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1 July 2007 Bird Coloration.
Alan H. Brush
Author Affiliations +
Abstract

The following critiques express the opinions of the individual evaluators regarding the strengths, weaknesses, and value of the books they review. As such, the appraisals are subjective assessments and do not necessarily reflect the opinions of the editors or any official policy of the American Ornithologists' Union.

Geoffery E. Hill and Kevin J. McGraw, Editors. 2006. Harvard University Press, Cambridge, Massachusetts. Vol. 1, Mechanisms and Measurements: ix + 589 pp., 32 color plates; ISBN 0-674-01893-1. Vol. 2, Function and Evolution: x + 477 pp., 32 color plates; ISBN 0-674-02176-2. Hardbound, $95.00 each.—In his now classic Animal Biochromes and Structural Colours (1953), Denis Fox surveyed the “physical, chemical, distributional & physiological features of coloured bodies in the animal world” in only 378 pages. A second edition (1976) expanded to 433 pages was considered equally comprehensive, scholarly, and complete. In 2006, just about a half century after Fox, Hill and McGraw require two volumes of 1,066 total pages, 23 authors, and 64 color plates just to survey birds! This is an important and timely contribution. Times have certainly changed, and it has all been for the good. By necessity, much of the work that Fox’s volume covered was done by a group of German workers beginning with Otto Völker and lasting to the start of World War II. Fox, trained in England but working at the Scripps Oceanographic Institute, studied fish, invertebrates, and birds into the early 1960s. His work on flamingos, in particular, set the stage for young ornithologists’ tentative ventures into feather pigment chemistry. The field began slowly, with the development of extraction techniques gentle enough to preserve the easily destroyed “lipochrome” pigments, separation on thin-layer chromatography, and measuring optical densities on primitive Beckman DB Spectrophotometers. It is astonishing, therefore, to follow Robert Montgomerie’s (volume 1, chapter 2) masterful romp through contemporary techniques and theory for measuring and quantifying colors in birds.

By the last decades of the 20th century, there was an explosion in the understanding of avian color perception, techniques to measure and analyze reflected light, the chemistry and physiology of pigments, and new and valuable insights into the basis and nature of structural colors. There have been dramatic technical advances in electron microscopy, and sophisticated tools to separate and identify pigments of just about every type (although there are some feather pigments still to be identified) and exploration of the role of plumage in behavior. The argument that the world of color for birds is different from ours is soundly made. All this and more is covered in these two volumes, which reflect the progress and growth in the field.

With a body of knowledge of this magnitude, the reader may want to tease out information relevant to a particular question or subfield. It is all here. Topics range from the plumage- pigment biosynthetic pathways to how they are absorbed, transported, and transferred across compartments. The consequences of dietary selectivity, seasonal metabolic changes, and their connection to the molt cycle, through the genetic input into the control of plumage patterns, and environmental factors that affect their display are all represented. Higher-level issues, such as the mechanisms of color perception, the determination of feather patterns, feather-tract specificity, and pigment content, are addressed. Further, the roles of colors in signaling such features as dominance or sexual selection or a dozen other functions are discussed. The list continues, and the chapter authors can be credited with taking a long view of the problem addressed. Consequently, the book makes major contribu- tions to numerous aspects of avian biology. It is well edited, with blanket coverage of the relevant literature. I noticed typos mostly in the references. Readers will delight to find authorities writing in areas and on material they know best.

Without doubt, this book is a milestone. All the contributions are relevant and all the authors conversant with their fields. I found several chapters to be extraordinarily stimulating and informative, because they either deal with a particularly knotty problem or represent major advances in the field. This is not to demean any of the other contributions. In one example, Gary Bortolotti (volume 2, chapter 1) grapples with one of the most diffuse aspects: natural selection and coloration. In a second, Alex Badyaev makes a masterful presentation of a new evolutionary synthesis of color displays (volume 2, chapter 8). Other authors explore, often at one or more levels, the plumage, its patterns, the ecological interactions, and mechanisms of its development and evolution.

