Unfortunately Nick (what is it they say about assumption ) neither body mass nor condition has any noticeable significance for overwinterers against fresh migrants.
Species like Chiffchaff and Blackcap in this instance undergo a partial winter moult and a complete summer moult, so birds from either would have identical plumage in spring. The effects of their spring migration has little if any perceivable bearing (through wear and abrasion) on their appearance once they finally arrive here.
As for body mass, you're right in that it's unperceivable to the human eye for a intents and purposes. Don't forget that your plump Blackcap (undoubtably heavier in weight due to your fat balls than those fresh in spring migrants) is likely to still appear plumper due to the effect of her trying to insulate herself with air in amongst her feathers against our cold winter. Whilst fresh spring birds may perhaps be slightly lighter in weight it's not perceivable in the field and many (most?) stop off to gorge themselves on route so arrive at a healthy weight anyway. I've seen many autumnal eastern vagrants in the UK (which have travelled thousands of miles) which appeared healthy enough and certainly had no external signs of malnutrition yet when caught were quite severely underweight and by the same token, retraps have highlighted the speed with which they can, under the right conditions etc, put that weight back on.
As for their song, it's unfortunately all supposition on our behalf. Sonograms may be a way forward but then how do we seperate those winterers and new arrivals in the first place? Back to square one
An interesting subject. Certainly not the last word on it. Let's see what spring and observations bring
Difficult I know but I have noticed the female Blackcap that has frequented my garden over the last month or so is particuly plump and in fine condition (presumably from all the fat balls she's been tucking into). Would it not be a sensible assumption that a bird that had travelled 1500 miles would not be in top condition and comparitively lighter (I appreciate trying to judge body mass in the field would be somewhat difficult but general "condition" should be judgeable !!)
From doing years of visiting the same spot on a weekly basis my records note that early in the season migrants songs tend to be quite half-hearted, just a few bars, as though they were practising and not too sure !! Potentially migrants in full song early in the season could be over-wintering birds (on the basis they have been "practising" earlier !!!)
A minefield, I know, but an interesting subject ! (hopefully the first will be Sand Martin and then this debate will be obsolete !!!!!!!!!!!!!!!)
And if you could Henry I'd be really interested to know! Personally and from experience with known overwinterers, I really couldn't begin to seperate them from true fresh spring migrants on song (not to mention plumage!)
Tough question Ian! From my experience in Cornwall, overwintering Chiffchaffs and Blackcaps often sing throughout the winter which makes it even tougher to detect fresh migrants down there. What I suspect are the true migrants sing with more gusto when they arrive, however this is speculation as I can't confidently tell wintering and long-distance migrants apart on plumage.
Chiff Chaff is the top one for me. The first one I hear always cheers me up a quintessential sound of spring. I know there are more glamorous birds but the first Chiffy and the first Cuckoo always do it for me. By the way old Cheshire custom the first Cuckoo you hear take your shoe off it brings good luck for the rest of the year. You look a divi but so what ? think of all that luck coming your way !!
) neither body mass nor condition has any noticeable significance for overwinterers against fresh migrants. 
