It’s One of the Biggest Events in a Bird’s Life. Scientists Can’t Agree How to Talk About It.

Feathers define birds as we know them. Whether fanned in courtship display, slicing the air, or gliding through water, birds’ plumage makes them unique among living animals. But as fascinating and glorious as feathers may be, they are merely dead tissue, and any damage to them is permanent. Too many impaired feathers spells disaster for a bird. Luckily, birds can replace their mangled plumes—and they do so at least once a year in a process called molting, often right under the noses of birders. Some species take the opportunity to sport seasonal looks, like the nearly radioactive red of a male Scarlet Tanager in spring. But most importantly, molt is a survival tactic, necessary to keep feathers in peak condition. Simply put: If a bird doesn’t molt, it will die.

When, where, and how often a bird replaces its plumes varies widely between and even within species, making molt a formidable research subject, though one that is key to understanding a species’ life history and conservation needs. Compared to other major events in birds’ lives, namely breeding and migration, molt is understudied. “It’s amazing,” says Erik Johnson, director of conservation science at Audubon Delta and author of a book on the molt of neotropical birds, “There are so many ornithologists that just don’t think about molt.” Yet it’s a vital part of all avian lives: “Not all birds breed every year. Not all birds migrate. But every single bird molts every year. It’s that foundational to what a bird is.” 

But ornithologists and molt-curious birders are likely to encounter a conundrum when diving into the subject: A schism divides the world of molt studies, with a major border line running down the Atlantic Ocean. Compare a scientific paper on molt from Europe to one written in the Americas, and you may be at a loss to find their common ground (and not only because Europeans spell it as “moult”). The two camps tend to use completely different words to talk about molt. 

While the system favored in Europe is tied to other events in birds’ lives—breeding, in particular—the terminology used in the Americas was designed by ornithologists Philip Humphrey and Kenneth Parkes in the 1950s to refer solely to molting itself. So, for example, when a tanager is turning scarlet, a European ornithologist would likely call it a “pre-breeding molt,” which results in a “breeding plumage.” An American using the Humprhey-Parkes system would deem the same transition a “prealternate molt,” yielding the tanagers’ “alternate” plumage. (Further complicating the picture, many bird banders around the world, including in North America, use a separate, intimidatingly complex system to describe age based on molt and the calendar year in which a bird hatched.)

At first glance, the main difference between the competing terminologies seems to be that the European strategy, sometimes called the “life-cycle approach,” is intuitive and easily mapped onto a bird’s life, while the Humphrey-Parkes (H-P) system is bewilderingly abstract: A freshly fledged bird is in its first basic plumage, sometimes quickly followed by a preformative molt, and perhaps later by a first prealternate molt—all within its first year of life. Even H-P proponents admit the terminology can be confounding, and years of conflated ideas and disagreements about how to apply the terms led to a significant revision to the system in 2003.

So why bother with a confusing, unintuitive way to talk about an already complicated process? Indeed, a 2022 paper implored the global bird community to get on the same page and adopt the “clearer and more understandable” life-cycle terminology, arguing that the simpler language could even enable recreational birders to understand molt and contribute to its study.

The life-cycle approach to molt, though, has a major problem. The system works well for the birds it was developed to describe: the small songbirds frequently banded and studied across Europe. By and large these birds replace all their feathers in the fall, after breeding (the “post-breeding” molt), and many molt again in the spring, some into flamboyant colors useful for attracting a mate (the “pre-breeding,” or, in older works, “pre-nuptial” molt). But things quickly get messy moving beyond northern songbirds. Raptors and large waterbirds don’t fit the pattern very neatly, and the system really starts to break down in the tropics, where birds’ lives are shaped by very different pressures and resources. Many tropical birds lack predictable breeding seasons and can reproduce throughout the calendar year, even interrupting their molt to do so. Some species undergo several molts in their first year, which the life-cycle approach struggles to classify. It’s not just a few outliers, either. Because of the overwhelming concentration of bird diversity in the tropics, the life-cycle system doesn’t work well for the majority of the world’s bird species.

