A lot of my fixtures had wasp nests inside....
More likely they entered the slipfitter area....thats where most commonly it happens....
McGraw Unidor are commonly known to get bird nests inside!
Westinghouse MO-8s gets bird nests a lot from an opening below the socket if the factory made cover got broken!
Older Silverliners seems to rarely get those junk get in, although it may be a bit different on newer ones, depending on the slipfitter design.