Mites - the wild bird population is crawling with them, so if your flock ever comes into contact with wild birds (and most of ours do!), you'll get them on occasion. It's not a matter of not keeping a clean coop. I use Eprinex twice yearly, and (knock wood) I haven't had a problem the last few years. HOWEVER, I HAVE been reading that mites build a resistance to it, just like many parasites and treatments. So, I've been considering stopping the use of Eprinex for a couple of years. Lots of folks are now using a flea med for dogs...okay, it's probably the most common one, but I can't for the life of me remember its name right now. Problem is, it's off label use (of course, so is Eprinex, but it's been used long enough that we have shared information and know the proper dosage), so there's not a lot of information out about the dosage and timeframe to withhold eggs, how often to use, etc.
SPECIALIZING IN: Jersey Giants. Black Copper Marans. Silver Penciled Plymouth Rocks. Isbars. Click to view my website.
Just for fun: First & second generation "olive eggers," and a few ducks - Welsh Harlequin, Black Swedish, Black Runner.