Moult & ageing of Palearctic Passerines

Based on text from Norevik, Hellström, Liu & Petersson. 2020. Ageing & Sexing of Migratory East Asian Passerines. Avium förlag AB, Mörbylånga.

The different aspects of moult in palearctic passerines have been described comprehensively in e.g. Jenni & Winkler (2020) and Ginn & Melville (1995). Here, we give a condensed and practical overview with the aim of facilitating the novice ringer’s first steps into the subject.

 

THE BASICS

 

A bird’s plumage has multiple functions: flight, insulation, and visual signalling, to name but a few. But, as the feathers are exposed to wear by physical processes, such as bleaching from sunlight, mechanical abrasion and degradation by parasites, the plumage needs to be renewed continuously in order to maintain its functions. The process of systematically replacing worn feathers is known as moult, and is usually undertaken at least once every year.

 

The fact that the extent of the moult often differs between young and adult birds, and that the juvenile feathers are structurally different from adult ones, gives us valuable tools that are useful when ageing birds. The basic principles are not difficult to learn – focus on the typical patterns and leave the (rather rare) exceptions aside in the beginning.

 

We will use the following terms (based on timing and extent) when defining different moult patterns:

 

Timing: Palearctic passerines have two main periods of moult: before and after the breeding season. This gives us the following three basic definitions:

  • Post-juvenile moult: Undertaken by young birds. During this moult, the bird replaces parts or all of its first (juvenile) plumage; the plumage that was grown in the nest. In most species it is conducted soon after fledging, close to the breeding grounds. It is generally completed before, or during the early stages of, the autumn migration.
  • Post-breeding moult: Undertaken by adult birds during late summer at the end of, and soon after, the breeding season (often close to the breeding grounds). It is generally completed before, or during the early stages of, the autumn migration.
  • Pre-breeding moult: Undertaken by both young and adult birds, generally in the non-breeding areas during winter but occasionally reaching into the spring migration. It is generally completed before, or during the early stages of, the spring migration. Through this moult, some species acquire a distinct breeding plumage.

Extent: Any of the three above-mentioned moults can be either complete or partial (see fig 1).

  • Complete moult: Involves the whole plumage.
  • Partial moult: Involves only parts of the plumage.

By combining the timing and the extent of a certain moult, it is possible to allocate a convenient terminology suitable for ageing purposes. For post-breeding and post-juvenile moults (both conducted on or near to the breeding grounds) we use an S for ‘summer’. For pre-breeding moults (conducted on the wintering grounds) we use a W for ‘winter’. When the moult is complete, we use uppercase letters and use a C for ‘complete’. Thus, summer complete moult is abbreviated to SC and winter complete moult to WC. Similarly, when the moult is partial, we use lowercase letters and use p for ‘partial’. Thus, sp for summer partial moult and wp for winter partial moult. The full moult pattern is first presented for summer and then winter – e.g. ‘SC, wp’ for summer complete, winter partial. When a moult is not undertaken in a given season, a dash (–) is used to indicate no moult.

 

THE COMPLETE MOULT

 

All Palearctic passerine species undergo a complete moult annually, either as a post-breeding or a pre-breeding moult (rare exceptions with two complete moults annually are known). Following a bird’s first complete moult, no juvenile feathers are left in the plumage.

 

In a few species – or in certain individuals within a species – the complete moult may be interrupted due to migration (most species are reluctant to moult during migration). Such patterns are referred to as: arrested moult – a complete moult that is halted before it has reached completion and is not resumed later; and suspended moult – a complete moult that is halted before completion but is resumed later.