I was especially impressed with Richard Prum’s review of his recent work on the nature of structural colors (volume 1, chapter 7). He carefully illustrates and separates pigment and structural colors, and then introduces a clear description of coherent light scattering as the basis for structural colors. This is a problem that has gone unsettled for years. The persistence of the problem is reflected in the names for the phenomena, which include Tyndall and Rayleigh scattering (both late-19th-century optical physicists). Prum goes on to describe a new approach to the analysis of nanostructure-based colors using Fourier Transforms. Both iridescent feathers and vividly colored skin are discussed. The physical basis for his arguments resides in the β-keratin-melanin-air space known as the melanosome. The analysis has been enhanced by advances in both electron microscopy and high-speed computing. The yield is a comprehensive understanding of the structural colors of feather barbs, skin, and eyes.

Another personal favorite is the chapter by I. C. Cuthill (volume 1, chapter 1) on color perception, which establishes the interface between the environment and behavior. Starting with an operational definition of color vision (“the capacity for discriminating between objects based on differences in the relative amounts of different wavelengths of light, rather than the absolute amount of light…”; vol. 1, p. 3), its psychophysical nature is carefully explained, which takes the reader to the boundaries of neuroscience and philosophy. Cuthill goes on to review the significance and implications of the fact that avian vision is tetrachromatic, rather than trichromic as in humans. As an important aside, Cuthill included recent information from genomics that identify and sort the differences in the multiple families of opsin, the protein portion of visual pigments. This is another example of a multiple family of genes coding slightly different functional elements. The careful explanation of the function of oil droplets as ocular filters is current, critical to understanding how birds perceive color, and helpful in integrating the recently documented use of ultraviolet (UV) wavelengths by birds. Vision in the UV raises new questions. For example, in plant-animal interactions, many flowers and fruits are detected by birds aided by patterns visible in the UV. For another example, plumages that appear sexually monochromatic to humans may be discriminated by birds as dichromatic in the UV portion of the spectrum.

Understanding the production and evolution of plumage patterns has been an ongoing challenge for decades. Now Kevin Omland and Christopher Hofmann (volume 2, chapter 10) review the use of phylogenetics to reconstruct a history of plumage patterns and colors. Pattern evolution, of course, includes phenotypic plasticity, a feature covered by Alexander Badyaev (volume 2, chapter 8). One of his insights is the introduction of modularity, “the ability to form and maintain a stable and largely independent network that retains its integration in different contexts and over evolutionary times” (vol. 2, p. 353). A consequence is to consider “plumage ontogeny and evolution as a series of hierarchically arranged and recurrent developmental and functional modules, from feather-germ precursors to coordination of pigment distribution among follicles to complex courtship displays” (vol. 2, p. 353). The focus is on the generation of the plumage in its entirety (but also applies to individual feathers) and the relationships with the visual, social, and evolutionary aspects of the environment. All levels, from the molecular through behavioral ecology, are considered throughout; and that may be the importance of the contribution of this set.

Reading Bird Coloration is like having large sequences of my scientific life flash before me. Revisiting the kinds of questions asked so long ago (my first publication on the feather tips of Cedar Waxwings appeared in 1963!) is only one aspect. The other is the successful framing of larger questions now that seemed almost incomprehensible then. Advances in laboratory techniques, combined with psychophysics, behavioral ecology, and evolution and development, have made this possible. The authors of Avian Coloration rephrase many theoretical questions posed over the past half-century and introduce new insights into others. With the new directions applying improved techniques and theoretical insights, answers to traditional questions cannot be far behind. Work on coloration has grown and expanded to encompass behavior, comparative biochemistry and physiology, the plumage as phenotype, and the control of these processes. We now have the intellectual tools to forge connections among functional mechanisms, development, complex behavior, and evolution. There is a deep satisfaction in all of this. Hill and McGraw have facilitated a major contribution to the field in its broadest sense.

Bird Coloration should be available at colleges, universities, natural-history museums, and nature centers. Personal copies would be invaluable to anyone interested in the myriad aspects of avian plumage. There are dozens of dissertation projects here.

Alan H. Brush "Bird Coloration.," The Auk 124(3), 1097-1099, (1 July 2007). https://doi.org/10.1642/0004-8038(2007)124[1097:BC]2.0.CO;2
Published: 1 July 2007
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