It means that something is obvious, like a cormorant (a bird nicknamed a shag) with its wings spread out to dry.
The Cormorant or common Shag,
Lays eggs inside a paper bag.
The reason you will see no doubt,
It is to keep the lightning out.
But what this unobservant bird,
Has failed to notice –
The hoards of wandering bears,
May come with buns and steal the bags to keep the crumbs!