By Bill Chaisson
The nesting season is winding down or is even over for some birds. I see ducks crossing the road that are like pint-sized versions of their mother. I hear and see house wrens that are following their parents around, cackling for food. I hear very little in the way of bird song on my post-dawn walks with the dog. This morning I didn’t hear a single warbler singing and only detected those never-stop species, the indigo bunting and red-eyed vireo, as I headed for the front porch.
The events of the breeding season—attracting a mate, building a nest, defending a territory, and feeding the next generation—require a lot of energy. Come the fall, migration will require a lot of energy for some species and building up a store of fat will require industry for others. In between these busy and demanding periods, birds molt their feathers and grow new ones, because this requires a lot of energy too.
Feathers are made of keratin, the same protein that constitutes mammalian hair, which is also molted periodically. Both birds and mammals evolved separately from reptile ancestors and feathers and hair represent integuments derived from scales. Snakes are the most dramatic example of the necessity of reptiles to molt, but they all do it, even turtles.
We don’t hear about intelligent design quite as much as we once did, but the molting of feathers, hair, and scales is another argument against it. Keratin, the material inherited from the reptiles as an outer layer, cannot repair itself and so must be discarded entirely and a new layer grown. The layers of tissue beneath the skin heal continuously and don’t need to be molted and replaced when they are damaged: they heal.
A more intelligent design would have been to deploy a system of integument that can heal itself without replacement, because that would demand less energy and resources. Instead the advantages of keratin, chiefly its impermeability, seems to have outweighed its downsides. When reptiles evolved from amphibians approximately 375 million years ago, they became primarily terrestrial animals by adding a layer of skin that prevented them from drying out. As this is a very important job and keratin does it admirably, the fact that it could not heal itself has been “dealt with” by the evolution of molting.
According to a page at allaboutbirds.org, there are three “systems” to describe molting in birds (one of which is too esoteric to go into). The first is the one known to birdwatchers: the molt from breeding into winter plumage and back again, which presumes two molts per year. The most obvious exceptions to this rule are among larger species like gulls and some raptors, which take several years to achieve their fully adult appearance.
The second system, the prevailing nomenclature among ornithologists, is called the Humphrey-Parkes system, which was introduced by the eponymous scientists in a 1959 paper (“An approach to the study of molts and plumages.” Auk 76:1–31). The objective approach of this system is initially confusing to North Americans: the winter plumage is called the “basic plumage” because the birds actually wear it for most of the year. The breeding plumage is called the “alternate plumage” by Humphrey and Parkes because it is worn for a relatively short period of the annual cycle.
Gulls provide an opportunity for the birdwatcher who wants to learn more about the Humphrey-Parkes system because first, there are so many gulls and they are so easily observed and second, their plumages are often clearly different from one stage to the next. To throw a curveball at you, there are “three-year” and “four-year” gull species with respect to their full molt cycle.
The herring gull (Larus argentatus) is the best place to start because of its ubiquity. It is a four-year gull, although in his 1934 guide Roger Tory Peterson shows you only the first and second winter plumages. David Allen Sibley shows you the third winter. The first molt is out of down and into juvenal plumage, which is sensibly called the “pre-juvenal molt.” (Note the spelling, which is specific to this context and everywhere else spelled “juvenile.”) Pre-juvenal molts occur in early summer.
Gulls begin replacing feathers again in September. These are referred to as “pre-basic molts” and are numbered by year. The first pre-basic plumage is worn from September to March, when yet another molt begins to take place and the gull undergoes its “first pre-alternate molt.” Here you can see Humphrey and Parkes reason for discarding “breeding plumage” from their system: many bird species don’t breed in their first post-hatching year of life (nor, for some, in subsequent years).
Molts are not instantaneous, but take place over weeks. The bird must eat in order to marshal the resources to make the keratin needed for all those feathers. Gulls begin their pre-basic molts throughout the late summer and fall (August-December) and begin their pre-alternate molts in the spring (March-May). Individual birds begin their molts at varying times during a season.
Allaboutbirds.org summarizes molt cycles with three general categories. Chickadees, flycatchers, hawks, hummingbirds, jays, owls, swallows, thrushes, vireos, and woodpeckers undergo one molt per year. Which is to say, they have not “alternate” plumage.
Several species undergo one full turnover after breeding, a pre-basic molt. They then experience a partial “pre-alternate molt” at varying times before the breeding season. This pattern is shared among Neotropical species—buntings, tanagers, and warblers—that assume their brighter breeding plumage (in most cases) before migration. The females molt too, even if their alternate coloration is unchanged.
Many open-country species undergo two complete molts per year because their feathers are more heavily damaged in the silicon-laden grassy environments where they live. Some, like the red-wing blackbird, look the same afterward. Others, like the male bobolink, are utterly transformed.
Bill Chaisson has been a birdwatcher from the age of 10. He is a former managing editor of the Eagle Times and now lives and works in the town of Wilmot. Contact him at firstname.lastname@example.org.