Can’t speak for any other brand but they are sonar shadows on the ‘Birds.

Sonar shows basically two things: how long it took the sonar signal to go out and bounce back from some object (bottom, fish, structure…) and the relative strength of that signal when it returns. This gets graphed on the display as depth or distance for us humans to understand and either a shade of color or multiple colors to represent the relative signal strength. Sonar shadows are showing that for that distance from the transducer, there was no sonar signal returned since there was something blocking it. Since the display is showing distance from the transducer; we are in effect looking at the sonar returns from a ‘heads down’ perspective.

Does that make sense?