In the summer, the stress on the birds, especially in our drought-ridden area, means they have lowered immune systems. One disease that often takes hold is trichomoniasis (trich)--most commonly in pigeons and doves, which are very susceptible to this disease. The raptors that feed on these birds also tend to get trich. This is not the same trichomoniasis as found in people.
Trichomoniasis is very contagious and highly fatal in birds. It progresses very rapidly. If you find a dove on the ground that is extremely emaciated, even though it has been eating, it is very possible trich is the culprit. Often you can see a yellowish, curd-like growth inside their mouths. If you see this, you know you have trich. A large number of extremely emaciated doves with no visible signs of injury are coming in to Wild Bird Rescue, suspicious for trich.
If you find a dead dove or pigeon in your yard that may have contracted trich, take down your feeders immediately and empty your bird bath. Leave everything down for 10 days. What you are doing here is dispersing the birds from your yard at least and allowing time for the infected birds to die. Before you refill your bird bath and feeders, clean them thoroughly and disinfect with a 1:10 bleach solution. I am not a big chlorine bleach fan, but it works well as a disinfectant.
Watching these birds waste away is heartbreaking. Clean your feeders regularly and watch for signs of trich in your backyard birds. You didn't cause the disease, but you can help prevent it from spreading.