Many years ago there was an issue with some peanuts containing the deadly Aflotoxin mould. Here at Wiggly Wigglers we have all our peanuts tested to ensure that they are Aflotoxin free.
The other problem with peanuts comes from their comparatively large size. Some say that it is possible for young birds to choke on whole peanuts. Because of this, the RSPB recommends that you offer peanuts in a special mesh feeder.
Finally, do not put out salted peanuts in your feeders, as salt in any quantity is not good for many birds.
In short, peanuts are a very popular food for garden birds: it's just a shame that they can't be grown in this country and have to be imported. For this reason I always promote the home grown alternatives such as Farmer Phil's Black Sunflower seeds which are, of course, grown right here in Herefordshire!