Good morning shivvy,
Rice and yellow split peas are often added the cheaper wild bird food mixes, mostly it seems to bulk up the weight and volume.
There has been an urban myth thats been around for 20 years or more, about dry rice being harmful to birds, which seems to have originated from a weeding invitation and is now considered erroneous, having said that I,ve noticed that the RSPB do not recommend dry rice (The Birdfeeder Handbook 1990).
If you are concerned, perhaps try a small qauntity to see if the eaten, or maybe soak some over-night. Birbs eat grit to help them digest, so I can't see it harming them I M O.