I would seem obvious that chicks need to be fed by their parents. If the parents are disturbed by humans encroaching the nest the parents will flee and feed less or even abandon the nest. In urban areas some birds have flourished eg tree sparrows, domestic pigeons and some starling species. These birds are more likely to be habituated to human contact and be much more tolerant of disturbance. In more rural areas, birds are much less tolerant of disturbance. I have to avoid going onto the roof of my house at the moment as black-necked starlings nest in a palm nearby and will not fly to the nest with food. They will sit and wait until I have gone out of sight before moving to the nest. They are clearly agitated, perching watching me, flying in circles calling and returning to the same perch. My presence disturbs the nest and endangers the chicks so I avoid the roof.
If you are aware of bird's normal behaviour it easy to see when they become agitated.
Dylan
