Not possible to age due to complete pre-breeding moult in both age classes. A few individuals retain single (or some) feathers during the pre-breeding moult (e.g. SS or GC), but these are usually not possible to use for ageing since the very same feathers are said to previously have been included in the post-juvenile/post-breeding moult (Jenni & Winkler 1994). A few birds also show other anomalities (e.g. PC that are more worn than GC, or minor difference in wear between groups of GC) but such are probably best explained by differentiated periods of moult during winter.