When Do Mallards Molt and Lose Their Feathers?

Mallards go through two major molts each year, but the most dramatic one happens in summer, when they shed and regrow all their flight feathers at once and become temporarily flightless. For females, this wing molt typically begins in late August, while males enter it a few weeks earlier, usually starting in June or July. The whole process is more complex than it first appears, involving not just the conspicuous summer wing molt but also a separate body-feather molt in autumn and winter that restores the drake’s iconic green-headed breeding plumage.

Two Molts, Not One

Mallards, like most ducks, cycle through two distinct molts each year. The first and most extensive is the prebasic molt, which happens after the breeding season and involves the replacement of body feathers and, critically, all the flight feathers in the wings. The second is the prealternate molt, a partial body-feather molt that takes place in autumn and early winter. Each molt serves a different function, and they happen months apart.

The prebasic molt is the one that gets the most attention because it leaves the bird unable to fly. During this molt, mallards shed all their primary and secondary wing feathers nearly simultaneously. New feathers grow in over the following weeks, but until they are fully developed, the bird is grounded. The prealternate molt, by contrast, is far less disruptive. It replaces body and head feathers but leaves the flight feathers untouched, so the bird can fly throughout the process. Both the prebasic and prealternate body molts tend to be more variable in timing and extent than older field guides sometimes suggest.

Eclipse Plumage and the Summer Wing Molt

If you have ever looked at a pond in July and struggled to find the green-headed drakes you saw in spring, eclipse plumage is the reason. Male mallards begin their prebasic molt shortly after the breeding season winds down. As they shed their bright breeding feathers, they replace them with dull, mottled brown plumage that closely resembles a female’s. This cryptic “eclipse” plumage is the bird’s look during its most vulnerable period, the weeks when it cannot fly.

The eclipse plumage of male mallards normally appears in June and July. Classic experiments showed it could be triggered earlier, in February and March, by exposing captive drakes to extended artificial daylight, which confirmed that photoperiod, the length of the day, is a key driver of molt timing.

Interestingly, those same experiments found that castrating male mallards did not prevent eclipse plumage from appearing in the first year but did prevent it in the second year. That result pointed researchers away from the idea that testicular hormones directly cause eclipse and toward a more complex endocrine picture involving multiple glands responding to seasonal light changes.

Females undergo the same wing molt, but their timing is different. Hens typically delay their prebasic molt until after their broods are independent or, in the case of failed nesters, until later in the summer. A study tracking female mallards in the western United States found that their wing molt began on average around August 26, with substantial individual variation of roughly three weeks in either direction, and ended around October 5. The average duration of wing molt for those females was about 41 days.

Why Feathers Must Be Replaced

Feathers are dead structures made of keratin. Unlike hair or claws, which grow continuously from their base, a feather stops growing once it reaches full length. After that, it can only deteriorate. Sun exposure, physical friction against vegetation and water, bacterial action, and ectoparasites all break down feather structure over time. A flight feather that was crisp and aerodynamically sound in October is noticeably worn by the following June. Because feathers cannot repair themselves, the only option is to shed the old one and grow a completely new feather from the follicle.

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This is why molt is considered one of the most important events in a bird’s annual cycle. For mallards and other waterfowl that depend on strong, intact flight feathers to migrate hundreds or thousands of miles, flying on badly worn feathers is not a safe option. The trade-off is stark: spend a few weeks unable to fly while new feathers grow, or try to migrate and evade predators on degraded equipment. Evolution has consistently favored the former.

What Happens During the Flightless Period

The flightless window for mallards lasts roughly three to five weeks, depending on the individual. During this time, the bird is stuck on the water or at the water’s edge. That vulnerability shapes nearly every aspect of behavior during molt.

One well-documented change is in body condition. Research on penned wild-strain mallards found that body condition, particularly lipid (fat) reserves, declined during the flightless period. But the researchers concluded this was probably more of a deliberate strategy than a sign of distress. Shedding body mass helps the bird recover its ability to fly more quickly once the new flight feathers are in, because a lighter bird needs less lift. Notably, the quality of food available during the flightless period did not affect how fast the primary feathers grew, which suggests the growth rate is internally regulated rather than driven purely by nutrition.

