size of an acorn.

Well, the two Mason-bees do not hesitate long in front of these
exchanged nests, not three feet away from each other. Each makes for
the site of her late home. One, the original owner of the old nest,
finds nothing but a solitary cell. She rapidly inspects the pebble
and, without further formalities, first plunges her head into the
strange cell, to disgorge honey, and then her abdomen, to deposit
pollen. And this is not an action due to the imperative need of
ridding herself as quickly as possible, no matter where, of an irksome
load, for the Bee flies off and soon comes back again with a fresh
supply of provender, which she stores away carefully. This carrying of
provisions to another's larder is repeated as often as I permit it.
The other Bee, finding instead of her one cell a roomy structure
consisting of eight apartments, is at first not a little embarrassed.
Which of the eight cells is the right one? In which is the heap of
paste on which she had begun? The Bee therefore visits the chambers
one by one, dives right down to the bottom and ends by finding what
she seeks, that is to say, what was in her nest when she started on
her last journey, the nucleus of a store of food. Thenceforward she
behaves like her neighbour and goes on carrying honey and pollen to
the warehouse which is not of her constructing.

Restore the nests to their original places, exchange them yet once
again and both Bees, after a short hesitation which the great
difference between the two nests is enough to explain, will pursue the
work in the cell of her own making and in the strange cell
alternately. At last the egg is laid and the sanctuary closed, no
matter what nest happens to be occupied at the moment when the
provisioning reaches completion. These incidents are sufficient to