Geoffrey E. Hill. 2002. Oxford University Press, New York. xiv + 318 pp. ISBN 0-19-5148487. Cloth, $65.00. —Why are star-forming galaxies red? Because enormous dust clouds absorb light of shorter wavelengths that is then re-radiated at longer wave-lengths. By analyzing that phenomenon, one can learn about the origin and behavior of stars and galaxies and, ultimately, the universe. Why are male House Finches (Carpodacus mexicanus) red? Because carotenoid pigments, acquired through the diet and deposited in the feathers, absorb the shorter wavelengths of the white daylight, whereas longer wavelengths are reflected. By analyzing that phenomenon, one can learn about the origin and behavior of birds and other living creatures including, ultimately, ourselves.

There are people (and funding agencies) that find the first of those questions fascinating and worth the enormous cost of a VLT (Very Large Telescope), while considering the second question uninteresting, even at the minor expense of RFB (Regular Field Binoculars). But there are also many people that, like me, would argue that the red forehead of a backyard bird has more interesting things to teach us than does 10-billion-year-old starlight.

I start this review in a somewhat confrontational way because the title of Geoffrey Hill's book, A Red Bird in a Brown Bag, implies the almost apologetic attitude that many behavioral ecologists have towards their study topics as compared to “real science” like astronomy or biochemistry (try to imagine an astro-physics book entitled A Red Dot in a Huge Telescope…). Luckily, there is nothing apologetic inside this tribute to the fascinating puzzle of evolutionary biology in general, and animal coloration in particular. Let us just hope that, despite its title, this book reaches the broad readership it deserves.

Geoffrey Hill takes us on an enjoyable personal and scientific journey, from how he first decided on House Finch plumage coloration as a Ph.D. project 16 years ago, through the many aspects of this problem that he and his students have tackled over the years, to the current frontiers in the study of avian plumage coloration. As a matter of fact, the House Finch is the frontier in many ways: Looking back at all the mechanisms of avian color signaling for which these little birds, in the hands of Hill and his collaborators, have been the first (and quite often the only) model system, it is no exaggeration to say that the field would be seriously set back if the House Finch proves to be an exception rather than a general example. Then again, even if the House Finch turns out to be one of a kind in some features, it has still provided a large number of novel hypotheses and testable predictions.

Personal memories and stories make great spices in scientific monographs, but there is always a risk of diluting or even concealing the scientific message. Hill has solved this nicely by keeping the ingredients apart, starting each chapter with a short, amusing anecdote (I especially liked the one about the merciless Terminator working undercover as a suburban dentist), before switching to focused and nicely illustrated accounts of the chapter topics—from pigment physiology and nutrition to sexual and social behavior, ending with comparative tests of sexual selection theory in the chapter, “Why red?”. The book is a pleasure to read from start to finish, but it also has enough repetitions of results and conclusions between chapters to allow readers with a more narrow interest to dive straight into the section of their choice.

The first chapter, on Darwinism and “Wallacism,“ describes how the views of dazzling bird colors have changed from divine creation (a “part of God's plan,“ as young Geoffrey was told by his mother) to the current models of sexual selection and visual signaling. The important contribution of Alfred Russell Wallace is emphasized. Although he rejected, almost ridiculed, Darwin's idea of female mate choice, he identified several of the other currently discussed adaptive significances of plumage coloration. Ironically, Wallace was also the first to suggest the link between color intensity and vigor that is now a central assumption of the sexual selection theory he dismissed.

After a chapter on general “housefinchology” and methods, filling in useful details for which there has been no or too little space in journal papers, the third chapter is devoted to color vision and color quantification. This is an important foundation for the chapters to come, not least to make sure that younger students, brought up in the age of portable spectrometers, do not dismiss the early House Finch research as flawed by its subjective color scoring. Hill makes a good case for the relevance and repeatability of his use of human color space, in the first years with color charts and more recently via tri-stimulus values measured with a miniature and rather low-resolution (10 nm segments) spectrophotometer, the Colortron. However, in my opinion (colored, of course), this chapter could have been a few pages longer and even more convincing. Why not use reflectance analyses directly to demonstrate in what way and how well the human color scoring captures objectively measured and computed parameters. Researchers familiar with spectrometry and colorimetry will know this, but there might be many readers (not to mention reviewers and editors) that dogmatically reject any animal color study without reflectance. (Somewhat reminiscent of how every comparative result without phylogenetic control was treated 10 years ago.)

