In the initial version of 'Shoe Girl', I unthinkingly wrote "rat's nest". Then, overthinking the situation, I decided a nest was likely to house multiple rats and therefore should be a "rats' nest". Belatedly I remembered Joe Internet and discovered that the colloquialism for a 'messy disorder' is ubiquitously spelt "rat's nest". So I'm in the process of changing it back again.
Although no longer relevant, I checked a 'nest made by feathery dinosaurs' for comparison purposes. The winner was "bird's nest", but "birds nest" is rapidly gaining in popularity.
And just to prove you can be wrong all the time, the Google summary of the Wikipedia (spit!) entry reads: "Bird's nest. A bird nest is a place where birds lay and hatch eggs."
For those who are downloading the story, when the next new chapter goes up, hopefully this afternoon UK time, you might like to refresh your copies of the earlier chapters.
AJ