In birds, moulting or molting is the routine shedding of old feathers. Also, the shedding of old skin in reptiles.

In arthropods, such as insects, arachnids and crustaceans, molting describes the shedding of the exoskeleton, typically to allow for further growth. See ecdysis.

Moulting in birds

Moulting in birds is a comparatively slow process, as a bird never sheds all its feathers at once; it must keep enough of its feathers to regulate its body temperature and repel moisture. Some species of wild bird become flightless during an annual "wing moult" and must seek protected habitat with a reliable food supply during that time. A moulting bird should never have any bald spots. If a pet bird has any bald spots, the bird should be brought to an avian veterinarian to search for possible causes for the baldness, which may include giardia, mites, or feather-plucking.

The process of moulting

1. The bird begins to shed some old feathers

2. Pin feathers grow in to replace the old feathers

3. As the pin feathers become full feathers, other feathers are shed.

This is a cyclical process that happens in many phases. In general, a moult begins at a bird's head, progresses down the body to its wings and torso, and finishes with the tail feathers.

Moulting in Arachnids

The exoskeleton of an arachnid is hard. This does not allow for growth. To overcome this problem, spiders will shed their old exoskeletons, allowing a new and larger one to replace it. The new larger exoskeleton fits underneath the old one, because it is still elastic. A layer of fluid separates the new and old skeletons. As the old skeleton is shed, the new one will expand to its full size, before hardening out on contact with air. The arachnid will have to flex its new exoskeleton, or it will become inflexible and less mobile until their next moult. Other reasons for moulting are damaged tissue and missing limbs. Over a series of moults, a missing limb can be regenerated, the stump being a little larger with each mould until it is of normal size again. After moulting, the exoskeleton is still delicate and the arachnid is vulnerable to predators or even its prey.

The Process of Moulting with Spiders, especially Tarantulas

Before moulting, some spiders will not eat for weeks, while others will eat up to the moment of moulting. Likewise, after a moult, some spiders will not eat for days or even weeks.

• The moulting commences, when a split appears on the side of a carapace part of the Cephalathorax (head and upper body)
• The split continues horizontally along the Opisthosoma (abdomen) and opens like a box
• The spider pushes its body out of the old skin, after which it withdraws its leg from it
• The spider will rest for a period of time, varying from minutes for spiderlings, to more than a day for a large mature Tarantula

Moulting in Reptiles

The most familiar example of moulting in reptiles is when snakes "shed their skin".

Moulting in Insects

Insects need to molt as they grow because their hard exoskeletons do not allow much room for expansion. Each stage in the development of an insect between moults is called an instar.

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