Four invented, flat objects face a viewer. This calculator shows angular sizes, not what a bird subjectively perceives.
Formula: angle = 2 × arctan(width ÷ twice distance), converted to degrees. Object positions are fixed at 0.5, 1, 1.5 and 2 m from the starting viewer position. A backward move adds the same distance to all four.
Try the size gradient at zero offset. Each angle matches. Move backward and the match breaks. Equal physical sizes instead get smaller in angular extent with distance.
Real stones are three-dimensional and can conceal one another. Light, head movement and visual processing also matter. These calculations isolate geometry only.