House Finch

There are quite a few finches in our backyard. The males are brown with red highlights. They are hard to distinguish from the purple finch. The purple finch also has red highlights- but they are a little more pronounced.
I am often confounded by the slight variations between species. We are studying telescope shiners- a species of minnow- which looks like almost every other shiner I have seen. The professor I am working with says there are two species he can delineate between. It probably is a difference in the DNA.
The problem we have right now is that these finches are coming down with Mycoplasmal Conjunctivitis. It is a swelling in the eye area that blocks off the blood flow to that area causing the feathers to fall out. The bird usually loses sight in that eye and becomes susceptible to predators.
I am puzzled by the patterns I am recording. The ones infected are all male and it is always the right eye. This isn't the same bird because I have been observing the progression of the illness in these individuals.
The finches come in groups. I have seen one group of two males and two females that came to our feeders for about three weeks. One male had the infection and the others seemed to protect him from the danger he couldn't see. I could get within two feet of him on his blind side but if the others saw they would fly into him and move him away. One day they came back without him. And then they moved on.