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Related waterfowl studied during their flightless molt shift their habitat preferences in telling ways. Dabbling ducks that are normally comfortable in open water move toward areas with dense shoreline vegetation, and they become much more sensitive to human disturbance. For species closely related to mallards, like gadwalls, habitat quality became more important than food supply during the flightless phase, reversing the normal priority.

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Mallards follow similar patterns. Molting drakes tend to gather in secluded wetland areas, sometimes undertaking “molt migrations” to remote or protected sites well before they actually become flightless. Females, as mentioned, often delay their wing molt and may move to different wetland areas after their broods disperse.

The Autumn and Winter Molt That Restores Breeding Plumage

After the prebasic molt replaces their flight feathers and puts them in drab eclipse plumage, male mallards begin a second, partial molt: the prealternate molt. This is the molt that brings back the glossy green head, chestnut breast, and white neck ring that most people associate with a drake mallard. It happens gradually through the fall.

A study of wintering dabbling ducks found that most male mallards, about 90%, had completed their alternate (breeding) plumage by mid-November. The timing of this plumage change lined up closely with pair formation, which makes biological sense: males need to look their best when competing for mates.

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Females go through the prealternate molt as well, but the visual change is far subtler because their plumage is cryptically patterned year-round. Data from females arriving at wintering grounds showed that those in mid-prealternate molt had relatively low body mass, averaging around 1,010 grams with only about 65 grams of lipid reserves. After completing the molt and pairing up, females gained considerable weight. By the time they initiated the next prebasic molt the following summer, they had built up to around 1,118 grams of body mass and 168 grams of fat.

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That fat accumulation matters. Females are essentially fueling up for nesting, and the timing of their body-condition cycle is tightly linked to where they are in the molt sequence. Paired females carried more fat than unpaired females throughout fall and winter, suggesting that being in good condition both helps attract a mate and reflects having finished the energetically costly molt.

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What Controls the Timing

Daylight length is the master switch for molt in most temperate-zone birds, and mallards are no exception. As days lengthen in spring, hormonal cascades triggered by light exposure ramp up reproductive activity. As the breeding season ends and days begin to shorten, those same hormonal systems shift the bird into molt mode. The artificial-light experiments on eclipse plumage mentioned earlier demonstrated this connection directly for mallards.

Thyroid hormones play a central role in the process. Research on a related passerine species showed that suppressing thyroid hormones completely prevented the pre-nuptial (prealternate) molt, while restoring thyroid hormone levels with either T3 or T4 brought molt back to normal.

⁹ While that particular study used white-crowned sparrows rather than mallards, the thyroid-molt link is broadly conserved across bird species, and it helps explain why molt timing tracks so reliably with seasonal light changes across different populations.

Sex hormones interact with the system too, but in less straightforward ways. The castration experiments on mallards showed that removing the testes altered eclipse plumage eventually, but not immediately. The current understanding is that no single hormone flips a “molt switch.” Instead, the interplay between thyroid hormones, sex steroids, and prolactin creates the seasonal rhythm, with daylight length setting the overall pace.

Geographic Variation in Molt Timing

Not all mallard populations molt on the same calendar. The late-August average start date for female wing molt mentioned earlier came from populations in California, and even within that state, there were differences between birds in the Suisun Marsh, the Central Valley, and the southern Oregon/northeastern California (SONEC) region.

In more northern regions, where the breeding season starts and ends later, molt timing shifts accordingly. Mallards breeding in the upper Midwest or Canada may not begin their wing molt until September. Conversely, resident populations in milder climates, or birds whose nests failed early in the season, may start molting earlier. The three-week spread around average start dates documented in California gives a sense of just how much individual variation exists even within one geographic area.

This variability has practical consequences. Hunters, wildlife managers, and birders who want to know when a local mallard population is flightless need to calibrate their expectations to the region and to local breeding outcomes that year, not to a single textbook date.

Why Mallards Drop All Flight Feathers at Once

Most songbirds replace their flight feathers a few at a time, maintaining the ability to fly throughout the process. Mallards and other waterfowl take the opposite approach, shedding all their primaries and secondaries nearly simultaneously. This strategy, called simultaneous wing molt, is unusual among birds and comes with the obvious cost of temporary flightlessness.

