I've never really thought of 'one' bird embodying the return of Spring, though hearing the Blackbirds starting to sing is probably the big one for me personally. It's more an overall impression from longer days, more birds singing, the trees coming into bud and flowers starting to appear.
Lots of birds to choose for a spring heralder here though; Cuckoo, Whitethroat, Blackcap, Willow Warbler, Sedge Warbler, Swift, Swallow, House Martin...
Skylarks should be the sign for me as I tend to see them a lot on my local meadow but though they're much more active at the start of spring and really making a lot of songflights the fact that I see them in the winter too and hear them singing a little in those cold days makes them less of a sign than birds that appear with Spring. Blackbirds are around all year too but somehow their song makes more impact on me - I've always been a sucker for Blackbird song, even before I got into watching birds.
Nightingale is another biggie in East Anglia for Spring, but I have to take a good long walk to hear one here.