I think overcrowding is a valid concern although for the most of it, the recommended guidelines are pretty 'commonsensical' and people do practise it already. However in the instances where photographers are massed (even at a safe distance away), the birds may already be fairly used to humans' presence. Otherwise they would already have flown off when they detect even a small number of people in the vicinity.
As in the case for any subject matter, it is usually only a few 害群之马 who spoils it for everyone else by engaging in kiasu actions that also cause birds distress as evidenced by the Tuas incident last year.
I doubt there would be a good measure to limit the number of people at such sites. It would probably have to be appealing to the community to voluntarily move on when there is already a substantial number of people in the area which would be dependent on the space available too.
Bro Turbo had a response that resonate my thoughts as well...
Who put the most stress on birds? I think the development of these lands put the most stress on the birds, a lot more than the birders do. The works in the area where the pintail was last year and half... the soon to come destruction at bidadari.... etc etc...
Its the press , obviously they have a way with words. I've been staying in Punggol for the last 3 years, and in the last year alone - one would be shocked at the sheer amount of habitats lost under the reasons of development.
But then most importantly is the heart of the photographers themselves. If they seek out to photograph nature without any thoughts of harming them for selfish reasons, i think we are already halfway there.
that said, i'm surprised there's so many with birding lenses, those aren't cheap stuff.