It doesn't look particularly desirable does it? However, the tree sparrow leaves some foecal sacs (that's poo, to you and me) of its young, in its nest on purpose. One theory behind this, is that the ammonia given off by the sacs, repels fleas and other such undesirable things.
Anybody who's ever cleaned out a nestbox that's been used by a blue tit or great tit , will testify that these are usually full of fleas and, unless you are properly dressed may end up with bites all over your arms!
Give me foecal sacs any day!
(Thanks go to volunteer Chris duFeu, for the opportunity to get a close-up view of a tree sparrow's nest).