Are they only doing it around the garden where the bird seed is? Even if they are not doing other places, since it seems to have started since you have been feeding the wild birds, you might want to stop that and see if it helps. (You probably don't want to encourage wild birds where the chickens are anyhow, since they can carry diseases and pests the chickens can get.) Do you free feed your hens / do they have food available all the time? If they do it when you are there, I'd consider spraying them with the garden hose when they go after the dogs, most chickens don't like that. (assuming the weather is warm enough).