I finished watching the videos and I want to change what I said before about secondary stories. I think what Will Terry is saying is that the primary story is a simple depiction of the text so in your case, any painting of a growing storm would be the primary story. Your could just paint wind whipping through trees and you would have satisfied the requirement of the text. The secondary story though is what the illustrator adds that is not necessarily in the text but that creates a new narrative within that scene. Your depiction of the kids and animals affected by the storm then would be all part of the secondary story because they are not explicit in the text and it makes you want to know more about the characters, and the storm’s effects on them, as well as the storm itself. There’s nothing in the text that says anything about kids or birds or people trying to hang on to a post but you created your own story within the story given to you by the text and that secondary story deepens the storytelling, engages the reader’s imagination, and evokes more emotion.
I may be wrong but I think that’s what he was saying in which case, all of the characters, including the birds are secondary stories. Does that make sense?