This favourite, remembered from my treasured paperback Faber Book of Comic Verse, is a wonderfully unlikely effort from Christopher Isherwood.
The common cormorant (or shag)
Lays eggs inside a paper bag,
The reason you will see no doubt,
Is to keep the lightning out.
But what these unobservant birds
Have failed to notice is that herds
Of wandering bears may come with buns
And steal the bags to hold the crumbs.