A new study of Thraupidae – the largest songbird family, representing nearly 10 per cent of all songbirds – has dispelled the long-held ‘transfer hypothesis,’ first conceptualized by Charles Darwin in 1871 and arguing that animals are limited in their options to evolve showiness.
Five tropical tanagers, clockwise from upper left: Golden Tanager, Swallow Tanager, Blue-winged Mountain-Tanager, Red-necked Tanager, Beryl-spangled Tanager. Credit: Peter Wendelken / Frank Shufelt / Keith Bowers / Vivek Tiwari / Priscilla Burcher.
The natural world is full of birds with brilliant colors, exaggerated crests and tails, intricate dance routines, or virtuosic singing. But it’s long been thought that these abilities are the result of trade-offs.
“Animals have limited resources, and they have to spend those in order to develop showy plumage or precision singing that help them attract mates and defend territories. So it seems to make sense that you can’t have both. But our study took a more detailed look and suggests that actually, some species can,” said Nick Mason, a PhD student at the Cornell Lab of Ornithology and the first author of a paper published in the Proceedings of the Royal Society B.
Mr Mason and his colleagues tested the idea of trade-offs by looking at Thraupidae (tanagers), a large family of 371 songbird species.
Thraupidae includes some of the most spectacularly colorful birds in the world (such as the Paradise Tanager) as well as more drab birds (like the Black-bellied Seedeater). The group also includes both accomplished and weak songsters alike.
The ornithologists examined museum specimens of 303 tanager species, using a spectrophotometer to measure nine aspects of plumage coloration, such as brilliance and contrast.
They took a similar approach to the birds’ songs, analyzing more than 2,700 recordings to measure 20 song variables including length, bandwidth, and number of syllables.
Finally, they compared how plumage and song complexity varied at each of the branches along a recently completed evolutionary tree of the tanager family.
“If there were going to be any group of birds at all that would show this trade-off, the tanagers would be a very good candidate, because there’s all this variation in song and plumage complexity. But when we dive into it and do some rigorous statistics, it turns out that there is no overall trend. Tanagers can be drab and plain-sounding, or colorful and musical, or anything in between,” Mr Mason said.
As a byproduct of the analyses, the team was able to put together:
– the Top-10 list of tanagers with the most colorful plumage:
1. Paradise Tanager
2. Opal-rumped Tanager
3. Glistening-green Tanager
4. Blue-winged Mountain-Tanager
5. Golden-hooded Tanager
6. Green-headed Tanager
7. Black-chested Mountain-Tanager
8. Opal-crowned Tanager
9. Grass-green Tanager
10. Black-chinned Mountain-Tanager
– and the Top-10 list of tanagers with the most complex songs:
1. Merida Flowerpiercer
2. Chestnut-bellied Flowerpiercer
3. Slaty-backed Hemispingus
4. Masked Flowerpiercer
5. Rufous-browed Hemispingus
6. Gray-bellied Flowerpiercer
7. Bananaquit
8. Lacrimose Mountain-Tanager
9. Drab Hemispingus
10. Plushcap.
“The study puts a significant dent in the idea of evolutionary trade-offs between plumage and song. It’s still possible that trade-offs take place at the level of genus, or that they influence species relatively fleetingly as evolutionary pressures appear and disappear,” Mr Mason said.
“But as a broad effect on an entire family of birds, a voice–plumage trade-off doesn’t seem to exist. One possibility is that the resources needed to develop fancy plumage are different from the ones required for complex songs, freeing tanagers to invest in both forms of showiness simultaneously.”
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Nicholas A. Mason et al. 2014. Elaborate visual and acoustic signals evolve independently in a large, phenotypically diverse radiation of songbirds. Proc. R. Soc. B, vol. 281, no. 1788; doi: 10.1098/rspb.2014.0967
