Snow Buntings in winter (left) and summer (right) plumage. Photos from left: Kent McFarland/Flickr (CC BY-NC 2.0); Aaron Budgor/Flickr (CC BY-NC-ND 2.0)

In the Field

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 of a bird is its plumage. But as complex and capable as feathers are, they don't last forever: They begin to break down after some time, forcing its owner to replace them. As a result, a bird’s appearance may radically change through the molting period—or just look odd and patchy. That, in turn, adds another layer of difficulty to species IDs. 

My advice to birders who are agonizing over these transitions: Embrace the molt. It's a fascinating behavior, just like migration and breeding, and it's one of the vital parts of a bird’s life; plus, it can add some interesting context to your next birding outing.

Let's look at some of the fundamentals of identifying molts.

Are molts obvious?

Yes, though more so in larger species and in longer feathers on the wings and the tail. When birds are molting, you can usually spot the missing primary flight feathers by looking for a gap in the outline of the wing. The molt typically starts with the inner-most 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 contrast with the faded old ones.

When does molting occur?

Molting is energetically expensive—as is migration and breeding. So, birds make sure these three activities don't overlap. For many of our North American songbirds, that sweet spot in the calendar is July into early August. Townsend’s 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 southbound migrations. Other birds such as Gray Flycatchers fly down to their tropical grounds first to wrap up the process there. Tree Swallows, meanwhile, may begin the swap up north, pause for migration, and then complete it after arriving at their wintering destinations. Many species also have a partial molt in late winter or spring, replacing their 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 replace all their feathers once, and will often replace some twice. But the bigger the feather, the more taxing it is to replace it. That's why huge birds such as eagles and pelicans don't grow a new set of flight feathers each year; they restore individual primaries and secondaries on the wings in a staggered manner, taking multiple years to refresh.

What is “juvenile” plumage?

The juvenile stage refers to the period right after a fledgling leaves the nest. For some species it lasts just a few days; for others it can take up to a year. This is also the only time in a bird’s life when all of its feathers grow in at the same time, giving it a particularly uniform appearance. Take the young Western Sandpiper, 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 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 immature (the awkward stage between juvenile and adult) look so “patchy.” 

Are molts the only reason why a bird's plumage changes?

Wear is another way. In autumn, the gorgeous Snow Bunting molts into a buff and white plumage. But during the winter, those buffy tips rub away to 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. Wear is mechanical deterioration from flapping and environmental elements that causes the feather tips to fray. Generally, paler plumes 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 feather's structure. Lice and bacteria also contribute to feather loss.

How do I become a molt-ID expert?

After learning the basics here, the next step is to get familiar with the habits, schedules, and variations of specific groups of birds. The Peterson Reference Guide to Molt in North American Birds is a great all-encompassing resource and field guide. You can also practice at home by noting the different plumages in the birds that come 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. Common species 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. So, if you're willing to challenge yourself and dive right in, grab a hot chocolate (or another warming “beverage”), hit a nearby lake or fast-food parking lot, and start training.

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