Returning from Uni, i went into the shed where my training wall is. Intent on actually using it i set about making some routes/problems and then went to get my shoes, that i'd left in the garage over the winter.
The Swallows, that reside in our garage every spring were happily buzzing around the roof space crapping at will. I was quite shocked to find my shoes had become the early attempts at a home, unfortunately for the poor Wren or whatever, i decided i wanted my shoes back.
So, if you have any old shoes - lop 'em in a shed and see what happens.