The broad, round-tipped, thick flight feathers, sharply contrasting black and white primaries and alula, nearly unifromly white secondaries, and pure white primary coverts all indicate this Snow Bunting (Plectrophenax nivalis) is a male in definitive basic plumage, meaning that it hatched in at least 2014 (this bird was captured in January 2016). Molts and plumages are among the most important features used for aging birds during the banding process.
Photo: Mark Conboy
This primer on molt in birds is read by all new volunteers and staff at Long Point Bird Observatory (LPBO). This is the most basic level of understanding that banders are expected to have before they can begin work in the banding lab. This primer assumes that the reader has at least a basic understanding of feather terminology (i.e., what are the greater coverts versus median coverts).
In bird banding, a comprehensive understanding of the strategies, timings, and mechanics of molt is necessary for aging birds with accuracy and precision. Molt is a cyclic pattern of feather replacement, and is highly variable between species, and exceptions occur to almost all of the "rules" that follow, but in general terms:
- Every species at LPBO molts all of its feathers at least once a year, usually after the breeding season.
- Some species have an additional annual molt before the breeding season.
- In most species, young birds molt differently than adults. For example, some species have two molts during their first 12 months of life, but only have a single molt every 12 month after that for the rest of their lives.
In general, molt is usually symmetrical, meaning, for example, that a feather actively molting on the left wing is simultaneously molting on the right wing. Similarly, if the outermost tail feather is actively molting on the left side of the tail, the outermost tail feather on the right side should be molting simultaneously.
Molt usually occurs in a predictable sequence and that sequence is reflected in the numbering system we apply to the wing and tail feathers. All of the wing’s flight feathers and the tail feathers are numbered in the order which they typically molt.
Wing and tail feather terminology and numbering.
Illustration: Adapted from the North American Banders' Study Guide (North American Banding Council)
In most species, the primary feathers molt from the innermost to the outermost, and the secondary feathers molt from the outermost to the innermost. The tail feathers, properly called rectrices, molt from the innermost to the outermost.
In order to age birds you will need to know what patterns of molt each species uses. Read the molt section in Pyle for each species as you encounter them. Contained within the molt section is almost all of the information you need to age a bird. Pyle tells you what kind of molt(s) each species undergoes and when it occurs. Pyle also notes exceptional (eccentric) kinds of molts that don't fit the general patterns discussed below.
There are three types of molts that you will regularly encounter at LPBO: preformative, prebasic and prealternate. These types of molts can occur in several different arrangements or molt strategies. The majority of LPBO birds follow two particular molt strategies:
1. Complex Basic Strategy
Natal Down
ê
Prejuvenal Molt à Juvenal/First Basic Plumage
ê
Preformative Molt à Formative Plumage
ê
Definitive Prebasic Molt à Definitive Basic Plumage
2. Complex Alternate Strategy
Natal Down
ê
Prejuvenal Molt à Juvenal/First Basic Plumage
ê
Preformative Molt à Formative Plumage
ê
First Prealternate Molt à First Alternate Plumage
ê
Definitive Prebasic Molt à Definitive Basic Plumage
ê
Definitive Prealternate Molt à Definitive Alternate Plumage
ê
Definitive Prebasic Molt à Definitive Basic Plumage
Note: There are two additional molt strategies that are rarely used by species encountered at LPBO, so won’t be addressed in detail here: Simple Basic Strategy and Simple Alternate Strategy.
Note: Molts are named with the prefix “pre”; so that the molt which leads to a bird’s juvenal plumage is called the prejuvenal molt. There are other molt and plumage terminology systems in use, but the one presented here is the best and is the one in most widespread use among banders today.
We will examine all of the molts and plumages and outline, briefly, how they can be used to determine the age of a bird, beginning with natal down. Natal down is the first kind of feather that nestlings grow. Some species hatch fully covered in down (precocial), others hatch naked and subsequently grow down (altricial). The development of natal down is not considered a true molt. True molts actually begin with the next stage of feather growth.
The prejuvenal molt occurs in the nest. It leads to the development of juvenal plumage. The prejuvenal molt is sometimes also referred to as the first prebasic molt and juvenal plumage is sometimes called first basic plumage. At LPBO we tend use the more intuitive terms prejuvenal molt and juvenal plumage. The prejuvenal molt is unusual in that all feathers are grown at the same time, thus it is a simultaneous molt. All subsequent molts in a bird's life are sequential molts, meaning feathers are replaced a few at a time, not all at once.
