Does a rainbow have any known impact on anything in nature, even the tiniest thing would qualify? Is there any evidence that birds might fly in that direction? Anything?

I don’t know if this counts as nature, but I had a ringneck parrot who stood near a crystal which made a rainbow on her shoulder. She saw the rainbow and tried to pick it up and was quite annoyed when I laughed.

Of course they have an impact. Where else would leprechauns stash their gold?