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Hi cyprinoid,

birds' nests are a very interesting resource not only for flies. From own experience and as already mentioned humidity plays a key role: for instance nests from birdhouses are usually to dry for rearing anything, which may also true for your nest in the garage.
A valuable reference for what might occurring you will find in the papers by Hicks. Full references and some other insights regarding bird's nest can be taken from here.

so long,
xylo