Here is what the Audubon Society has to say about feeding peanut butter to birds:

http://birds.audubon.org/faq/peanut-butter-or-uncooked-rice-harmful-birds

Is peanut butter or uncooked rice harmful to birds?
Peanut butter, by itself, is not harmful to birds. Some people dilute it with birdseed or cornmeal to reduce its stickiness, but that is not necessary. Uncooked rice is also not harmful. Uncooked grains are a staple for the diet of species of many birds, so throwing rice at weddings is not going to harm any birds.