House Martin eggshell

I don't pretend this has any photographic value at all.   It is purely a record for myself.  We have three House Martins nestboxes.  One has been commandeered by Sparrows, one I have seen adults going in and out.  I assumed the third was empty.  This morning I found this half eggshell below the third box.

In case anybody doesn't see the significance of this let me explain.  When an egg hatches the parent birds fly a distance away with the shell before dropping it.  This is to avoid giving predators a sign as to where the nest is.  I can only assume that either the parent dropped it accidentally leaving the nest or given the nest is very high and has a tiny entrance they felt safe just dropping the shell.