 

A few things that are useful to keep in mind:

  • Primary coverts are usually (but not invariably) moulted together with their corresponding primaries. The greater coverts, on the other hand, are all shed at the same time and have no association with the moult of the secondaries.
  • The last feathers to finish growing are usually the innermost secondaries. This may be useful to check in early autumn when it may be difficult to separate fresh juvenile birds from newly moulted adult birds of equally fresh plumages. In adult birds, the inner secondaries may then still show a sheath at the base of the feather, indicating that they are in the very final stages of growth.
  • Even though complete moults are meant to be, as their name implies, complete, it is not too uncommon to find ‘forgotten feathers’ – a single feather (often a covert, an alula feather or a secondary) that, for some reason, was left unmoulted.

 

THE PARTIAL MOULT

 

A partial moult does not include the whole plumage. The extent is highly variable between species, seasons and individuals, ranging from just parts of the head to (in extreme cases) most feather groups, even including some primaries or secondaries. However, as a general rule, a partial moult does not include primaries, primary coverts, secondaries and in many species no (or just a couple of) rectrices. A partial moult always results in a moult contrast, meaning a contrast between different generations of feathers.

 

Most partial moults typically include some wing coverts. In species with more restricted moults, this may be confined to a few lesser coverts, but many species frequently include median coverts and a variable number of inner greater coverts as well. These feathers are sufficiently large, well-structured and well-marked to make them useful when it comes to finding moult contrasts. This is why the wing coverts often play a central role in the ageing of passerines.

 

A few helpful notes:

  • A moult contrast is much easier to find if you know where to search for it. For this reason, it is highly recommended to learn the normal extent of the partial moult for each species that is handled. In many bird genera, e.g. thrushes, pipits and buntings, most, or even all, species show the same moult pattern and similar moult extent.
  • A moult contrast is easier to find when it is situated within a feather group (e.g. within the greater coverts), instead of between feather groups (e.g. between moulted greater coverts and unmoulted primary coverts).
  • In some species, the inner greater coverts often differ in pattern compared to the adjacent feathers, even within the same generation. This often create impressions of false moult contrasts. It is commonly seen in pipits, wagtails and buntings, but also to a lesser extent in other species. In order to avoid such mistakes, attention should be paid to structure and wear as well.
  • As a general rule, northerly breeding birds often have more restricted partial moults than southerly ones. This is often also the situation within populations of the same species.
  • Any accidentally lost feather is regrown straight away, and this may result in unexpected moult contrasts in the plumage. Such feathers may be useful for comparison to remaining feathers, as the new feather usually has an adult-type structure and shape.

 

THE MOULT SEQUENCE


Since passerines are dependent on their ability to fly, moult of the larger feathers (primaries, secondaries and rectrices) progresses bit-by-bit in a given sequence in order not to create gaps in the plumage so large as to impede flight. The larger feathers are therefore usually moulted in ‘waves’, where a certain feather is not shed until the precedent feather has reached a suitable length. In most species, the moult starts with the inner primaries, which are moulted descendantly (from the body outwards). Moult of secondaries starts after some inner primaries have been replaced, and the secondaries are then moulted ascendantly (towards the body). The sequence of the replacement of rectrices is more variable.


In order to understand the moult history of a bird, and to know how to interpret the moult contrast you see, it is vital to know the hierarchy in the partial moult. In short, this hierarchy can be summarised as follows:

  1. lesser and median coverts
  2. greater coverts
  3. alula, tertials and tail
  4. [single secondaries and primaries; only in rare cases in a few species].

This is best read backwards: Single secondaries and primaries are only included (occasionally, in a few speces) if the other feather groups are moulted. Alula, tertials and tail are only included if most of the greater coverts are moulted. Greater coverts are only included if most of the lesser and median coverts have been moulted.


Another important hierarchy is found within the greater coverts: it is the inner feathers (closest to the body) that has priority. Regardless if a bird has moulted two, four, seven or nine greater coverts, it is generally the inner ones that has been included. In other words, any retained juvenile greater coverts are likely found in the outer part of the wing. This rule does not apply to lesser and median coverts – these feathers are moulted in a more random sequence.


Overall, some variation is seen, but the above basic pinciples are generally valid for most birds.


