On this episode I talk about molt, the process by which birds replace their feathers each year. Replacing feathers is mandatory for all birds, and different species have different strategies for accomplishing this essential annual task. All birds undergo a basic molt every year, where they replace all of their feathers. The plumage they attain after this molt is called their “Basic Plumage.” In the most commonly used terminology, the Modified Humphrey-Parkes terminology, the molt by which feathers are changed is named for the plumage attained, so the molt into basic plumage is called the “pre-basic molt.”

Some birds have an additional molt each year where they replace some of their feathers to achieve a different appearance. Many birders call this their “breeding plumage” and this plumage is often brighter and more showy than the basic plumage (but not always). This plumage is called the “alternate plumage” and the molt into alternate plumage is called the “pre-alternate molt.”
So birds either have “basic” or an “alternate” molt strategy.

All birds need to grow a first set of feathers that they need to be able to safely get out of the nest, and this is called the “juvenile plumage” so the process of growing these feathers is called the “pre-juvenile molt.” We usually don’t see this first molt except on web-cams or when watching a nest specifically, so it is not commonly thought about, but every bird has to go through a pre-juvenile molt to get their first set of feathers. In some species this first set of feathers, the “juvenile plumage” is retained for a full year or more until the first pre-basic molt into the first basic plumage at a year or more of age. Other birds undergo a special molt in their first year, usually shortly after fledging, into what is called a “formative plumage.” These species are said to have a complex molt strategy. Reasons for evolving this strategy are unclear, but probably allow a quick set of head and body feathers to be grown so that the birds can get out of the next faster than if they needed to grow a strong set of feathers that could last a full year. Then after fledging they replace some of most of these first set of feathers to look more like adults and to have feathers suited to last a full year. Maybe the initial juvenile feathers allow them to be obviously different and easily recognized by their parents to be fed while very young. The real reason may vary and is not completely understood, but most North American songbirds use this complex molt strategy, and are said to have a complex molt strategy.
So now you know that birds can be split into 4 groups of molt strategies:

Simple Basic: these birds grow a set of juvenile feathers that they keep until their first basic molt more or less a year later, and then have one complete basic molt each year after.

Simple Complex: these birds have one additional molt in their first year, from juvenile into a special one-time formative plumage, and then just one pre-basic molt each following year where they replace all of their feathers.

Simple Alternate: These birds retain their juvenile feathers through their first year until a first pre-alternate molt some time later, and then have one full “pre-basic molt” every year into their “Basic plumage” and another “pre-alternate molt” each year into their “alternate plumage.”

Complex Alternate: These birds have a one-time “Pre-alternate molt” after fledging into a “Formative Plumage” and then follow the same molt sequence described in the simple alternate strategy above.

Examples of birds with each strategy are below:

Simple Basic Simple Alternate Complex Basic Complex Alternate
Albatrosses Large White-headed Gulls Spotted Towhee Most common strategy for
Barn Owls our North American passerines
Petrels

Using this concept of thinking about the plumages you see in birds can really help understand what you are seeing. If you see an essentially perfectly feathered Turkey Vulture soaring overhead in July or August you can deduce that it is this year’s first cycle bird, because essentially all of the adults will have some degree of flight feather molt going on. In the winter if you are in a place where Osprey are wintering, only the first cycle juvenile birds will have all of their wing flight feathers intact, whereas all of the adults will have some feathers being replaced.
You know that many of our migratory song birds have spring and early summer plumages that are quite different from how we see them in the fall. In fall they are in their basic plumage, all new feathers. In the spring they have undergone their alternate molt and have some head and body feathers that are replaced and their appearance is noticeably changed.
There is a lot more to know about molt, but here I just want to touch on the basics. For lots more see either Howell’s book or this article he wrote on molt.
Here is the ABCBirding post I put up after one of Ken’s classes.
As I mentioned in the podcast episode, I’ll be travelling for a few weeks, and may not get another episode published until I’m home again. Until then, good birding and good day!