Killdeer Nest

This is the time of year I stumble on nests and young.  It is always an ethical question to either ignore a photo opportunity or back off completely. The mother is instinctually tied to the nest but there is an unseen line in the sand which when crossed will result in the loss of the nest.  What do I do?  I usually try to get a couple quick shots and back off.  Have my actions resulted in loss of the nest? I'm sure they have.  What do you do when you come across a nest?