Personally i'm not really attracted to them as a food.
Don't know much about them either. Feces would be a problem then again the saliva seems to be the key ingredient to the nests and they are harvested from a specific type of bird:
How it's served:
The nest used in bird's nest soup are composed almost entirely of saliva with little or no plant material.
In urban areas, such "bird houses" may be considered a nuisance by neighbours due to the loud bird calls and bird feces.
The quotes are from the second link. It looks like the birds build their nests out of saliva but drop the feces somewhere else which kind of makes sense.
Saliva can be dangerous...long ago (2006? 2007? maybe earlier) some guy in Vietnam got infected by feeding his fighting cock with his own mouth but these birds are not aquatic and they probably don't hang around with to much other birds.
They live of insects they catch in the air.
Many of the regular infections in chicken flocks come through environmental sources like water shared with ducks or particles shed by much more commonly infected wild birds nesting in their buildings.
If i was into birds nests i wouldn't worry about bird flu too much.
Remember the insistent: properly handled chicken is safe.
Here's a recipe for birds nest soup:
Preparation: Soak bird’s nest in cold water
overnight. Drain and rinse. Spread softened nest
pieces on plate; pick out prominent pieces of
“foreign” matter (e.g. feathers, twigs) with tweezers.
Now the fun starts:
You'd buy that local. Is HK RFID-ing their birds? Ganzhou?
Remember to properly handle that chicken!
Cooking: Bring rich chicken stock for soup to boil.
Immediately add bird’s nest; simmer 30 minutes.
I do wonder why China came up with the requirement & it could be some sort of bullying and who knows what lesser conflicts remain between China & Malaysia?
As a serious anti bird-flu measure it's BS but it's fun to run into these subjects you never ever think about in normal life.