This verse is often given as Anonymous, but is actually an early work by Christopher Isherwood.
This is how I remember it. There have been a couple of versions in print, and I doubt that my memory matches either exactly.
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 come by, with buns
And steal the bags to keep their crumbs
[Edit: I got it all right, except the last couplet, which as originally published went
Of wandering bears may come with buns
And steal the bags to hold the crumbs
I prefer my mis-remembered version.
It seems he also revised it for a later publication, but the first version is better.]