AGEING BY MOULT PATTERN


With few exceptions, Palearctic passerines can be assigned to two age classes: young birds (1cy in autumn or 2cy in spring) and adult birds (2cy+ in autumn or 3cy+ in spring). Generally, young birds can be distinguished as long as they retain some juvenile feathers in their plumage, i.e. until their first complete moult.


Juvenile feathers, grown while the bird is still in the nest, generally differ from subsequent feather generations by:

  • their shape (often slightly narrower and more pointed, sometimes also differing in length)
  • their structure (less densely barbed, resulting in a slightly looser and transparent structure)
  • their resilience to wear (partly as a result of their less dense structure, juvenile feathers wear quicker than adult feathers).

These juvenile traits are not present in the second, post-juvenile, feather generation, which (like all subsequent feather generations) is of adult type: broader, blunter tipped, denser, glossier, darker and with better resilience to wear.


By knowing the moult pattern of a species, it is possible to deduce if and how a bird can be aged at a given time of the year:


Example 1:

It is spring, and you have a male Blackbird Turdus merula in your hand. Moult in this species is conducted only once every year – before the autumn migration. This means that the bird carries the same plumage throughout the annual cycle, and therefore the same characteristics can be used for ageing both in autumn and spring. Adults replace the whole plumage in a summer complete (SC) moult, while young birds replace just part of their plumage in a summer partial (sp) moult. The latter commonly include head, body, lesser, median and inner greater coverts, meaning that a moult contrast normally should be found within the greater coverts (GC). When opening the wing, you notice differences between the inner and outer GC; the latter being brownish, looser and more worn. It is straightforward to find the exact point within the GC where juvenile and post-juvenile feathers meet. This is the moult contrast, which is the expected outcome of the sp moult in young birds, and the evidence required to age the bird as a second-calendar-year (2cy).


Example 2:

It is autumn, and you have an Icterine Warbler Hippolais icterina in your hand. In this species, both adult and young birds have a summer partial (sp) moult, where part of their plumage is replaced before the autumn migration. This indicates that moult contrasts are likely to be found in both age groups and, hence, the presence of a contrast is not relevant for ageing. Instead, attention should be directed to the feathers that were not included in the summer partial (sp) moult. In young birds, those feathers are just a few weeks old, and should therefore (despite being of poorer quality) still be rather fresh. Adult birds (moulting WC) have feathers that are many months old and have endured both spring migration and a full breeding season, meaning that they show bleach and wear. The bird in your hand has a body plumage that does not appear to be worn, but close examination of the wing reveals a single moulted tertial that stands out from the rest by being brighter green and fresher, although it does not appear denser in structure. The whole wing (except for the replaced tertial) seems to have a brownish hue and the edges and tips of the wing coverts, as well as the tips of the primaries, show some abrasion. The tail is similarly worn, although the rectrices are rather broad and rounded. All these clues suggest that the wing and tail is of a worn adult generation, and that these feathers were grown during a winter complete (WC) moult. Hence, you can safely conclude that the bird is an adult in its second-calendar-year or older (2cy+).


THE MOULT PATTERNS


Using the terminology outlined above, most Palearctic passerines group into a few main categories according to their respective moult pattern.


Adult: SC, –

Young: SC, –

This moult pattern is shown by larks, starlings, some sparrows and a few other species. As soon as the SC moult is completed, no juvenile feathers are present in the plumage. Hence, young and adult birds can then no longer be separated using plumage characters. Since this moult often occurs on (or near) breeding grounds, most birds on autumn migration must be aged as 1cy+. During the late summer and early autumn, any remaining, as-yet-unmoulted, feathers are worn (one year old) in adult (2cy+) birds, but usually rather fresh (only weeks or a few months old) in young (1cy) birds. During spring all birds are aged as 2cy+.


