Molting, the replacement of all or some of a bird’s feathers, occurs in response to a mixture of hormonal changes brought about by seasonal changes. This process serves to replace worn feathers (they cannot repair themselves) and can play a part in seasonal camouflage as well as attracting a mate.
All of our small songbirds have a complete molt, replacing all of their feathers in late summer. In addition, many species have a partial molt (replacing body feathers but not wing or tail feathers) in the spring.
According to David Sibley, American Goldfinches begin to molt all of their (alternate/breeding plumage) feathers in September, with the males replacing their brilliant gold feathers with much duller feathers by November. Come spring and the breeding season, male goldfinches replace their dull (basic/non-breeding) body feathers (but not the wing or tail feathers) with new, bright feathers.
Imagine my surprise when I spied a brilliantly colored American Goldfinch at my feeder this week. According to ornithologist George Clark, it’s usually March before one starts to see an American Goldfinch in breeding plumage. One can only wonder what prevented this individual from molting its breeding plumage in the fall. (Photo: male American Goldfinch, winter plumage; inset – male American Goldfinch in breeding plumage in January)
With birds, molting refers to the loss of old, worn feathers and the growth of new ones. A molt can involve all of the bird’s feathers (complete molt), or just some of them (such as wing or tail feathers – a partial molt). Most birds have a complete molt once a year (chickadees, hummingbirds, owls, etc.), or one complete molt and a partial molt before the breeding season (buntings, tanagers, warblers, etc.), or two complete molts per year (Bobolinks, Marsh Wrens).
Complete molts often occur in late summer and early fall, after the breeding season is over. When you think about it, the timing of this “prebasic” or “postnuptial” molt makes a great deal of sense. Growing new feathers takes an inordinate amount of energy; food is plentiful now, the demands of breeding are over and for many birds, migration isn’t quite under way. It is the perfect time to look for molted feathers on the ground. (Photo: molted Red-tailed Hawk tail feather)
The larval stage of a butterfly or moth is spent doing little but eating. Only as a caterpillar will these insects have chewing mouthparts, and they waste no time in using them. As they eat, caterpillars increase in size and their skin (exoskeleton) becomes tighter and tighter, as it doesn’t grow larger. The caterpillar grows a new, larger exoskeleton underneath the outer skin and then sheds, or molts, the old one. Most caterpillars molt five times. At first, the new exoskeleton is very soft and not very protective, but it soon hardens. The shed exoskeleton is often eaten before the caterpillar ingests more plant food.
There are names for the caterpillar’s stage of development in between each molt, called “instars.” When the caterpillar hatches from its egg, it is referred to as a “first instar” caterpillar. After its first molt, the caterpillar is referred to as a “second instar,” and so on up until the exoskeleton is shed for the final time, revealing the chrysalis (if it’s a butterfly).
The Monarch in the photograph is a very new 4th instar instar caterpillar (see antennae which haven’t hardened). It has shed three times. Its third exoskeleton (which it has just shed) is on the milkweed leaf behind the caterpillar. To see a real-time video of a Monarch molting go to https://www.youtube.com/watch?v=EbHyq3RwtxI.
(Thanks to Otis Brown for his keen eye in finding this Monarch caterpillar before it ate its just-molted skin. Also to Ba Rea ((www.basrelief.org) for her instar confirmation.)
In much of New England Green-winged Teals, our smallest dabbling (feed mainly on the surface rather than diving) ducks, are present either during the breeding season, or over the winter, but most of Vermont and New Hampshire have to settle for glimpses during spring and fall migration. Often these ducks stop to rest and refuel on shallow ponds, marshes and flooded fields. While occasionally one or two are spotted amongst a group of mallards, typically they are found in small groups that often congregate in large flocks.
The pictured male is soon to complete its fall “prealternate molt,” in which all body feathers except the innermost feathers of wing are replaced. When finished, he will sport brilliant cinnamon and green head feathers as part of his alternate, or breeding, plumage. Next July he will molt these feathers and acquire a duller basic, or non-breeding, plumage. (Photo by Jeannie Killam)
Like other arthropods, spiders have a protective hard exoskeleton that is flexible enough for movement, but can’t expand like human skin. Thus, they have to shed, or molt, this exoskeleton periodically throughout their lives as they grow, and replace it with a new, larger exoskeleton. Molting occurs frequently when a spider is young, and some spiders may continue to molt throughout their life.
At the appropriate time, hormones tell the spider’s body to absorb some of the lower cuticle layer in the exoskeleton and begin secreting cuticle material to form the new exoskeleton. During the time that leads up to the molt (pre-molt period), a new, slightly larger, inner exoskeleton develops and is folded up under the existing exoskeleton. This new soft exoskeleton is separated from the existing one by a thin layer called the endocuticle. During the pre-molt period the spider secretes fluid that contains digestive enzymes between the new inner and old outer exoskeletons. This fluid digests the endocuticle that separates the two exoskeletons, making it easier for them to separate.
Once the endocuticle is completely digested the spider is ready to complete the molt. At this point a spider pumps hemolymph (spider blood) from its abdomen into its cephalothorax in order to split its carapace, or headpiece, open. The spider then slowly pulls itself out of the old exoskeleton through this opening.
Typically, the spider does most of its growing immediately after losing the old exoskeleton, while the new exoskeleton is highly flexible. The new exoskeleton is very soft, and until it hardens, the spider is particularly vulnerable to attack.