In Chapter 7, what did Bartle remember Murph telling him about some canaries?

Bartle remembered Murph telling him a story about some canaries. Murph's father had brought the caged canaries home from the mine and let them loose in the hollow where they lived. The canaries flitted and sang for a while before they perched on top of the cages. Murph's father had probably thought that the birds would have been ready for freedom.