We have chosen to use the following terms and definitions which we believe are easy to understand for new ringers and (with some simplifications) are rather practical to apply to passerines of northern Europe:

• Post-juvenile moult: Conducted by the young bird (often soon after fledging, close to the breeding grounds) and is generally completed before, or during the early stages of, the autumn migration.
• Pre-breeding moult: Conducted sometime between the autumn migration and the spring migration, generally on the wintering grounds.
• Post-breeding moult: Conducted by the adult (2cy+) bird during late summer by the end of, and soon after, the breeding period (often close to the breeding grounds) and is generally completed before, or during the early stages of, the autumn migration.

Any of the above mentioned moults can be either complete or partial:

• Complete moult: Involves the whole plumage, including primaries and primary coverts.
• Partial moult: Involves parts of the plumage, often only the body contour feathers and some wing-coverts.

In a few species (or in single individuals), the complete moult may be interrupted due to e.g. periods of migration. Such pattern is described according to the following:

• Arrested moult: A complete moult that stops before it is completed, and is not resumed later on.
• Suspended moult: A complete moult that stops before it is completed, but is resumed later on.

Any moult that is not complete results in a moult contrast which is a rather central term for a bird ringer, meaning a contrast between different generations of feathers. Careful study of the plumage, combined with knowledge of the species’ moult pattern, is the base for ageing birds. This is the reason why each species-chapter in this guide starts with a short, but important, presentation of the moult pattern – both as a moult-key, and in more detailed (but still easy to understand) text. In the moult-key we use single letter abbreviations for summer (S or s), winter (W or w), complete (C) and partial (p). The codes ‘sp’ and ‘WC’ should thus be read as ‘summer partial moult’ and ‘winter complete moult’ respectively.

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.