Noticed this too actually. There were three new masts put up on the edge of the moors near me over winter and then come March we had dead starlings all down the back lane behind my street. Council said it was "natural causes" which is the standard brush-off they give for anything they dont want to investigate properly.
Not saying I know exactly what the mechanism is, I'm still looking into the frequency stuff and trying to understand what millimetre wave emissions actually do at a biological level. Found a couple of papers that suggest birds navigate using magnetoreception and that certain RF frequencies can interfere with that, but I cant find anything peer reviewed that directly links tower installation timescales to die-off events.
If anyone has actually mapped the tower rollout dates against reported die-offs in their area I'd genuinely be interested to see that data. Not just anecdote, proper location and date correlations. Someone must have done it by now.