The Common Cormorant

"The common cormorant or shag
Lays eggs inside a paper bag
The reason you will see no doubt
It is to keep the lightning out
But what these unobservant birds
Have never noticed is that herds
Of wandering bears may come with buns
And steal the bags to hold the crumbs."

Christopher Isherwood

Though according to QI the cormorant isnt a shag. Or vice versa