As a Yank, we would have called someone like Lord Haw-Haw, A GAS BAG! As to the whitewash, I can't speak as to Rooks. I CAN speak about Atlantic Sea Gulls.
We went to the sea each summer, in the days of old fashioned Telephone poles. The poles were prime roosting space, because they had iron spikes driven into the poles to use in climbing. Each Spike was appropriated by one or two Gulls, and the sidewalk beneath? I think you can imagine. Every morning, my Dad or Brother would go out with a hoe and remove the mess, then hosing off the sidewalk.
I think if "Haw-Haw" Joyce had gotten Dive Bombed by a Quartet of fully loaded Sea Gulls, he would have given up and SURRENDERED!
I do hope he won't upset Henry

I don't hink that is the answer. These are country men, not Cockneys.
Wind and whitewash.
Rooks live in large colonies in the tops of trees. They roost in the wind-tosseded upper branches, and often soar around on the wind. The reference to wind implies he talks a lot.
The whitewash is a reference to the bird lime that streaks the trees and ground below the trees where rooks roost. In other words "he talks sh*t".
"great minds think differently"

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