Juvenal plumage is grown in the nest. Its primary function is to make young birds mobile so they can leave the nest, reducing the risk of depredation and giving them the ability to follow their parents while they forage. Juvenal plumage is short-lived, lasting for only a few weeks in most LPBO species, before being replaced by formative plumage. Juvenal plumage, aside form usually looking very different from all subsequent plumages that a bird will have, has particular characteristics that make it distinctive. Understanding these characteristics is imperative to aging birds in non-juvenal plumages (i.e., formative, basic, alternate):
- Body feathers are whispy, with widely spaced barbs; they have the appearance of being low-quality feathers.
- Primary feathers are usually narrow with a somewhat pointed tip.
- Retrices are usually narrow and come to a noticeably pointed tip.
- Flight feathers are usually dull coloured, with limited or no coloured edging.
- Flight feathers and rectrices are usually thin, sometimes to the point of being translucent.
- Growth bars line up across the wing and tail feathers.
- Fault bars, when present, line up across the wing and tail feathers.
Note: Growth Bars are feather pigment bands that, when visible, can be useful for aging birds. Growth bars develop because daytime and nighttime metabolic processes take place at different rates in birds, so that pigment deposition is more concentrated during the day and less concentrated at night. In some species or individuals, growth bars can be difficult to see, but in other species or individuals they can be easily detected. Because the prejuvenal molt is simultaneous, juvenal plumage growth bars line up across the wing feathers and rectrices. Conversely, all other molts are sequential, so growth bars don't line up across the feathers that have grown during the preformative, definitive prebasic or prealternate molts. Growth bars in the rectrices need to be used with caution because it is possible for a bird in definitive basic plumage that accidentally lost its tail to adventitiously regrow those feathers simultaneously, in which case it may have growth bars similar to a bird in juvenal plumage. Fault bars are growth bars that developed during periods of stress, so that pigmentation is particularly weak. They are usually very obvious, but their occurrence is somewhat infrequent. In addition to being weakly pigmented, fault bars are also usually structurally weak, meaning they are prone to breaking, leaving the feathers looking unnaturally truncate. The same characteristics and cautions apply to fault bars as to normal growth bars.
Growth bars in Carolina Wrens (Thryothorus ludovicianus) and other wrens show up as dark bands on the wing feathers and rectrices. Growth bars are usually far less obvious in other birds, but the principle is the same. The top photo is of a bird in definitive basic plumage, showing dark growth bars slightly staggered on the wing feathers. The bottom photo is of a bird in formative plumage, showing dark growth bars that line up across the inner greater coverts and across the primaries and secondaries.
Photos: Marie-Anne Hudson (Piranga)
The preformative molt replaces the juvenal plumage with the formative plumage. The most important thing about the preformative molt is that it is typically a partial molt. That means most or all body feathers, but usually no primaries, secondaries, tertials or retrices are replaced (in some species a few tertials or the central rectrices may be replaced).
Note: Part 1 of Pyle doesn’t use the term preformative molt, but instead uses the term first basic molt. This volume was written before a new (and more accurate) terminology came into use. When Part 1 of Pyle says first prebasic molt, it really means preformative molt. When Part 1 of Pyle says first basic plumage, it really means formative plumage. You should attempt to always use the new terminology and not the terminology used in Part 1 of Pyle. Part 2 of Pyle, having been more recently published, uses the new terminology.
Formative plumage is roughly analogous to what most field guides call immature plumage. Understanding that formative plumage follows the partial preformative molt is crucial to aging LPBO birds! So, once again: typically, the preformative molt is partial, involving the replacement of only some juvenal plumage with formative plumage, while retaining other parts of the juvenal plumage. Here, the concept of molt limits becomes important. A molt limit is the contrast between different generations of feathers. In formative plumage a bird will show contrast between retained juvenal and replaced formative feathers. Detecting molt limts is the key to aging birds by plumage. Typically, molt limits occur in the wing feathers. In many LPBO species, juvenile primaries, secondaries, tertials and primary coverts are retained, while the greater, median and lesser coverts are replaced (fully or partly) by formative feathers. Thus there may be a visible molt limit between the retained juvenile feathers and replaced formative feathers in birds that have completed their preformative molt. Formative feathers are usually brighter in colour, rounder-tipped, broader, and thicker. In general, they look very much like definitive basic feathers (see below). When we detect a molt limit that's the result of a preformative molt, we know that the bird hatched in the most recent breeding season.
