Molt is a critical element of the lifecycle of birds, insects, spiders, mammals, and more! Learn the how, when, why, and where of molt.
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What is molt?
Molt, in birds, is the growth of new feathers, likely as a response to environmental and social pressures. It is such a critical element of the avian annual cycle that while not all birds may breed or migrate, essentially all birds will undergo a molt each 'bird year.' Each major molt, or new feather growth, creates a new plumage for birds. Plumage is the term used to describe a bird's entire feather coat or assemblage.
Some birds will undergo feather replacements in response to injury, accidental feather loss, disease, etc. These molts are usually restricted to the affected feather tracts only and are not classified the same as the life cycle molts discussed here. For example, Gray Catbirds can lose their tails while escaping predators.
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Why do birds molt?
Birds molt in as a response to the environmental wear of feathers. Sun, wind, rain, parasites, etc all wear feathers to a state that decreases their functionality. In North America, many species of birds will undergo two molts each annual cycle. One of these molts results in a plumage referred to as a non-breeding or basic plumage. The second molt produces a plumage frequently coined as the breeding or alternate plumage. The breeding plumage has traditionally been considered solely for its role in social aspects of bird life, such as mate selection; new research provides an argument that the plumage seen in spring and summer likely still evolved first as a response to environmental wear on feathers.
When do birds molt?
The timing in bird molt varies across species, latitudes, longitudes, and ages. However, typically all altricial birds will undergo a molt in the nest. Altricial birds are those born in a less advanced state (naked, unable to walk or fly). In many species, we see a second molt shortly after leaving the nest. This can begin weeks to months after departing the nest. A bird may then go a full year (if they have no breeding molt) before undergoing what is defined as the definitive molt, where they have the appearance of an adult bird. This molt will likely occur in late summer or early fall for most North American species.
The table below offers four examples of North American songbirds and the timing of each of their molts. Please note each molt does not take the entire period listed. The range is simply offering a timetable for when each of these molts may occur for each species of bird.
Prejuvenal Molt (baby)
Preformative Molt (teen)
Prealternate Molt (breeding)
Definitive Molt (adult)
*The Northern Cardinal has an additional molt between its molt in the nest and its teenager molt. This molt is called the presupplemental molt, and cardinals will begin to take on a more adult-like appearance after completing this molt and the preformative (teenager) molt.
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Where do birds molt?
For baby birds, their first molt occurs in the nest. For formative-aged birds, their 'teenage' molt often occurs after they have departed the care and protection of their parent birds. It may occur near their nesting grounds, on their migration route, or while on their wintering grounds. As for the adult, or definitive molt, it often occurs after birds have finished raising chicks or defending territory. Some birds will disperse away from nesting grounds, then molt, while others may molt on migratory or wintering grounds. It is so dependent on species and location, that a single answer cannot cover all species of birds.
When do chickens and domestic birds molt?
Molt in the wild ancestors (Red Junglefowl) of domestic chickens would have likely occurred after the nesting season, resulting in non-breeding plumage from June to September. This particular plumage is referred to as the eclipse plumage in waterfowl and gallinaceous birds or game birds such as chickens. However, the domestication of chickens has led to this plumage not occurring as it did in wild populations, so domestic fowl no longer exhibit this plumage. Instead, they are likely to begin full feather replacement after the end of the egg-laying season, whatever months those are, depending upon your location.
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Do insects, spiders, and other arthropods molt?
Yes! Arthropods can molt their exoskeleton! However, while molting in birds is the growth of new feathers, molting in insects and other arthropods is the shedding of the exoskeleton as the animal grows. There are far too many examples of arthropods molting, but when they do molt, the new exoskeleton underneath is soft and often discolored, like the boxelder bug below.
Do mammals molt?
In mammals, molt is the seasonal replacement of hair or pelage. In mammals, pelage cycles, or the molt of hair and fur, are typically triggered by the photoperiod. The photoperiod is the amount of time each day that an organism receives light. So, the change in the length of daylight likely triggers the hormonal response in mammals to begin molting. For example, a long-distance migrant, like the hoary bat, molt their pelage before the autumn migration. However, in this mammal (Zach), the pelage is being molted as its age progresses. Yes, I'm making a hair-loss joke. (Please, let Hims or Keeps work.)
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