Howell: Molt in North American Birds

For long decades, guides to North American birds treated seasonal and age-related differences in plumage as fixed, discrete phenomena, with hardly a hint as to just how an individual bird’s feathers might pass from immature to adult, from winter to summer, from breeding to non-breeding. Even the banding manuals implicitly instructed us to consider the colors of a plumage rather than the timing and the circumstances of its origin, so that for most species–especially passerines–we were left in those benighted pre-Pyle days with a relatively ineloquent and relatively imprecise set of age-and-sex categories into which we had to fit the victims of our nets and potter traps.

Matters improved and eyes opened with the publication in 1987 of Peter Pyle and Steve Howell’s Identification Guide to North American Passerines, which immediately rendered the erstwhile standard texts by Dwight, Roberts, and Wood obsolete; suddenly, banders and ambitious birders had a tool that let them use not the state of a bird’s plumage–properly speaking, its aspect–but rather its relationship to other, preceding and subsequent plumages to age and sex the individual with sometimes startling precision. We learned, in a moment of almost blinding clarity, that we should be paying attention to the transitions between plumages rather than to the “plumages” themselves: that we should, in other words, be concentrating on molt rather than on its product.

I’d venture to assume that nearly every birder has heard of molt, that regular process by which birds replace old, worn feathers with new, strong ones.  And I’m  equally certain that even today not one North American birder in ten fully recognizes the many ways in which even a simple understanding of the phenomenon can increase our enjoyment, and our knowledge, of the birds we watch in the field.

Photo: WINGS Birding Tours Worldwide.

In a series of groundbreaking articles in Birding, Birding World, Western Birds, and elsewhere, Steve Howell has been at the forefront of introducing recent discoveries about molt into our field practice as birders. Now, with his new Peterson Reference Guide, Molt in North American Birds, Howell offers birders a concise and beautifully written summary of much of what is known about molt as a phenomenon, followed by chapters dedicated to the molt “strategies” followed in every family of birds breeding in North America north of Mexico. The result is a volume that every birder will learn a great deal from–and one that, by raising repeatedly questions still unanswered by students of molt, will inspire birders, banders, and ornithologists to learn even more about this fascinating and often complicated subject.

Howell begins with a too modestly titled 65-page “Introduction” to the subject at hand, offering first a series of interrogatories on such fundamental topics as the nature of feathers and the difference between a plumage–a set of feathers produced by a molt–and an aspect–the appearance of a bird’s feather “coat” at a given time. Richly and attractively illustrated (most of the stunning photos are Howell’s own), this catechism of molt should be required reading even for birders uninterested in the phenomenon’s broader implications for birding.

In the remaining 45 introductory pages, Howell discusses the patterns and strategies of molt observable in North American birds, defining plumages using the “modified” Humphrey-Parkes system and introducing the four fundamental sequences of molt. The Basic strategies are distinguished from Alternate strategies by their lack of a pre-alternate molt (and thus of an alternate plumage); Complex strategies differ from Simple strategies in the “insertion” of an “extra,” pre-formative molt in the first molt cycle (a cycle lasts approximately a year in most familiar species). Thus, birds exhibit a Simple Basic strategy, a Complex Basic strategy, a Simple Alternate strategy, or a Complex Alternate strategy.

The strategies, and the molts and plumages they comprise, are described with admirable clarity in Howell’s text, though in this section–unlike anywhere else in the book–I found some of the illustrations and their captions less immediately comprehensible. The series of photographs tracing the progress of molts and plumages of a nuttalli (italicized, please!) White-crowned Sparrow is itself informative, though in a quibbling moment I might have preferred a subspecies where the adults’ head pattern is brighter. Unfortunately, the caption to Figure 29, showing a bird undergoing its pre-alternate molt, might easily be misread. At least the last sentence should have been rewritten, to begin “In this first-cycle bird, most of the body feathers are still formative plumage….” As the caption stands now, it could be read–as I did at first–as suggesting that all Nuttall’s White-crowns somehow retain bits of their formative and of their juvenile plumages after their limited pre-alternate molt.

Figure 36, which uses paintings familiar to many readers from David Sibley’s Birding Basics, suffers from a lapse in design: the horizontal lines separating the illustrations of each strategy from their label  should be below, not above, the accompanying diagram; as things stand, the label “Simple basic strategy” seems to belong to paintings showing a complex basic strategy, and the label “Simple alternate strategy” appears to go with paintings showing a complex alternate strategy. No matter how many times I’ve looked at this opening, my eye still mis-allocates the paintings and their labels.

Perhaps the most important illustration in the book, Figure 35, presents graphically the differences so well described in prose among the four molt strategies. But the caption here fails to do justice to the ingenuity of Howell’s diagram, leaving the reader–at least this reader–uncertain whether some of the graphic details are intended to convey meaning or, and this seems less likely, are simply inconsistencies in design. For example, the caption informs us that a broken line distinguishes body feathers from flight feathers (an important distinction, given that in all but the simple basic strategy, body feathers in at least the first cycle are molted more often than remiges and rectrices); but what does it mean when that line becomes a solid line in the diagram? And does the pre-alternate molt under the complex alternate strategy in fact commence later and last longer than the homologous molt in birds employing a simple alternate strategy–as the diagram seems to depict? This is a tremendously useful diagram, but the caption should address, if only to deny, the significance of such small differences in the graphs.

Just as useful is the table occupying pages 26-28, a clear overview of the molt strategies observed in each family of North American birds. The rarest, and perhaps the oldest, is the simple basic strategy, in which birds molt directly from juvenile plumage to basic plumage, without a formative “immature” plumage in between; the commonest–exhibited by nearly all passerine families and a range of non-passerines, too–is the complex alternate strategy, in which young birds molt from juvenile plumage into an “extra” formative plumage, and birds of all ages undergo an almost always incomplete pre-alternate molt.

This table serves as a quick guide to the family chapter that follow, each of which begins with a short account of the family’s taxonomic affinities and general natural history. There follow concise but easily understandable descriptions of each molt, often including a fascinating question or two about apparent anomalies in a species’ or group’s molting behavior: why, for example, asks Howell, do skimmers have a pre-formative molt, which tends to be more typical of long-distance migrants? Howell’s answers to these questions, when he has them, are always cogent, and the cases where he can offer only speculation are precisely those where careful and observant birders and ornithologists could make a real contribution to our knowledge of molt.

Because the systematic section of the book remains at the relatively abstract level of family, birders tempted to look here for shortcuts to species-level identification may be disappointed. There are useful hints here and there, but the significance of this splendid book is far greater than helping us distinguish the eastern crows or the western empids. With his characteristic clarity, completeness, and good humor, Steve Howell has given the birding world an entree into one of the great mysteries of bird biology, and the time spent studying Molt–and molt–will be more than rewarded by the increased sophistication and enjoyment with which we will be able to look at even the commonest birds. Molt, says the author, “offers a fascinating window through which to appreciate how the lives of birds are built.” Many thanks to Steve Howell for opening that window to the rest of us.