Juvenile male Brewer's Blackbird, Fernhill Wetlands, Forest Grove, Oregon on 28 July 2010 by Greg Gillson.
I think that I am like a majority birders in that my understanding of molt was never really very strong. I could, though, recognize the four annual plumages of the larger gulls leading from chick to adult. And I recognized breeding and non-breeding plumages of many birds that changed dramatically from summer and winter. And I recognized that juvenile shorebirds have plumages unlike the adults.
But until recently, and perhaps now still, I don't have as good a handle on plumages and molts as I could have, or that I would like.
David Irons, on the BirdFellow blog recently posted a 3-part article on an introduction to molt limits. David didn't specifically define the term "molt limits," but it is simply the contrast between different ages ("generations") of feathers on the bird, especially wing and tail. Perhaps this is most obvious on blackbirds, as the above photo demonstrates.
If we know the state of molt we can know the age of the bird--at least for the first two years for most species. Knowing the age of the bird helps in the identification of shorebirds, gulls, fall Empidonax flycatchers, and some others. It also tells us about things such as survivorship of first year birds, where birds go after breeding, whether adults or first year birds wander more widely, winter in different areas, etc.
In a personal extreme example, an immature albatross was discovered off Oregon in September 2008. It was in the Wandering Albatross group, only seen in North America once before. The taxonomy of albatrosses is in flux. While right now the North American scientists in the A.O.U. (American Ornithologists' Union) treat several populations as one species--Wandering Albatross--such is not the case in other countries. In the future, the A.O.U. may well split this complex into 3-5 species. Only by looking at the exact sequence of old and new feather generations in the wings were we able to determine that the bird was likely 5 years old and pin down the breeding island (Antipodes). In the future, I may have Antipodean Albatross on my list, rather than Wandering Albatross or "unknown Wandering-type Albatross," a highly undersireable thing for any birder, I can tell you.
Even without knowing too much about molt, most birders would be able to tell you that this Brewer's Blackbird (above) is a young bird (hatched perhaps only a month or two ago). It is obviously molting into adult plumage. And, with that black plumage and tell-tale pale eye, it is a male.
In this case, we can look at the molt and tell more. Using the Humphrey and Parkes terminology (and, please, if I have this wrong someone please tell me!) The fluffy brown feathers are the juvenal plumage--the very first feathers birds get after their downy chick state.
The next plumage after juvenal is the first basic plumage. It is acquired by the prebasic molt. And that is the state of the Brewer's Blackbird above. The first prebasic molt is not a complete molt. Some juvenal wing and tail feathers will not be replaced. Thereafter, every subsequent prebasic molt will be complete.
In this bird that doesn't have a more colorful alternate (or breeding) plumage, it molts once per year, in the late summer/early fall. It molts from basic plumage to basic plumage in an endless cycle.
Other birds (for instance, Yellow-rumped Warbler) have two molts per year and switch between the brighter alternate plumage (breeding) and duller basic plumage (non-breeding). Alternate plumage is often only a partial molt--just the body feathers, not wing and tail.
OK, this has gone longer than I intended. The blackbird above has new black, shiny feathers on the greater secondary coverts, old juvenal feathers on (most of) the median secondary coverts, and new feathers on the lesser and marginal coverts on the shoulder. Some body feathers near the sides are new. There is one new tertial feather and 3 black primary feathers mixed in with the "old" brown juvenal feathers. If I blow up the photo I can see at least 3 black greater primary coverts. This molt is not yet over and will undoubtedly include more feathers in the coming weeks.
For more information online see the Ontario Field Ornithologists' web site and print out and study Ron Pittaway's April 2000 article: Plumage and molt terminology. This article discusses older and newer plumage terms, calendar year terminology, molts, banding codes, Humphrey and Parkes terminology, and the two main molt and plumage sequences.
[The title of this article alludes to "trash birds," a facetious term applied to any bird considered common or uninteresting, especially to those birders seeking new species to add to their list. Obviously, for the purpose of learning molt and plumages, this bird is not a trash bird.]