Myrtle Warbler (Setophaga coronata) in formative plumage. The primaries, secondaries, primary coverts and alula are retained juvenal feathers, while the greater and median coverts are replaced formative feathers. This molt limit, between the duller juvenal feathers and brighter formative feathers could only occur on a bird that hatched in the most recent breeding season.
Photo: Marcel Gahbauer (Piranga)
Note: When looking for molt limits, it is important to be aware of pseudolimits in some species. Pseudolimits are apparent contrasts between feathers, giving the impression of a true molt limit, but they are merely inherent differences between feathers of the same generation, thus are not indicative of a bird's age. Only a handful of LPBO species regularly have confusing pseudolimits, including many sparrows and Brown Thrasher (Toxostoma rufum).
Brown Thrasher (Toxostoma rufum) in definitive basic plumage. The apparent contrast between the inner and outer greater coverts is normal for Brown Thrashers in all plumages; it is a pseudolimit and is not useful for aging.
Photo: Peter Pyle (Piranga)
The definitive prebasic molt replaces formative plumage with definitive basic plumage. In contrast to the preformative molt, the definitive prebasic molt is almost always a complete molt in LPBO birds. Every single feather on a bird’s body is replaced during a complete molt: every body feather, every flight feather, and every rectrix. For LPBO birds, the definitive prebasic molt usually occurs on the breeding grounds, right after breeding is completed, though exceptions do occur. The reason the molt is called definitive is because it occurs every year of the bird’s life, except for its first year, when the preformative molt takes place at roughly the same time. The preformative molt occurs only once, replacing the juvenal plumage, while the definitive prebasic molt occurs every single year thereafter.
Myrtle Warbler (Setophaga coronata) in definitive basic plumage. All of the feathers of the wing are broad, round-tipped, and thick. There are no molt limits, meaning that this bird hatched not in the most recent breeding season, but at least in the season before that.
Photo: Marcel Gahbauer (Piranga)
Definitive basic plumage is roughly analogous to what field guides call adult plumage. It differs from formative plumage in that no juvenal plumage is mixed in with the basic feathers. When a bird is found to have only definitive basic plumage we know that it is one that hatched not in the most recent breeding season, but at least in the season before that. It is impossible to age most LPBO birds with any more precision than that because the definitive basic plumage occurs in birds that are one year old, two years old, three years old, etc; it is the same every year following the preformative molt. Definitive basic plumage has particular characteristics that make it distinctive from juvenal and formative plumages. Understanding these characteristics is imperative to aging birds:
- Body feathers are tight and crisp with dense barbs; they have a high-quality appearance.
- Primary feathers are usually broad and a fairly rounded at the tip.
- Retrices are usually broad and come to a noticeably truncate end.
- The feathers of the wing are usually colourful, usually with thick coloured edging.
- Flight feathers and rectrices are usually thick.
- Growth bars do not line up across the wing and tail feathers.
- Fault bars, rarely present, don't line up across the wing and tail feathers.
The complex alternate molt strategy has two additional types of molt that are not part of the complex basic strategy. These are called prealternate molts and they result in the development of alternate plumages, which are roughly analogous to the breeding plumages illustrated in field guides. Less than half of LPBO species have a prealternate molt and in all cases it's a partial or incomplete molt. Prealternate molts usually take place on the wintering grounds either in winter or early spring.
Note: In banding terminology there is an important distinction between partial and incomplete molts. As discussed earlier, partial molts include most or all body feathers, but usually no primaries, secondaries, tertials or retrices are replaced (in some cases a few tertials or the central rectrices may be replaced). By contrast, incomplete molts usually include the replacement of all body feathers as well as some primaries, secondaries, tertials and/or rectrices. Partial prealternate molts are far more common among LPBO species than are incomplete prealternate molts.
