Steve N. G. Howell, Peterson Reference Guides, HMH, 2010
National Geographic Bird Coloration
Geoffrey E. Hill, National Geographic Society, 2010
Dateline: July 30, 2010. The earliest arrivals are here. Their numbers will peak in perhaps three weeks. Having concluded their vital business in the arctic they have set out on their long journey to winter habitats in South America. It’s the time of year when I leave a half-dozen shorebird guides on my night stand to brush up on my identification skills in anticipation of the coming rush hour at Jamaica Bay Refuge. Shorebirds have become an abiding passion for me at least partly because their identification is a perpetual challenge and a continuing learning opportunity.
They arrive from South America in Spring in their alternate (“breeding”) plumage which, in many individuals, has not yet reached its peak coloration. The differences in their appearance are due to the varying progress of the molt among individual birds and to the difference in ages of the birds. The evolutionary purpose of such diversity seems, from our perspective, to be solely for the purpose of keeping us birders on our toes. But there is, of course, much more to it. Returning birds are harder to identify than those we see in Spring because there are many more variations. A mud flat with a mixed flock of perhaps a thousand birds will include Western, Semi-palmated, and a few Least sandpipers. Add a few Sanderlings, Red knots, Ruddy turnstones, and other usual suspects and a birder has a lot of sorting to do. Although most of the small sandpipers will be Westerns around these parts at this time of year, you can’t take anything for granted. The plumage we learned in May has undergone considerable wear, has less color, and gives a “dirtier” or more “diffuse” appearance in August. Our picture is further complicated by the presence of many birds in varying stages of their pre-basic molt (in which they change to their “non-breeding”plumage) and by juveniles in fresh formative plumage. Although sorting through all this can be confusing, learning it isn’t impossible.
Paying close attention to birds with complicated plumages such as shorebirds or wintering gulls will make you a better birder and deepen your insight into our natural world because feathers define birds. As a novice birder I thought about feathers as a beautiful devise by which I could name birds and by which they could choose mates. My understanding of molt was limited to “breeding” and “non-breeding” plumage. However, as with all things in nature, deeper investigation yields a far more interesting, more complex, richer picture. Without feathers birds couldn’t fly. They wouldn’t be able to regulate their body temperature, and they would have to devise other strategies for avoiding predators, attracting mates, and for getting bird watchers to look at them. (After all, only herpetologists would bother looking at naked birds, and we all know that herpatologists are “different.”) Feathers are marvels of invention. They are aerodynamic. Each flight feather is a perfect airfoil. Pigments and structure determine how we experience them and add strength. Feathers define body shape. Being as light as, well, feathers, they seem insubstantial, yet they are remarkably durable and have incredible structural integrity. But they do wear out, and they must be regularly replaced. A birder who takes the time to learn about feathers and molt will absorb some physics, a little biochemistry, some knowledge of natural selection and evolution, and a dollop of mystery.
Although academic ornithologists have been seriously studying feathers for a long time, the subject has remained locked in the domain of academia since most of the published material has appeared in scientific publications which are inaccessible to the general reader. Fortunately, more and more of these studies are being redacted for the general reader. Just as Donald Kroodsma has shined a light on the mysteries of bird song with his two recent books on the subject, we are now blessed with the publication of two outstanding books about plumage and molt.
The first is Molt in North American Birds by Steve G. Howell (Peterson Reference Guides, HMH, 2010). Howell is the co-author of a previous volume in the growing Peterson Reference Guide series, Gulls of The Americas whose excellent introduction whetted my appetite to learn more about the complex subject of molt. This new volume is composed of two sections: an extensive introduction and family accounts.