The advantage is speed. By growing all flight feathers at the same time rather than sequentially, mallards get through the entire wing molt in roughly five to six weeks instead of the months it would take to replace feathers one or two at a time. For a bird that needs to be migration-ready by autumn, compressing the molt window into midsummer is worth the risk of a few flightless weeks on a protected wetland.

Evolutionary analysis has found that bird lineages with simultaneous wing molt have a higher rate of evolving permanent flightlessness compared to lineages that molt their wing feathers sequentially.

¹⁰ The reasoning is that species already spending part of the year flightless have less evolutionary “distance” to cover on the path to giving up flight entirely. Mallards themselves show no signs of heading that direction, but the pattern is a reminder that their dramatic molt strategy sits at one end of a continuum with real evolutionary consequences.

Molt as a Detox Pathway

A less obvious function of molt has to do with contaminants. Mercury, which accumulates in birds through their diet, binds tightly to the keratin in growing feathers. When a bird grows a new set of feathers during molt, a substantial fraction of its mercury burden gets locked into those feathers. Once the feathers are fully grown and metabolically dead, the mercury is effectively sequestered. Over time, as feathers are shed and replaced, the bird rids itself of accumulated mercury.

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For mallards living in wetlands where mercury contamination is a concern, which includes many agricultural and industrial waterways, molt acts as a regular detoxification cycle. This is also why researchers collect feathers to assess mercury exposure in wild bird populations. The feather essentially records a snapshot of the bird’s contaminant load during the period when it was growing. A bird that molts more feathers at once, as mallards do during their simultaneous wing molt, flushes out more mercury in a single pulse than a bird replacing feathers gradually.

Why Molting Sites Need Protection

Because flightless mallards are confined to whatever wetland they chose before losing their flight feathers, the quality and security of molting habitat matters enormously. A bird stuck for five weeks on a pond that dries up, gets heavily disturbed by boats or foot traffic, or lacks adequate food is in serious trouble.

Researchers studying postbreeding movements of female mallards and gadwalls in California emphasized that conservation and active management of the wetland areas most used during molt could directly improve postbreeding survival, which in turn supports larger breeding populations the following year.

³ The challenge is that many important molting wetlands in the western United States face competing demands from agriculture, development, and water allocation, and the birds’ needs are most acute in July through September, precisely when water resources are most strained.

For backyard pond owners or park managers who notice mallards gathering in midsummer, the practical takeaway is straightforward: those birds may be about to enter or already be in their flightless period. Minimizing disturbance, keeping dogs away from the water’s edge, and maintaining vegetation cover along shorelines can make a real difference during the few weeks when the birds have no option to leave.

1
BioOne Complete / Waterbirds. Molts and Plumages of Ducks (Anatinae)
2
Journal of Experimental Biology. On the Eclipse Plumage of the Mallard (Anas Platyrhyncha Platyrhyncha)
3
The Journal of Wildlife Management. Postbreeding movements and molting ecology of female gadwalls and mallards
4
PubMed Central. The effects of annual cycle stages and life-history traits on feather abrasion
5
Oxford Academic (The Condor: Ornithological Applications). Effects of Body Condition on Molting in Mallards
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Ibis. Species‐specific habitat use of wing‐moulting waterbirds in response to temporary flightlessness
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The Canadian Field-Naturalist. Timing of pair formation and male acquisition of alternate plumage by three wintering dabbling ducks
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The Condor: Ornithological Applications. Body Composition of Female Mallards in Winter in Relation to Annual Cycle Events
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PubMed Central. Effects of thyroid hormone manipulation on pre-nuptial molt, luteinizing hormone and testicular growth in male white-crowned sparrows (Zonotrichia leuchophrys gambelii)
10
The American Naturalist (University of Chicago Press). Simultaneous Wing Molt as a Catalyst for the Evolution of Flightlessness in Birds
11
Environmental Toxicology and Chemistry. Rapid depuration of mercury in songbirds accelerated by feather molt