Sometimes mites proliferate on a hen that is already weakened with some other sickness. When I dust I just do the vent, tummy and under the wings. Need to redo it in a week.
Oh I meant to add I use a blush brush. My daughter saw me buy one in the drugstore and said, in a shocked voice. "What! I've never seen you use make up?" And I answered, "no it's to put Dri-Kill on chickens." So she laughed and said, "good that's more like the mum I know."
A blush brush works really well, dipped in a cup of dust, then brushed on vent, tucked under wings. Gets right where you want without excess flying all over the place.