Old friend of mine was a Typhoon fighter pilot during WW2. He said that when German radar controlled anti-aircraft guns were on to him, the first warning was a clicking or similar noise in his headphones (VHF).
He was leading a formation attacking German forces on D-Day. He heard the noise and immediately changed heading. But for some reason one of the formation pilots in his line astern continued straight ahead and was immediately shot down by remarkably accurate AA fire.
However, in the case of birds and radar the theory was the birds were subject to heat from the weather radar - not "noise."