ajohn

Note that the white spots on the mantle are a feature of juvenile plumage (in all flycatchers). They are useful because Dark-sided often migrates in juvenile plumage, unlike other species - but even dark-sided should not show these white spots at this time of year.

I agree with Gary, however, that this appears to be Grey-streaked. The colour and structure fit better for this than dark-sided, and it is obviously the commoner species at the moment.