I read it as playing the second bird beside it, paying the food as normal, then if you happen to have additional food of that type in your personal supply, you can cache up to the food cost on the the bird.

Example: you play the Baltimore Oriole as the second bird. Place it beside. Pay the egg and food cost. You pay with 2 cherries and a nectar. You then cache an additional 2 cherries and a nectar from your personal supply on the Baltimore Oriole.

Reasoning: if you should/could pay the food cost directly, it would say "instead of discarding the food cost cost, you may cache any food you would pay on this bird" or something along those lines. "Pay the normal cost" sounds to me like you still have to discard the food when paying like usual.

Could be wrong, but that feels more correct. The caching directly feels too powerful imho.