@marcelcosta I wish I could give you a general answer but it varies so much across species and, critically, situations. Distances that would be no problem for most of the year can be intensely stressful during nesting season.

It's all about knowing the species you are photographing and assessing the behavior. Here, I was laying on the ground and concealed in the grass with only the lens protruding. Mother was unconcerned with my presence and feeding normally until the other birders arrived.