How birds keep secrets in color

Songbird plumage reflects UV light, but is less obvious to avian predators

By | April 27, 2005

Songbirds are able to communicate with potential mates using plumage colors while remaining inconspicuous to avian predators, Swedish researchers suggest in PNAS this week. They do so by using colors that the larger birds are less able to discriminate from the background.

Ultraviolet plumage coloration, which reflects light in the range of 355–380 nanometers, has long been known to serve as a secret communication channel in songbirds, exploiting a shortfall in the mammalian visual system. But it has not been clear how avian predators, which can see ultraviolet, are excluded.

Ornithologists Olle Håstad, Jonas Victorsson, and Anders Ödeen, all based at Uppsala University, present evidence that small passerines such as the robin Erithacus rubecula, brambling Fringilla montifringilla, and golden oriole Oriolus oriolus exploit differences in the maximum sensitivities of their own visual systems and those of their potential bird predators.

Using retinal models—an equation that describes how the cones in the retina react to light of a given wavelength—for the songbird and predator visual systems the researchers compared the reflectance of the head and chest plumage of 18 species of songbirds to that of their typical Swedish forest habitat. Against the appropriate background, the plumage was significantly more visible to the songbirds than predator birds, they report.

"I'm really pleased to see this work published, because I always thought that the notion of UV signals being a private channel [of communication] never squared with the fact that avian predators of birds can see UV," Innes Cuthill, professor of behavioral ecology at the University of Bristol, UK, told The Scientist. "This paper shows that, yes, there is potential mileage in the argument, because raptors aren't as good at discriminating colors in the UV waveband as passerines," he explained.

Evolutionary biologist David Harper, based at the University of Sussex, UK, agreed that the study "introduces an interesting idea that songbirds can communicate with each other without being conspicuous." However, he also expressed concern over some aspects of the paper, particularly the lack of detail regarding the methodology and some of its assumptions. "This is one of those cases where we have to curse word limits," he said. "Hopefully, future papers in less prestigious journals will be more enlightening."

Peter McGregor, a behavioral ecologist at Cornwall College Newquay, whose own research has centered on animal signalling, noted the "striking comparison with bioacoustics," in particular the private "seeet call" of some bird species. But he also echoed Harper's concerns. While studying sound is relatively straightforward, he told The Scientist, understanding color and the visual sense is much more challenging. For example, plumage signals are "omnidirectional and always on", in addition to being subject to large variations in light regime throughout the course of the day, season, or year.

McGregor pointed to the study's reliance on retinal models of both songbird and predator visual systems. Just looking at retinal pigments isn't enough. "Retinas are hooked up to brains, and brains can do all sorts of flashy processing," he said. In addition, there is a crucial distinction to make between what is signal and what is information; only the former is the result of selection. "Håstad et al. have found a correlation, not direct evidence of a private communication channel."

Ödeen admited this is only the start, but emphasizes the nature of the differences between songbird and raptor/corvid visual systems. "We are looking at the tuning of maximum sensitivities. Raptors are sensitive some way into the UV [range], but their maximum sensitivity lies elsewhere," he told The Scientist. "As the title of the paper suggests, songbirds are less conspicuous, not inconspicuous."

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