There is no scientific data to prove that the periodic use of salted peanuts does harm to birds. This is partly because of the very small amount of salt usually found on peanuts and the fact that peanuts typically make up a small percentage of a feeder bird's daily diet. The availability of fresh water also helps mitigate salt's dehydrating effects.
But it is fact that large doses of salt can be fatal to songbirds. Without additional research to set the levels of salt ingestion that are irrefutably safe, our recommendation will be to provide unsalted peanuts for feeding birds whenever possible.
Response adapted from material from WBU, Inc.