Het is sowieso opvallend dat de vogel, in tegenstelling tot de vorige 2 gevallen, niet meer in grotendeels juveniel kleed was. Dat viel ook Magnus Hellström op, en met de beste foto's van gisteren maakt hij er een waarschijnlijk adulte vogel van:
The Dark-sided Flycatcher is a rather unique passerine when it comes to ageing. To start with, the species leaves their Siberian breeding grounds largely unmoulted – either in fresh juvenile plumage (1cy birds) or in worn breeding plumage (adult birds) – and perform their post juvenile (partial) or post breeding (complete) moult on stopover sites further south, perhaps in N China(?). Hence, young birds turning up in Europe during autumn are expected to show a more or less completely juvenile plumage (typically pale spotted over the body, like other flycatchers before their moult). Indeed, both previous European records have concerned 1cy birds in such juvenile plumage. This strategy is not shared with Asian Brown Flycatcher and, hence, is actually an important and helpful tool in the separation of the two species. Later on, during the winter, both age groups seem to perform a rather limited partial moult. The present Dutch bird does not show a juvenile body, but instead shows the unspotted plumage typical of post-juvenile (or subsequent post-breeding) feather generations. That gives us the following options: 1) a 1cy bird that (untypically) already performed its post-juvenile moult, or 2) a (typical) 2cy+ bird. Like other flycatchers, the partial post-juvenile moult in Dark-sided Flycatcher includes the body, all lesser/median coverts, but only a few inner greater coverts. Hence, the central and outer greater coverts are retained juvenile, just like the tertials. The juvenile coverts/tertials shows a contrasting pattern with distinct pale edges and a distinct and large pale spot by the tip, which makes them easily recognised. Smaller pale spots are also seen at the tips of the primary coverts. This should be compared to the adult type coverts/tertials that, in contrast, shows more diffuse pale edges rounding the feather tips without creating a wide spot, and all dark primary coverts. Applying the above to the Dutch bird clearly place it as an adult individual. However, I mentioned above that the Dark-sided Flycatcher is rather unique. The reason for this is that the delayed moult means that 2cy birds retain their still juvenile wing during much of the autumn, until their first complete post breeding moult is conducted. And since the juvenile and adult feather generations are rather easily separated, it is actually possible to separate 2cy from 3cy+ during the autumn, which is very rare indeed among Palearctic passerines! In 2cy autumn, the still juvenile wing is over a year old and very worn, compared to the moderately worn 3cy+. This assessment requires good photos, and perhaps we should be careful and wait for better images of the Dutch bird. But until then, my preliminary view is that the wing seems to be in reasonably good condition and is best fitted with a 3cy+. So, in short, early autumn birds are aged as follows: 1cy: fresh juvenile wing 2cy: worn juvenile wing 3cy+: moderately worn adult wing
/Magnus Hellström, Ottenby Bird Observatory, Sweden
