A reader recently emailed me, asking about avian molting. She specifically was interested in the plumage differences she was observing in her late-summer scarlet tanagers. Photos were provided, which included young-of-the-year birds at her feeders that looked totally different from adult tanagers. In fact, the youngsters looked like an altogether different species!
Years ago, when I was conducting my graduate school research in North Dakota, I was surveying a grove of cottonwood trees and other assorted woody vegetation near one of my study wetlands. I observed a yellowish bird that had the body shape of an oriole, and so I watched it for a while through my binoculars. It took me some time to identify the bird because of its unusual plumage markings and color, but it turned out to be a juvenile male orchard oriole.
Many of you undoubtedly notice the annual mass migration of Canada geese flying north during late May and into June. For some observers, the phenomenon is a curious spectacle that’s misunderstood. Some people wonder why geese are migrating at such a seemingly oddball time. The answer is simple. They’re going to molt.
Known as the “molt migration,” non-breeding or otherwise unsuccessful breeding adult Canada geese migrate north in late spring to locate isolated water bodies that provide refuges and isolation for molting.
In the case of waterfowl like Canada geese, molting includes losing primary wing feathers that, without them, renders the birds flightless for a period of time before new feathers grow and replace the molted feathers.
Molting some or most of a particular bird’s feathers is as common as other animals that shed fur. Feathers, after all, are simply modified hair. The primary protein that makes up feathers and fur, including nails, claws, and talons and other structures, is keratin.
All of these structures, feathers included, wear out, get damaged, and need to be replaced. However, unlike fingernails for example, a damaged feather doesn’t heal or replace itself. A feather doesn’t grow continuously either. Feathers need to be replaced, hence, molting.
Molting is highly variable amongst both individual birds within a species and between species, but most birds fall into one of three basic categories. For instance, some birds, such as black-capped chickadees, ruby-throated hummingbirds, owls, and woodpeckers undergo one complete molt a year.
Other birds, such as tanagers and warblers, go through one complete molt and one partial molt per year. All of the feathers of these birds molt soon after the nesting season is completed.
Then, just prior to the following breeding season, both male and female birds in this group undergo a partial molt that gives the male of the species their brilliant breeding plumage.
The third type of molt pattern observed in birds are two complete molts each year. Birds falling into this group are typically birds that inhabit dense vegetation as a major component of their lifestyle. Birds such as red-winged blackbirds, bobolinks, and marsh wrens are a few species that experience substantial wear and tear on their feathers as a result of moving through vegetation. Thus, the need to replace worn feathers is greater than other species of birds that don’t damage their feathers to this extent.
A molting bird, or the molt patterns of birds, can also clue us in on the relative age of a bird. Think for example the bald eagle. Here’s a species of bird that you know about that takes five years before obtaining its distinctive white head and white tail feathers.
Between the ages of four and five, a mottled appearance becomes evident as the young bird begins to transition from a mostly dark brown body to the adult form.
Many species of gulls also undergo an annual series of “metamorphoses” until adulthood, as the birds’ appearance changes each year. And so, as is the case for herring gulls, an astute observer can tell if a bird is one, two, three, or four years old merely by what molt period the bird is currently in. At the age of four, most gulls have obtained their adult plumage.
There’s a lot more to a feather than one would think. Designed to protect birds from the elements, to enable most birds to fly, and to enhance their success to reproduce and raise offspring are the obvious elements, yet the manner in which birds molt these critical structures are fascinating to think about, too, as we get out and enjoy the great outdoors.