My african grey has started plucking chest feathers out, WHY?

There can be a variety of reasons why this has started. It can range from a medical problem to a behavior problem. If there has been any changes in the house this can cause stress to the bird. They respond in different ways. Some will start screaming, others pluck or develope other destructive behavior. It can also be a medical problem. If he has had contact with another bird he could have contracted something from them. It would be best to start with an exam by your vet to look for and rule out any medical issue. This would require bloodwork and a stool check to be done. These would look for problems with the liver or kidneys and also look for intestinal parasites which can cause feather picking. If these are negative then it may be necessary to check for heavy metal poisoning. If he has gotten any new toys recently that are metal, it could be one of them that is the culprit. Although many toys are sold as bird safe they can contain metals like lead or zinc which can lead to the feather picking or worse. If all the labwork is normal then you can start to work with him like this is a behavior problem. There are medications that can be used depending on what your vet feels would be most useful. Talk to your vet, let them know about everything that is going on with your bird, new people, new place in house, new routine, pets, anything that has changed from before he started to pluck. This will help to narrow down what recommendations your vet will give you.