I don't know if this is possible, but here's the scenario:
Last night, I kept getting "No signal" appearing on my TV. (Freeview) Primarily it was on the stations that aren't the traditional BBC/ITV/Channel 4 & 5 lot. Kept retuning, to no avail and eventually gave up for Match of the Day.
This afternoon it started doing the same thing and I popped outside to see if the aerial had perhaps moved. It hadn't, but there were four juicy, fat pigeons sitting on it. Chucked a stone at them and off they flew. The TV came back more of less immediately.
Coincidence? Or is there any possibility of pigeons stopping the signal?