It is often not a problem when people are close to birds nests or young birds. As tomoatofamily mentions, birds breeding in urban parks often have large numbers of people nearby. However, the great majority of these people walk straight past the nest or baby - the adult can sit nearby for a short time and then return to feed the chick or shelter the eggs. This does not make the chicks 'stronger and more able to survive', it is just that they become less scared by the presence of people.
The problem is that it is often very difficult to judge when you are causing disturbance. Imagine you see a nest or a chick and go closer to take a look. Maybe you don't notice the adult nearby, so you spend a lot of time near a chick - all the time the adult is sitting close by with food that it does not bring to the chick - if this happens too often the chick will not get enough food to survive. Maybe you have not noticed that there is a cat or a bird of prey nearby - as you go to the nest or chick, the predator watches you, and then knows exactly where to go to eat the chick.
Often the presence of people near breeding birds is not a problem, but sometimes it can be a disaster for the birds. It is always in the best interest of the bird that people spend as little time as possible close to breeding birds. For this reason, I recommend that you do not attempt to photograph nests or chicks.
EricB recently posted details of a recent code of conduct for bird photography which provides advice to minimise disturbance. Check this link:
http://www.hkbws.org.hk/BBS/view ... &extra=page%3D1