Understanding the Basics of Bird Molts
As the seasons change, so do birds' feathers. A birder should be prepared.
One of the most defining featuresof a birdis itsplumage. But as complex and capable as feathersare, they don't last forever: They begin to break down after some time, forcing its owner to replace them. As a result, abird’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 tobirders who are agonizing over these transitions: Embrace the molt. It's a fascinating behavior, just like migration and breeding, and it'sone of the vital parts of a bird’s life; plus, it canadd some interesting context to your next birding outing.
Let's look at someof the fundamentalsof identifying molts.
Are molts obvious?
Yes, though more so in larger speciesand in longerfeathers on the wings and the tail. When birds are molting, you can usually spotthemissing by lookingfor a gap in the outline of the wing. The molt typicallystarts with the inner-mostprimary 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 theycontrast with the faded old ones.
When does molting occur?
Molting is energetically expensive—as is migration and breeding. So, birds make sure these threeactivities don't overlap.For many of our North Americansongbirds, that sweet spot in the calendar isJuly 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 theirsouthbound migrations. Other birdssuch as Gray Flycatchers fly down to their tropical grounds first to wrap upthe process there. Tree Swallows, meanwhile, may begin the swap up north, pause formigration, and then complete itafter arriving at their wintering destinations.Many species also have a partial molt in late winter or spring, replacing theirhead 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 speciesreplace all their feathers once, and will often replace some twice. But the bigger the feather, the moretaxingit is to replace it. That's why huge birds such as eagles and pelicans don'tgrow a new set of flight featherseachyear; they restoreindividual primaries and secondaries on the wings ina staggered manner, taking multiple years to refresh.
What is“juvenile” plumage?
The juvenile stagerefers to the period right after a fledglingleaves thenest.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. Takethe , 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, itonly changes feathers sequentially, which means it always has something old and something new on its body. That's why late-winter gulls that areimmature (the awkward stage between juvenile and adult) look so“patchy.”
Are molts the only reason why abird's plumage changes?
Wear is another way. In autumn, the gorgeousSnow Bunting molts into abuffand whiteplumage. But duringthe winter, those buffy tips rub away toreveal 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 ismechanical deterioration fromflappingand environmental elementsthat causes the feather tips to fray. Generally, paler plumeswear 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 afeather's structure. Lice and bacteria also contributeto feather loss.
How do I become a molt-ID expert?
After learning the basics here, thenext step is to get familiar withthe habits, schedules, and variations of specificgroups of birds.is a great all-encompassing resource and field guide. You can also practice at home by noting the different plumages in thebirdsthat cometo 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 speciesthat 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 nearbylake or fast-food parking lot, and start training.
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