related but allopatric Neotropical passerines (“taxon pairs”) to compare 2 methods for measuring song divergence between isolated populations: statistical analysis of 7 acoustic traits measured from spectrograms, and field playback experiments that “ask the birds themselves” if they perceive foreign song as conspecific or not.

We report 4 main findings:

1. Behavioral discrimination (defined as failure to approach the speaker in response to allopatric song) is nonlinearly related to divergence in acoustic traits; discrimination is variable at low to moderate levels of acoustic divergence, but nearly uniformly high at high levels.
2. The same nonlinear relationship held for both song learners (oscines) and nonlearners (suboscines).
3. Song discrimination is not greater in taxon pairs ranked as species compared to taxon pairs ranked as subspecies.
4. Behavioral responses to allopatric song are symmetric within a taxon pair.

We conclude:

1. that playback experiments provide a stronger measure of species recognition relevant to premating reproductive isolation than do acoustic trait analyses, at least when divergence in acoustic traits is low to moderate; and
2. that playback experiments are useful for defining species limits and can help address the latitudinal gradient in taxonomy, which arises because species are defined more broadly in the tropics than in the temperate zone.

To this end, we suggest that 21 Neotropical taxon pairs that are currently ranked as subspecies, but that show strong behavioral discrimination in response to allopatric song, merit classification as distinct biological species.

[Reformatted for greater clarity]

Benjamin G. Freeman and Graham A. Montgomery
Using song playback experiments to measure species recognition between geographically isolated populations: A comparison with acoustic trait analyses
The Auk 134(4):857-870. 2017 DOI: 10.1642/AUK-17-63.1

© 2017 American Ornithological Society.
Published open access.

Cinnamon Becard Pachyramphus cinnamomeus. Song

Photo credit: Matt Brady/Macaulay Library.
Song credit: David L Ross, Jr/Macaulay Library.

Chestnut-crowned Becard Pachyramphus castaneus. Song

Photo credit: Ryan Treves/Macaulay Library.
Song credit: Paul A Schwartz/Macaulay Library.

Golden-bellied Warbler (Choco) Myiothlypis chrysogaster chlorophrys. Song

Photo credit: Peter Kaestner/Macaulay Library.
Song credit: Andrew Spencer/Macaulay Library

Golden-bellied Warbler Myiothlypis chrysogaster chrysogaster. Song

Photo credit: Fernando Angulo Pratolongo/Macaulay Library.
Song credit: Mark B Robbins/Macaulay Library

Bird song serves two purposes so far as breeding is concerned. Firstly, it helps a male establish and defend a territory so excluding other males and reducing competition for females. As a secondary benefit, this also helps establish and exclusive feeding ground for rearing the young. Secondly, it helps attract a female who may then exercise a degree of sex selection based on song and courtship ritual.

The press release announcing publication of this paper, from the University of British Columbia, helpfully explains the reserchers' methodology.

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. For this experiment, the researchers 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 behaviour 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.

Historically, scientists have identified new species by finding birds that look different enough or occupy different geographic locations.

“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,” said Freeman. “We know so much about birds but this demonstrates that we still have a lot to learn.”

This research is part of a larger pursuit to learn about the evolution of bird songs and why birds develop different songs.

New UBC research suggests bird songs isolate species.

The press release also includes examples with sound and vision. The first pair, the Cinnamon Becard and Chestnut-crowned Becard have a different appearance and somewhat different songs, but the Cinnamon Becard from west of the Andes responds to the song of the Crestnut-crowned Becard from the Amazon basin. In these related species, a geographical barrier has lessened the selection pressure to discriminate on the basis of song.

By contrast, the second pair, from different populations of Golden-bellied Warbler (Myiothlypis chrysogaster), although very similar and regarded as subspecies, do not respond to one another's songs. Myiothlypis chrysogaster chlorophrys is from the Pacific slope in western Equador whilst Myiothlypis chrysogaster chrysogaster is from the Amazon basin in eastern Peru.

It is clear from this study that bird song plays a major role in some related species in establishing barriers to hybridisation and should therefore be used as a factor in taxonomic classification. Where discrimination on the basis of song is a major component, then the pressure to evolve different plumage and/or display rituals may be reduced, making it difficult to identify what are effectively different genetic populations and so, biologically, different species.

Incidentally, if you haven't yet discovered the Macaulay Library, established by the Cornell Lab of Ornithology, and I hadn't until I read this paper, it contains a wealth of media material on wildlife, not just birds. As their 'About' page says:

The Macaulay Library is the world’s premier scientific archive of natural history audio, video, and photographs. Although the Macaulay Library’s history is rooted in birds, the collection includes amphibians, fishes, and mammals, and the collection preserves recordings of each species’ behavior and natural history. Our mission is to facilitate the ability of others to collect and preserve such recordings, and to actively promote the use of these recordings for diverse purposes spanning scientific research, education, conservation, and the arts.

Access to audio and video recordings in the archive is available for research, educational, and commercial use. If you would like to contribute to the archive, you can submit your bird recordings and photos via the eBird/Macaulay Library media upload tool. Recordists with videos or non-bird recordings can contact us to to discuss the possible archival of their material. New recordists can receive training at our annual sound recording workshop and learn how to become expert field recordists, or use our online resources for quick tutorials.

Enjoy!

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