The introduction explains the progression of feather replacement in birds from hatching throughout their lifetimes. It introduces a new (to most birders) vocabulary which replaces “breeding” and “non-breeding” with the more neutral “alternate” and “basic” to define plumages. “Basic” plumage refers to the first (after juvenile) definitive plumage by which most of us know birds. “Alternate” refers to any other patterns which occur on a regular basis during breeding season. Species which inhabit their breeding territories throughout the year tend to appear only in their basic plumage, while birds which migrate to breeding territories develop an alternate plumage. The introduction explains replacement strategies and the four standard strategies -- “simple basic”, “complex basic,” “simple alternate,” and “complex alternate” – by which all birds regularly replace their plumage throughout the year. Howell’s introduction also answers a series of questions which will occur to you as you spend more time in the field – What triggers molt? How long does it take to complete a molt? How long do feathers last? What is the cost (in terms of calories and ability to avoid predators) of molt? The style is unavoidably dense, but this section is only 50 pages. Working through the introduction will make you a much better birder because you will gain a better understanding of what you are looking at when you are trying to sort through any one of the species whose appearance is highly variable like gulls, shorebirds, Yellow-rumped warblers, sparrows, etc..
After working through the introduction, I suggest saving the family accounts to prepare for field trips. Study the section on Parulidae next spring as you prepare to go out looking for returning warblers; and you will be astonished at how much your skills improve and how much more fun you will have.
Geoffrey E. Hill’s National Geographic Bird Coloration is about everything else you want to know about feathers. Hill is an ornithologist and a professor of biology at Auburn University who has devoted the large part of his career to studying feathers and was the co-editor of a two volume academic publication on the subject. We owe him our gratitude for distilling his work into a publication aimed at birders.
The first chapters of Bird Coloration include a primer on the physics of color and an enlightening discussion of the visual world of birds – how their visual perception of the world dramatically differs from ours. You want to know this because it partially answers the question, “what function is served by color.” Our ability to discern color is actually quite limited in comparison to birds. The human retina contains three types of cones which are sensitive only to the three primary colors. All other colors are the result of stimulating more than one type of cone to varying degrees. A bird’s retina contains a fourth type of cone which is sensitive to ultraviolet. We have no way of knowing how birds actually perceive the visual world, but we do know that many feathers have ultraviolet peaks which signal something to birds which we cannot see.
Hill includes extensive discussions of the pigmentary and structural basis of color in birds and the nature of iridescence which is caused by light refracting through and reflecting off the micro-structures of feathers. A crow is matte black because melanin is deposited near the surface of the feather. A grackle appears iridescent because the melanin is deposited deeper in the structure of the feather, so we experience light which is coming simultaneously from two sources – that which is reflected off the feather’s surface and that which is reflected off the inner microstructures of the feather. Since the two sources are traveling different distances, various wavelengths cancel each other out because they are out of phase and other wavelengths amplify each other. The appearance of iridescence is also due to the fact that the color changes with our angle of view. The gorgets of hummingbirds are extreme examples of iridescence which is caused by the multiple layers of fine feathers of the gorget which provide a much richer source of iridescence.
Pigments, it turns out, serve quite a few functions. In addition to adding color and pattern, melanin strengthens feathers and provides some protection against bacteria and other parasites. Hill sites an example of an albino Great frigatebird, found in 1985, whose feathers were so worn by friction with air molecules that the bird could no longer fly. This example explains why light colored birds which spend most of their lives in the air (like White pelicans and gulls) have black wing tips. The tips of the wings are placed where they are subject to the most turbulence and are most susceptible to wear. Adding melanin to the ends of the flight feathers provides protection from wear and distinctive coloration and pattern. This strengthening function of melanin also explains why bird populations who live in humid, bacteria and parasite rich environments tend to be much darker than those of the same species who live in drier climates which support fewer bacteria and parasites.
Color and pattern form the basis of sexual attraction in many species and signal social status and dominence. Many species which undergo more than one pre-basic molt reach sexual maturity before attaining their first basic adult plumage. Researchers have found that a prolonged sub-adult plumage phase protects these young birds from agression by older more experienced males within their group.
Bird Coloration also includes detailed discussions of the genetic, evolutionary, hormonal and environmental factors which affect color and pattern, the evolutionary reasons for the development of cryptic and bright colors in birds, and the role of color in choosing mates. The book is well illustrated and is peppered with “Birder’s Notes” which provide birders with tips on using variations in color and pattern to make more accurate identifications.
Bird Coloration is far too rich to discuss adequately in a short article such as this but I hope that I have whetted your appetite for this subject enough to inspire you to read more. Few areas of study will do as much to advance your birding skills as the study of plumage and molt. These two volumes belong on the shelf of every birder.