I've had the best results with Ivormec cattle pour-on after reading about it on here. I still use the Sevin dust for the coop. The wild birds must be dropping them in the grass or something... I was fine for a year, didn't add any birds... then the little creepy things showed up.
I applied it with an eye dropper... two person job. One person holds the bird and moves the feathers to find skin, the other person applies the drops. I did 1 drop in each spot.. bare spot under each wing, back of the neck, and under the vent. Bugs have been gone for 3 weeks so far.