DIFFERENT MOLT STRATEGIES

Most birds molt their flight feathers in a fairly straightforward way, as explained in the last blog. The feathers on each wing molt sequentially, starting with the innermost primary and usually ending with the innermost secondary. This is called sequential wing molt.

But there are two other ways to do it.

Very large birds (above about 1 kilogram in weight) that need to be able to fly at all times, for example eagles, cormorants, and herons, can retain feathers for more than one year and molt only some of them each year. The largest among these take about three years to replace all the flight feathers. This molt strategy is called stepwise wing molt.

In many large birds, you can see the different feather generations in an outspread wing because the older feathers are more worn and thus a bit paler. These differences should be evident in this Turkey Vulture wing specimen from the Slater Museum collection.

The other strategy is simultaneous wing molt. If predation is the only reason they need to be able to fly, and birds can avoid predation, they can give up flight for a period of time and drop all their flight feathers simultaneously. This is the case in anhingas, ducks and geese, loons, grebes, and larger alcids, all of which can remain in the water during this period, out of range of their potential predators. Note this group includes the very large swans and geese, which perhaps could not fly very efficiently with gaps in their wings.

The female Mallard is typical of midsummer ducks with all flight feathers missing. The Common Murre also is in full wing molt, usually obvious because the wingtips can’t be seen above the tail.

This Anhinga has dropped all of its flight feathers and most of its tail feathers simultaneously and is in the process of growing them back.

Dennis Paulson

IT'S TIME TO MOLT

All birds have a complex coat of feathers, thousands of them, which they use for insulation, display, camouflage, and flight. Look at a feather closely and you’ll see that it looks flimsy, yet it does its job very well by meshing with others of its kind. Flight feathers can hold up birds such as Trumpeter Swans that weigh in excess of 30 pounds and provide thrust and lift for continuous flight in birds on migrations that extend thousands of miles.

But these feathers, while strong, are not indestructible. Inexorably, a feather wears over its lifetime, and if a bird grew only one coat of them, eventually wear would take its toll. If not naked, a bird would look rather frazzled. Over time the feathers would lose their strength and insulating ability as the microscopic structures that hold them together wore off. Look at the same feather when about a year old, and you can easily see that wear. This Little Gull wing in the Slater Museum collection, from a one-year-old bird just molting into its second set of wing feathers, shows you how worn feathers can become before they are replaced.

The solution to this problem is the annual molt that all birds undergo. All the feathers on the body are replaced each year, usually soon after breeding, which would be in fall in our north temperate zone. Not only are the contour feathers of the body replaced, but all wing and tail feathers are replaced as well, except in the very large birds in which this isn’t energetically possible (see next blog post).

It takes a lot of physiological energy to grow a feather, so molting is a fairly slow process. A complete body molt takes as long as a month for an average songbird. Individual flight feathers take about three weeks to replace completely, so a complete wing molt may take a month or more. These constraints are very important to migratory birds, as they may not be able to migrate until they have finished growing all their flight feathers, and they molt only after breeding, with its own energetic demands, has been completed.

Wing molt is typically sequential. The innermost primary is shed, and its replacement begins to grow. Before it completes its growth, the second primary is shed and its replacement begins to grow. Etc. The molt progresses out the primaries and, at some point, begins in the secondaries, where it moves from the outermost (adjacent to the primaries) inward. This gull is in the middle of primary molt, with the two outermost feathers from the previous generation and the feather just in from those growing in. You can also see that some of the outermost secondaries have been shed.



This adult Black Turnstone shows body molt (worn brownish feathers being replaced by crisp blackish ones) and wing molt (worn brown primaries and newly grown blackish ones, with a gap where the intervening feathers have been shed and are regrowing). Most birds would look something like this in fall.

Of course, in birds that change plumage color between breeding and nonbreeding times, molt of the body feathers must occur twice each year, spring and fall. This molt is one of the most obvious ones to those of us who look carefully at birds. This juvenile Short-billed Dowitcher is just beginning its molt into its dull first-winter (much like adult) plumage; the scapular feathers are often the first to molt.

Dennis Paulson