There is a classic post about how Liam found that skull and lost his stuffed monkey at the same time.
Read it here.
Flashing forward six years...I kept seeing the wrens perched on the skull with food in their bills, and it took me a little while to realize that they had made a neat little nest down inside the brain case. Before long we could hear the baby wrens piping in there, and soon they were big enough that the adults would pop onto the skull, poke a spider or caterpiggle into the foramen magnum, and be gone. They were a very sneaky wren family, and I never saw them fledge. I think they did it during a thunderstorm when I was inside all day. When we took the skull down to look there was feather sheath dust in the nest, which generally means the babies made it to fledging. Being wrens, they kept their nest spic and span.
There is nothing like showing an animal like that to your kids. I'm so glad we did, when we could, because animals like this one don't come along every day.
Neither do boys like Liam.