Homing pigeon experts attach great importance to the eyes of homing pigeons. The main parts they study are pupils, eye marks, inner openings, bottom sand, top sand, etc. Unfortunately, in addition to these parts, there is also an external sealing sand that cannot be underestimated but has been ignored. Outer sealing sand, its specific location is on the outer edge of the surface sand (see picture), and it is most concentrated and obvious at the two corners of the pigeon's eyes.
Outer sealed< span style="font-size: 1em;">Sand, some are in the shape of one or several arc-shaped lines, some are in the shape of flakes, or lines or flakes coexist. So I call it "outer sealing sand". Some people compare the function of the pigeon's character circle or eye mark to the light pattern of a camera. If this is the case, then the outer sealing sand should at least be used as a "second aperture".