For example, ultraviolet (UV) vision is waved off as insignificant in this system because House Finch plumage reflects very little in UV. This is illustrated with a reflectance figure (the only one in the book), showing spectra from one red and one orange male that indeed are less intense in the UV. But what counts in color perception are relative differences, and here I would have liked to see reflectance variation with averages and standard errors, just as we demand of biometrical data. Supposedly, there would be a reassuringly small variation in UV compared to the variation in position ("hue") or steepness ("saturation") of the “red” slope. Furthermore, correlations between such variables and the human-subjective measures reported throughout the book would better back up their biological relevance. Yet, those are minor problems for the simple reason that most results are positive—even conservative—in the sense that, for example, mating preferences, effects of diet supplementation, parasites, and so on, have emerged despite (not because of) the human-subjective color scoring. The more overarching risk of Type-II errors seldom applies, and I can not help but envy the striking plumage variation in this species, from drab yellow to bright red, clearly visible to man and bird alike.

The second part introduces the central theme: the proximate control and signal function of carotenoid pigmentation. Although the dietary origin and physiological importance of carotenoids was known several decades ago, and their potentials as quality advertisements had been shown by John Endler, it was Geoffrey Hill that profoundly introduced those red-hot ideas to behavioral ecology. In chapter 4, he explains what carotenoids are and how they are acquired, metabolized, and deposited in the plumage. In the preface, readers without interest in such proximate details are advised to skip over this ‘technically challenging’ chapter, but I would encourage everyone seriously interested in animal color signaling to read this chapter carefully. Apart from a minor error in a figure (should be only one hydroxyl group on beta-cryptoxanthin) and maybe too-sweeping conclusions about metabolism and storage, this is a uniquely accessible introduction to the biochemical basis of avian carotenoid signaling in the wild, with the House Finch in focus. Its red color results from a mixture of 13 different carotenoids, some direct-deposited and some metabolically modified. A “red” pigment (3-hydroxy-echinenone), produced from a dietary “orange” pigment (beta-cryptoxanthin), is shown to be the main determinant of plumage redness ("hue"), thereby linking a particular carotenoid to the sexually selected color variation. This is the obvious way forward to better understanding of selection and constraints on carotenoid signaling. Better, that is, than the already amazingly complete picture of the House Finch system.

The rest of Part Two is a welcome review of how this picture has been painted, step by step, by feeding experiments to identify the nutritional and physiological control of coloration mentioned above, and by mate-choice trials and correlates of pairing status in the wild that point to mate attraction as the main advantage of being red. It is pleasing to have the many carotenoid supplementation experiments, with and without the influence of parasites or food stress, strung up on one line to show how several mechanisms interact to make the red pigmentation an honest ("uncheatable") signal of health and condition. At the end, Hill reviews the recent ideas of direct tradeoffs between the antioxidant and coloring functions of carotenoids. He finds this unlikely to be important, as judged from the often excessive levels of carotenoids in bird blood, and from the lack of effects of carotenoid supplementation on immune responses in a experiment on goldfinches. A recent study of Zebra Finches (Taeniopygia guttata), however, found the opposite to be true, so this debate has only just begun.

As regards sexual selection and fitness consequences in the wild, the results are fewer and more incomplete, but served together, and with additional, unpublished data, the story is convincing. Over several years, successfully paired House Finch males had redder color and (when measured) larger and more symmetrical color patches. A positive correlation between male redness and offspring production also corroborates the indications that colorful males pair earlier, with higher quality females, and feed their young more, in addition to the famous initial relationship with food provisioning to the incubating female.

Sexual selection also operates through direct competition among males for access to females or resources that attract females. Status signaling can thus be an alternative or additional function of a sexual “ornament.” The House Finch is unusual in that the two processes seem to oppose each other (i.e. brown males are subordinate to red males, at least in aggressive competition over food). Chapter 9 reviews the work that shows that brown plumage is not a status signal in itself and that no simple explanation can be found in testosterone levels; despite positive effects on dominance and negative effects on coloration in captive flocks, testosterone levels were higher in red than in drab males in the wild.

The third part takes on the ultimate goal of evolutionary biology, to explain organic diversity—in this case, the variation in ornamental coloration among populations and subspecies. A historical account of House Finch taxonomy and biogeography is given as background to 12 geographical populations that are analyzed with respect to color and extent of pigmentation. In the last chapter, Hill describes the current runaway and indicator models of sexual selection together with his own “combo” model of honest signaling and stasis interrupted by periods of cheating and runaway elaboration. By mapping patch sizes and female preferences (established in captivity) onto a tentative phylogeny, he then attempts to distinguish between those processes in trait elaboration. Although some readers may not agree with some of the predictions or interpretations, this is an inspiring and thought-provoking concluding chapter. Concluding the book, that is, not the research project. No way. Not even a sign that it was slowed down by this book project.

In the epilogue, Hill describes the exciting new directions that the study of this and other colorful birds is now taking, in particular the refined analyses of selection on multiple signal components, and of carotenoid biochemistry and nutrition. Finally, Hill expresses the standard with that “this book will serve as a starting point for future research,” and it certainly will. Just follow that taxi.