Why are ecologists unsettled on whether or not any truly commensal interactions among organisms?

Ecologists are unsettled on whether or not any truly commensal interactions among organisms take place or not because it depends on the kind of organisms. When a bird makes a nest in a tree and lays eggs, the eggs are definitely safe. This procedure does not harm the tree directly, but it can stop the sunlight from reaching some parts of the tree. This actually shows that the bird has benefited from the situation but the tree has lost out.