Adult: SC, –

Young: sp, –

This moult pattern is shown by some robins, most thrushes, most tits, and finches. During the autumn migration, adult birds (2cy+) show a fresh and uniform plumage, while young birds (1cy) have a plumage with moult contrasts between juvenile and post-juvenile feathers. Due to the lack of a pre-breeding moult, birds in the spring will show the same plumage, though it will be more worn than during the autumn. Hence, the presence of a moult contrast can be used to age young birds (2cy) during the spring as well, since adults (3cy+) show a uniform plumage from their post-breeding moult.


Adult: sp, WC

Young: sp, WC

This moult pattern is shown by many swallows, many reed warblers and some leaf warblers. During the autumn migration, both age classes show moult contrasts. Adult (2cy+) birds show retained wing and tail feathers that are many months old (from last winter) and have endured both the spring migration and the breeding season. They are consequently obviously worn. Young (1cy) birds, on the other hand, have wing and tail feathers no older than a few weeks (grown in the nest), that are therefore very fresh. During the spring migration, the age classes cannot be separated since all birds have undertaken a WC moult resulting in a uniform adult-type plumage; thus aged as 2cy+.


Adult: SC, wp

Young: sp, wp

This moult pattern is shown by wagtails, most pipits, many robins, most leaf warblers, flycatchers and most buntings. During the autumn migration, adult (2cy+) birds show a uniform and fresh plumage, while young (1cy) birds have a plumage with moult contrasts between juvenile and post-juvenile feathers. During the spring migration, following the wp moult, both age classes show moult contrasts somewhere in the plumage, depending on the extent of the partial moult. Focus should then be directed to the older (retained) feather generation (often primary coverts, outer greater coverts, primaries and rectrices) – are these of an adult or a juvenile type (see the general description under Ageing by moult pattern above)? Additionally, if a young (2cy) bird has undergone a wp moult that was less extensive than the sp moult last summer, the bird will actually show three generations of feathers (juvenile, post-juvenile and pre-breeding), whereas adults (3cy+) will only ever show two generations of feathers (post-breeding and pre-breeding). The occurrence of three feather generations is diagnostic of a 2cy bird, while two generations could be either 2cy or 3cy+.


Arrested and suspended moult

A few passerine species (in Scandinavia Ortolan Bunting Emberiza hortulana and Barred Warbler Sylvia nisoria and some individuals of Sand Martin Riparia riparia, Tawny Pipit Anthus campestris, Common Whitethroat Curruca communis, Common Grasshopper Warbler Locustella naevia and Golden Oriole Oriolus oriolus) show more complicated moult patterns, where the complete moult is halted during migration. This often creates moult contrasts that are difficult to assess and understand. See relevant literature for a fuller description.


THE IRIS

 

The colour of the birds’ irises undergo a general development during the first years of living. The general rule (with exceptions of course) is that young birds show a dull and rather cold greyish iris that turns warmer brownish during the first year. During autumn, the difference is often readily apprehended through an eyepiece in good light conditions, but already during their first spring most species have developed a colour that is more difficult to tell from adults (3cy+). Thorough and long term studies of birds of known ages (i.e. recaptures) may give experience enough to, for some species, separate three age categories (which is only exceptionally done on passerines when ageing is based on plumage), but our advice is to be cautious in this respect. We suggest that logged ages should primarily be based on the assessment of the plumage and that the colour of the iris (or any other soft part) is treated as a supplementary character to add to the general impression. In this guide we have included photographs of irises rather irregularly, and this is mainly because of the difficulties to depict the character under studio conditions.


A WORD OF ADVICE

 

As implied above, the basic principles of the moult patterns are not complicated or difficult to learn. However, it should be stressed that in order to successfully apply the theory in reality, a lot of practice under the supervision of experienced mentors is required. Furthermore, remember that exceptions from these basic rules do occur on rare occasions. Single feathers may be ‘forgotten’ during complete moults, feathers lost accidentally will result in replacement of odd, single feathers, and aberrant individuals may show other anomalies. Though exceptions do occur, they should not be allowed to cloud the overall picture!


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