The first prealternate molt is partial (or in some cases incomplete) and results in the first alternate plumage. The first prealternate molt replaces some feathers of the formative plumage. The first prealternate molt usually is limited to feathers of the head as well as the greater, median and/or lesser coverts in many LPBO species. In other species, such as American Goldfinch (Spinus tristis), almost all of the body feathers will also be replaced, but such extensive molts are not the norm for LPBO species. Incomplete molts may also include some primaries, secondaries, tertials and/or rectrices. Birds in first alternate plumage may thus show molt limits between three generations of feathers in their wings (e.g., juvenal primaries, secondaries, tertials and primary coverts, formative outer greater coverts, and first alternate inner greater coverts, as seen in some Myrtle Warblers [Setophaga coronata]). When we detect molt limits that are the result of a first prealternate molt, we know that the bird hatched in the most recent breeding season.
Myrtle Warbler (Setophaga coronata) in first alternate plumage. The primaries, secondaries and primary coverts are retained juvenal feathers. The second outermost greater covert is a formative feather, while the rest of the greater coverts and median coverts are first alternate feathers. These molt limits, between three generations of feathers, duller juvenal feathers, a slightly brighter formative feather, and brightest first alternate feathers, could only occur on a bird that hatched in the most recent breeding season.
Photo: Marcel Gahbauer (Piranga)
Myrtle Warbler (Setophaga coronata) in definitive alternate plumage. All of the feathers of the wing are broad, round-tipped, and thick. The primaries, secondaries and primary coverts are definitive basic feathers, while the greater coverts and median coverts are definitive alternate feathers. A molt limit of this kind means this bird hatched not in the most recent breeding season, but at least in the season before that.
Photo: Marie-Anne Hudson (Piranga)
Note: There is another type of molt that is rather rare among North American songbirds, which I haven't yet addressed: the presupplimental molt, which leads to the supplimental plumage. Presupplimental molts can vary widely between species. For example, one species may molt only wing feathers, while another species may molt only body feathers; thus, presupplimental molt is sort of a catch-all term which is applied to any molt, whether it be partial, incomplete or complete, that occurs in addition to the prejuvenal, preformative, prebasic and prealternate molts. Only two LPBO species are known to undergo presupplimental molts of any consequence: Northern Cardinal (Cardinalis cardinalis) and Indigo Bunting (Passerina cyanea). Refer to Pyle for details about aging these species.
When aging birds by molt, put all of the above information together to decide what age to give the bird:
- Read the molt section in the species account in Pyle so you know what to look for at a given time of year.
- Eliminate all plumages you are not likely to encounter at the time of year.
- Open a wing and look at the primaries, secondaries, tertials, primary coverts and greater coverts in particular, for molt limits. Determine molt limits by comparing the shape, colour, thickness and amount of wear between feathers.
- If molt limits are present, determine if they are the result of a preformative molt or a prealternate molt. If molt limits are not present determine if the feathers are uniformly juvenal or non-juvenal.
- Open the tail and look for molt limits. Determine molt limits by comparing the shape, colour, thickness and amount of wear between feathers. Pay particular attention to the central and outer rectrices.
- If molt limits are present, determine if they are the result of a preformative molt or a prealternate molt. If molt limits are not present determine if the feathers are uniformly juvenal or non-juvenal.
- Look at the body feathers. Determine if the body feathers are juvenal or non-juvenal. Use the plumage criteria given in Pyle to see if additional clues about age can be found in the colours of body, wing feathers and rectrices.
- Look at other characteristics such as colour of the eyes, orbital skin, legs, beak and/or mouth lining. Determine if the bird has a gape, is in breeding condition, and/or if the skull is completely ossified (look for these last three criteria at the appropriate time of year).
- Come to a final decision about the bird's age based on a combination of criteria.
There are plenty of additional resources about molt:
Piranga (NatureInstruct)
North American Banders' Study Guide (North American Banding Council)
The Variety of Molt Strategies (Steve N.G. Howell)
Finding Order Amid the Chaos (Steve N.G. Howell)
As you read through these documents, and as you begin to work in LPBO's banding lab, you will see that aging birds by molt is far more nuanced and fraught with exceptions than I've presented here. This makes banding a source of perpetual learning and challenge, something that any serious bander will embrace, rather than dismay.
