Sometimes mites/lice will only get one bird, sometimes several, sometimes the whole bunch, just depends. Often if a bird is having other issues, is weakened by something else, the mites/lice seem to find them. And some mites only live in the coop and come out at night while they are roosting to feed on the birds, so checking at night with a flashlight or lantern after they've gone to roost is a good idea. The tape on the roosts, as suggested, is also a good idea. It may not be an issue, but whenever there is something going on with a bird it's a really good idea to rule that in or out. It could be the cause of a problem or make something else worse if they are there.
