Understanding the Basics of Bird Molts
As the seasons change, so do birds' feathers. A birder should be prepared.
One of the most defining features听of a bird听is its听plumage. But as complex and capable as feathers听are, they don't last forever: They begin to break down after some time, forcing its owner to replace them. As a result, a听bird鈥檚 appearance may radically change through the molting period鈥攐r just look odd and patchy. That, in turn,听adds another layer of difficulty to species IDs.听
My advice to听birders who are agonizing over these transitions: Embrace the molt. It's a fascinating behavior, just like migration and breeding, and it's听one of the vital parts of a bird鈥檚 life; plus, it can听add some interesting context to your next birding outing.
Let's look at some听of the fundamentals听of identifying molts.
Are molts obvious?
Yes, though more so in larger species听and in longer听feathers on the wings and the tail. When birds are molting, you can usually spot听the听missing 听by looking听for a gap in the outline of the wing. The molt typically听starts with the inner-most听primary feather and works out; similarly, the central tail feathers tend to be shed first. At the same time, the plumage on the body is also being swapped. In black or darkly colored birds, the new feathers are noticable as they听contrast with the faded old ones.
When does molting occur?
Molting is energetically expensive鈥攁s is migration and breeding. So, birds make sure these three听activities don't overlap.听For many of our North American听songbirds, that sweet spot in the calendar is听July into early August. Townsend鈥檚 Warblers, for instance, go through a complete molt during this time, after they're done mating, nesting, and tending to their chicks, but before they embark on their听southbound migrations. Other birds听such as Gray Flycatchers fly down to their tropical grounds first to wrap up听the process there. Tree Swallows, meanwhile, may begin the swap up north, pause for听migration, and then complete it听after arriving at their wintering destinations.听Many species also have a partial molt in late winter or spring, replacing their听head and body feathers but not their flight feathers. That's how a male Scarlet Tanager can switch to olive-green for the winter and back to luminous red for spring and summer.
Do all birds molt once a year?
In general, smaller species听replace all their feathers once, and will often replace some twice. But the bigger the feather, the more听taxing听it is to replace it. That's why huge birds such as eagles and pelicans don't听grow a new set of flight feathers听each听year; they restore听individual primaries and secondaries on the wings in听a staggered manner, taking multiple years to refresh.
What is听鈥渏uvenile鈥 plumage?
The juvenile stage听refers to the period right after a fledgling听leaves the听nest.听For some species it lasts just a few days; for others it can take up to a year.听This is also the only time in a bird鈥檚 life when all of its feathers grow in at the same time, giving it a particularly uniform appearance. Take听the , for example: It's crisp and clean-looking, almost like a recently detailed and upholstered car.
After a bird goes through its first juvenile molt, it听only changes feathers sequentially, which means it always has something old and something new on its body. That's why late-winter gulls that are听immature (the awkward stage between juvenile and adult) look so听鈥減atchy.鈥澨
Are molts the only reason why a听bird's plumage changes?
Wear is another way. In autumn, the gorgeous听Snow Bunting molts into a听buff听and white听plumage. But during听the winter, those buffy tips rub away to听reveal the underlying black and white breeding suit of the male.
Are wear and fading the same thing?
There's a small, but important difference between the two terms.听Wear is听mechanical deterioration from听flapping听and environmental elements听that causes the feather tips to fray. Generally, paler plumes听wear more quickly because they lack melanin, a pigment that strengthens cells and protects them from damage. Fading, on the other hand, is a photochemical reaction, where UV radiation from the sun breaks down a听feather's structure. Lice and bacteria also contribute听to feather loss.
How do I become a molt-ID expert?
After learning the basics here, the听next step is to get familiar with听the habits, schedules, and variations of specific听groups of birds.听听is a great all-encompassing resource and field guide. You can also practice at home by noting the different plumages in the听birds听that come听to your feeder. Look for darker and fresher feathers and contrast them with the paler ones.
The best part is, because molting is so staple and universal, you don't need to seek out rare birds to study the results.听Common species听that change radically from one plumage to another include juvenile Brown-headed Cowbirds in late summer and American Goldfinches, which go from yellow to brown and back to yellow. Meanwhile, gulls are notorious for their diversity of molts and can prove tough to ID in summer and fall.听So, if you're willing to challenge yourself and dive right in, grab a hot chocolate (or another warming听鈥渂everage鈥),听hit a nearby听lake or fast-food parking lot, and start training.
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