If a robin has lost its redbreast in summer, and the bird looks more like a pigeon, then it is possible that it has been parasitized, which means that it has been invaded by a parasite. The parasite is a fly that lays its eggs in the robin's body, and the eggs hatch into larvae. The larvae also feed on the bird's blood. The robin will be infected and will die. The parasite is a fly called the red-clawed fly. There are a few things that can be done to help your robin. You can help the robin by getting rid of any dead birds that have been found on your property. You can also take it outside and allow it to feed on natural sources of food. If you see a dead robin, or if you are concerned that it's been parasitized, you can call a wildlife rehabilitator for help.