“The terminology gets complex because the biology is complex, not just because we want to complicate our lives,” says Miguel Moreno Palacios, an ornithologist at Universidad de Ibagué in Colombia. As a young ornithologist, training with scientists from the United States and Europe, Moreno Palacios learned quickly that much of what he was taught from northern systems would need tweaking—or replacing. He’s not compelled by the argument that it’s easier to grasp a “simpler” way of talking about molt, even if it doesn’t hold up in large parts of the world. “That’s not a good reason,” he says. “That’s not how science works.”

__________

Because of the overwhelming concentration of bird diversity in the tropics, the life-cycle system doesn’t work well for the majority of the world’s bird species.

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Humphrey and Parkes attempted to head off these problems 65 years ago, with a fresh framework they hoped would “throw new light” on the study of molt, which they argued was being held back by the muddling of concepts. As one example they offered the Rock Ptarmigan, found in the arctic and alpine regions of several European countries (and in North America across the arctic tundra). In Scotland, Humphrey and Parkes reported, the bird breeds in its brown “summer plumage.” Meanwhile in Greenland the same species breeds with snowy white “winter” feathers. (And in Scandinavia ptarmigans breed in “particoloured, partimoulted dress.”) “Obviously,” the pair wrote, “in this species no one plumage may correctly be called a ‘nuptial’ plumage in the functional sense.”

Humphrey and Parkes had likely seen for themselves the shortcomings of older molt terminology, first in their work with museum specimens from the tropics, and later in the field. As curators of natural history collections, they were taxonomists used to thinking about lineage, and besides accounting for the world’s many “aberrant” birds, their proposal had a deeper mission: to map molt onto the evolution of birds as a class of animals. Considered this way, the H-P system is simpler than a “life-cycle” approach that needs myriad exceptions and qualifications to work for every bird around the world. Humphrey and Parkes instead looked to the “fundamental pattern” they saw across all species of birds and went from there. The different ways birds molt today, they asserted, can be seen as clues to the branches that have split and sprouted from the avian tree of life over millions of years.

More than half a century later, H-P boosters are still inspired by that vision. “I don’t think they even fully understood how great their system was,” says Peter Pyle, a biologist at the Institute for Bird Populations and the author of a pair of exceedingly detailed identification guides that opened the door to molt for many North American ornithologists, bird banders, and even enthusiastic birders (Pyle also helped developed universal age codes for bird banders, based on the H-P system).

Earlier this year Pyle and co-authors published a new paper that foregrounds and builds on Humphrey and Parkes’s evolutionary perspective. An illustration shows how (the authors hypothesize) different molt strategies evolved along different lineages of birds. Pyle hopes that homing in on molt as an evolved, and evolving, trait can help make the system feel more intuitive. The evolutionary lens also calls attention to thought-provoking details—like the increasingly clear evidence that molt is a carry-over from ancestral reptiles shedding their skin. Similarly, the appearance of extra molts in many distantly related groups of birds suggests the strategy evolved several different times in avian history. These molts, called prealternate in the H-P system, are the ones that some birds now take as an opportunity to don more brilliant plumes, useful for attracting a mate. Since so many of these more-frequent molters are long-distance migrants or live in harsh environments, lifestyles that are hard on feathers, it seems likely that this strategy evolved first and foremost as a needed rejuvenation. Flashy courting outfits are just a bonus—dearly appreciated by birders.

If your head is spinning from the dueling molt terminology, there’s no need for distress, says Johnson. Instead of worrying too much about which system to use, he recommends just tuning in to molt itself. When and where do your local birds molt? Can you see it in action? Learning about molt, he says, will make you a better birder and add to your appreciation of the birds around you and the challenges they face. But if you want to go deeper on the subject, definitely stick with Humphrey-Parkes, Johnson says, because you can use it anywhere in the world: “It’s a system that is inherently global." 

Given the life-cycle approach’s lapses in the tropics and for many groups of larger birds, it’s unclear why the entire field of ornithology hasn't embraced the H-P system—though Pyle does have a somewhat less-than-scientific theory: “Europeans hate it when Americans come up with something better than they have.”

This year’s paper was Pyle’s latest attempt to persuade his colleagues across the Atlantic to “see the light,” he says. “We’ll see if it works.”