The jokes pretty much write themselves.

Some sources are saying swamps, others are pointing at industrial areas.

My own guess would be somebody busted a tank of the odorant they add to natural gas to make it smell like that. Pure gas has no odor.

They add this mercaptan odorant in tiny amounts to the natural gas. If someone broke open a tank of the pure stuff, you can bet it could stink up a whole city.

The flaw in this theory is you'd think it would have been immediately reported, unless somebody's trying to avoid lawsuits.

Large numbers of dead birds can have many causes. I suspect that somebody poisoning annoying pigeons would not want to publicize his actions any more than somebody who busted open a tank of stink.