Why flies?

God in his wisdom made the flyAnd then forgot to tell us why.

-Ogden Nash

This little ditty has been going through my head for the past several weeks.  We have frequent fly infestations here, the latest of which has take over Ryan's bathroom.  They do seem to prefer our bathrooms in general (big mystery there) but particularly this bathroom because we have a pigeon's nest in our fan.  A couple months into our stay here, we covered the fan with a plastic bag and sealed the edges.  This was immensely helpful until they cleaned the outside of the building and all the nest (or at least all the excrement) was sprayed into that bathroom.  We covered our faces in towels (for fear of bird flu, and of course, the smell) and disinfected it until it sparkled!  But then came the flies.  We turned on the bug spray plug, and this morning, here is what we found (enlarge pics for best* viewing):