Bird Coloration: Vol. 1, Mechanisms and Measurement; Vol. 2, Function and Evolution

Edited by:
  • Geoffrey E. Hill &
  • Kevin J. McGraw
Harvard University Press: 2006. Vol. 1, 544 pp. £59.95;Vol. 2, 496 pp. £59.95 0674018931, 9780674018938, 0674021762, 9780674021761 | ISBN: 0-674-01893-1

Who can fail to be beguiled by the extravagant plumage colours of birds of paradise, peacocks and parrots? But there's more to feathers than showy displays. These remarkable, lightweight structures, which evolved 150 million years ago, perform a range of functions, both within individuals and across species. For example, they are one of the best thermal insulators known; they have several aerodynamic properties, creating refined aerofoil structures, producing lift and reducing drag; they can be waterproof; they are resistant to wear and abrasion, and are self-repairing; they perform a wide variety of sensory functions; and they create humid habitats for parasites and symbionts. Finally — and this is the focus of Bird Coloration, a two-volume work edited by Geoffrey Hill and Kevin McGraw — feathers produce a plethora of colours that demand an evolutionary explanation.

Simply put, an animal's coloration results from a trade-off between crypsis, for protection from predators, and gaudiness, for mating advantages. But the whole story is much more fascinating, and understanding it requires a firm grasp of both sexual-selection theory and colour vision. Sexual selection is a term coined by Darwin to explain the evolution of traits — often extravagant ones, such as the peacock's tail — that appear to confer disadvantages in natural selection but offer mating advantages. It was largely ignored for more than a century, but over the past three decades it has been one of the most intellectually vibrant areas of evolutionary biology. Its powerful body of theory now provides evolutionary explanations for a vast range of behaviour, ecology and life-history variation. For example, in humans it can explain mating patterns and even sporting and musical aptitude.

The test-bed for much of the work on sexual selection has been birds and their coloration. Why? Birds have grabbed biologists' attention in a way that nematodes have not, so there is a vast knowledge base on which to draw. In addition, the gaudy coloration and extravagant displays of birds require explanation and are amenable to testing. Vision plays a key role too — indeed, birds have perhaps the most advanced visual system of any vertebrate.

Putting on a show: a peacock displays its colourful tail. Credit: G. DOUWMA/GETTY IMAGES

An explosion of interest was fuelled in the early 1980s by Bill Hamilton and Marlene Zuk's remarkable theory that much bird coloration evolves from sexual selection on traits that indicate condition, particularly disease status and heritable resistance to parasites. Many studies have since showed that feathers and fleshy ornaments can be condition dependent, and so provide potential mates with reliable information about quality.

It is surprising that until only a decade ago, the study of avian coloration proceeded in ignorance of colour science. A mini-revolution has since occurred in how colour is considered, measured and analysed in behavioural, ecological and evolutionary studies of birds. This change is reflected in Bird Coloration. Previously, almost all studies of avian coloration were made on the assumption that birds see colours in much the same way as humans. This assumption was flawed. We now know that many, if not most, diurnal birds are sensitive to ultraviolet wavelengths, to which humans are blind. Moreover, colour vision in birds arises from four classes of retinal cone cell, whereas humans have only three, probably giving many birds an extra dimension to their colour vision. In addition, experiments have shown that ultraviolet reflectance and illumination invisible to humans influences birds' decision-making in several contexts, including mate choice, foraging and provisioning chicks.

The two volumes of Bird Coloration provide an excellent up-to-date overview of the topic. Despite having 20 contributors (including many prominent people in the field), the book is generally well focused, although the editing could have been tighter in places.

The science of bird coloration today is genuinely interdisciplinary, drawing on behaviour, ecology, pigment chemistry, the physics of light, the microstructure of reflectors, physiology, immunology, molecular genetics and vision research. Most of these areas are well represented in the book. I particularly enjoyed the chapters on topics that have had relatively little attention in the past few years, such as the numerous non-sexually selected functions of avian coloration. These also provide a reminder of the many issues that remain to be unravelled. Consider, for example, even well-studied species, such as the zebra finch or the blue tit. There are still no convincing explanations for their combinations of colours and their proximity and areas.

Colour science is an important but complex subject. Those studying behaviour and ecology are now taking this on board, as several chapters demonstrate. How are receptor signal-to-noise ratios dealt with, and does the Vorobyev and Osorio model best describe most avian colour spaces? What are the advantages and disadvantages of the different approaches to analysing reflectance and illumination spectra, and how do these vary with the different level and type of biological questions posed? How precisely are different cone inputs combined and weighted? What is the taxonomic distribution of spectral sensitivity in birds? How are photonic crystals genetically coded? These are all areas of current intrigue, and quite a few are well covered here.

A sound understanding of colour vision, then, underpins our understanding of bird coloration. This can be a knotty area, with most hard evidence coming from colour-mixing experiments, retinal physiology and sequence data, so contributors specializing in these areas would have been welcome. However, bird coloration is a huge topic and anyone organizing an overview should be heartily congratulated.

Another overview can be found in a special issue of American Naturalist due out this month. Both publications offer much for those studying coloration in other taxa too, because birds provide model systems for investigating animal coloration. Besides, birds are just so exquisitely beautiful.