It was an almost constant deluge, from dusk to dawn, with particularly busy periods either side of midnight - but you could pick any 30-second frame of the spectrogram and they they are, tssup-ing over in their teaming masses, often really low. How many birds were involved, we'll never know, but a minimum of 2,256 registrations gives an idea of the numbers streaming over.....
My recorder is an old, non-directional hand-held MP3 recorder, in a clingfilm-covered plantpot in the garden - imagine how many would've been picked up by a fancier set-up (directional mic, parabola etc.)? No matter - my cheapo DIY set-up is serving me well.
It's been a particularly productive spring for nocmig Redwing registrations, with each night's counts (up to the end of March) now uploaded onto Trektellen here.