An Evolutionary Perspective On Moult

Submitted by Michi on 2 February 2024.

Text by Meredith Swett Walker

The picture above features a simplified version of the artful and informative illustration from the perspective piece “Moult terminology: envisioning an evolutionary approach” by Peter Pyle, Steve N. G. Howell, Danny I. Rogers. and Chris Corben. Twenty years ago these same authors originally proposed the term 'preformative moult' and defined four strategies to help elucidate the evolution of moults in birds (Howell et al. 2003; Condor, 105:635-653). The illustration, by artist and biologist Lauren Helton, depicts an avian evolutionary tree with moult strategies mapped onto it.

Our understanding of the evolution of moult strategies is still developing and Pyle and coauthors emphasize that the conceptualization they present here is a working hypothesis. The four main moult strategies defined by Howell et al. (2003) are mapped onto the tree: “simple basic strategy” (SBS; no preformative or prealternate moults), “complex basic strategy”(CBS; a preformative moult but no prealternate moults), “simple alternate strategy” (SAS; a single moult in the first cycle and prealternate moults in later cycles), and “complex alternate strategy” (CAS; a preformative moult in the first cycle and prealternate moults in all cycles).

Seeing moults presented in an evolutionary framework, one is struck by the prevalence of the CBS strategy and intrigued by the departures from it. The authors propose that the CBS is basal to all avian lineages and provide discussion and speculation on how the other three strategies may have evolved. For example, the SBS was formerly thought to be basal, but instead a preformative moult appears to have been lost in lineages exhibiting the SBS from those exhbiting the CBS, for families and species that grow strong juvenile feathering. Taxa exhibiting the CAS appear to have evolved prealternate moults to replace worn feathers (particularly in migratory species), but some species have taken adaptive advantages of these extra moults to develop more cryptic plumages for flight-feather moult or more colourful plumages for courtship. Some species that exhibit the SAS may have evolved this strategy through the merging of ancestral preformative and first prealternate moults along lineages exhibiting the CAS.

An evolutionary perspective makes the endeavor of understanding avian moult less daunting. "Once one envisions the evolutionary bases of H-P, which can be difficult to do for those of us who grew up on the life-cycle system, the categorization and study of moults becomes much less confusing, much more satisfying, and actually rather enjoyable," says Pyle.  

Examples of birds with different moulting strategies:

     Tinamous (Order Tinamiformes) are a family of ratites, the most primitive group of flightless birds that also includes ostriches, rheas, and emus. The fact that tinamous and other basal bird taxa include preformative moults suggests that this moult could have evolved from reptiles, which may undergo an extra shedding of skin in the first year as body size grows rapidly (but more study is needed on this). As such, and since these basal taxa lack prealternate moults, the authors propose that the Complex Basal moulting strategy (CBS) is ancestral to all bird lineages. This Little Timaou (Crypturellus soui) was photographed in Cali El Faro, Colombia on January 23, 2023 by Doug Greenberg.

         Some avian taxa, such as albatrosses, American vultures, storm-petrels, and the Ivory Gull (Pagophila eburnea), lack both preformative and prealternate moults, and thus undergo the Simple Basic Strategy (SBS). The authors had originally proposed that this strategy was ancestral; however, as taxa undergoing the SBS have evolved along avian lineages that exhibited the Complex Basic Strategy (CBS), it seems more likely that the preformative moult was 'lost' evolutionarily, and that preformative moults and CBS are thus basal to all avian lineages. Birds exhibiting the SBS typically grow stronger juvenile feathers during prolonged nestling periods precluding the need for their replacement during the first year. This Ivory Gull was photographed in Sweden on December 11, 2004 by Thomas Landgren.

         Prealternate moults are extra inserted moults that have evolved within moult cycles of various avian lineages; species with these moults in both the first and later cycles undergo the Complex Alternate Strategy (CAS). The moults themselves likely evolved due to the need to replace worn feathers more than once per year, especially in migratory species exposed to more solar radiation on an annual basis. In some taxa such as Tyrannid flycatchers, most vireos, and wrens, prealternate moults result in little change in plumage colouration or pattern. Other species have taken advantage of prealternate moults for adaptive purposes, for example, in ducks, ptarmigan, and possibly jaegers, prealternate moults result in duller or more cryptic plumage for camouflage during moult of flight feathers. In many other CAS taxa, such as grebes, loons, wagtails, American warblers, and Red-backed Shrikes (Lanius collurio), males or both sexes take advantage of prealternate moults to develop colourful plumage for display and sexual selection during the prebreeding season. This Red-backed Shrike was photographed in Slovakia on June 2nd, 2016 by Radovan Václav.

           Finally, some species have an inserted prealternate moult in second and later moult cycles but show only one moult (preformative and/or first prealternate) in the first cycle, and thus undergo the Simple Alternate Strategy (SAS). The SAS may have evolved along avian lineages in one of several ways. In some taxa, such as loons, pelicans, and ibises, prealternate moults may have been gained along lineages exhibiting the Complex Basic Strategy (CBS) in species that do not breed until two years old or later. The moults likely evolved due to the extra wear to feathers that result from harsh breeding environments and later resulted in colourful plumage for mate selection. For others, such as some alcids, larger gulls and shorebirds, lineages that exhibited the Complex Alternate Strategy (CAS) may have lost the preformative or prealternate moult in the first cycle or, as seems more likely, had these two moults merge into one. In only some cases, such as for the Horned Puffin (Fratercula corniculata), does it seem clear that one moult (in this case preformative) may have been lost while the first prealternate moult was retained. No landbirds, including passerines, exhibit the SAS, perhaps relating to most of these species initiating breeding at a year of age. These Horned Puffins were photographed on St. Paul Island in Alaska on May 30, 2015 by Isaac Sanchez.

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