What Is Moulting? Replacing a Covering That Cannot Be Repaired
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Feathers, fur, skin and exoskeletons all wear out and most cannot be mended, so animals replace them periodically. That replacement is expensive, leaves the animal vulnerable while it happens, and has to be fitted into a year that already contains breeding and migration.
Why replacement rather than repair
The tissues concerned are dead once formed. A feather is produced by a follicle and is thereafter a structure of keratin with no blood supply and no cells, so damage accumulates and cannot heal, and the same applies to hair and to the outer layers of skin. An arthropod exoskeleton is rigid and cannot expand, so growth itself requires shedding it. In each case the only route to a functioning covering is to grow a new one and discard the old. The costs are substantial. Making new feathers requires a large quantity of protein, and birds in moult show measurably elevated energy requirements and frequently lose condition. The process takes time, during which the covering is incomplete and insulation, waterproofing and flight are all compromised. And the animal is more vulnerable, which is why many species become secretive during moult and why several ducks and geese hide in dense cover while flightless.
The strategies
Species schedule the process differently and the variation reflects what they can afford to lose:
- •Sequential moult in most birds, replacing flight feathers a few at a time in a set order so that flight is retained throughout, which takes weeks or months
- •Simultaneous moult in ducks, geese, swans and several others, which drops all flight feathers at once and leaves the bird flightless for a period, and which is only viable where a safe refuge exists
- •Partial moults, replacing body feathers only, which many species perform additionally before breeding to acquire display plumage
- •Arrested and suspended moults, where the process pauses for migration and resumes afterwards
- •Ecdysis in arthropods, where the whole exoskeleton is shed and the animal is soft and defenceless until the new one hardens, which is when most predation on crabs and insects occurs
- •Continuous replacement in mammals, where hair is shed and regrown on individual cycles, producing seasonal coats rather than a discrete event
Fitting it into the year
The annual cycle of a migratory bird contains breeding, migration and moult, each demanding and none compressible indefinitely, and how a species arranges them is a substantial part of its ecology. Some moult on the breeding grounds before departing. Some migrate first and moult on the wintering grounds where food is abundant. Some perform a partial moult at each end. Some stop at a staging area specifically to moult, which makes those sites critical habitat that is easily overlooked in conservation planning because the birds are present only briefly and are inconspicuous while there. Conflicts between the demands are resolved differently by different species and the resolution constrains everything else, including how late a failed breeding attempt can be replaced. Climate change has shifted the timing of food availability in several systems, and mismatches between moult timing and resources are among the documented consequences.
What a moulted covering records
Discarded material is informative and is used as a non-invasive sample. Feathers record the isotopic composition of the food eaten where and when they were grown, so a feather collected in winter reports on the summer grounds, which is used to determine where migratory populations breed without catching anything. They record contaminant exposure including heavy metals. They carry DNA sufficient for individual and species identification, which is used in monitoring and in enforcement against wildlife crime. Shed arthropod exoskeletons record size and can be counted to estimate population. Moult patterns themselves age a bird, since the sequence and extent of feather replacement differs between age classes and is the basis of ageing techniques used by ringers. And the state of a covering indicates condition, since interrupted moult leaves growth bars in feathers that record days of poor nutrition, which is a record of stress written directly into the structure.
The takeaway
Feathers, hair and exoskeletons are dead once formed and cannot heal, so the only option is replacement, which costs protein, takes weeks and leaves the animal vulnerable. Most birds replace flight feathers gradually to stay airborne, while ducks and geese drop them all at once and hide. A discarded feather records where its owner was feeding when it grew, which locates migratory populations without catching anything.