This was in a poetry anthology I had as a child:

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 the unobservant birds
Have never noticed is that herds
Of wandering bears may come with buns,
And steal the bags to hold the crumbs.

The anthologist, Louis Untermeyer, was apparently ignorant of the word "cormorant", and so introduced the poem by explaining that a cormorant was a fantasy bird invented by the poet.