| SPECIES | NATURAL BILL | EXPERIMENTAL RELEASERS | REFERENCES |
| Herring Gull | Yellow bill with red patch | Patch √ Head shape × Patch redness √ Head colour × Bill length √ | Tinbergen and Perdeck, 1950 |
| Laughing Gull | Plain red bill | Bill redness √ Head shape √ Shape of bill √ Head presence √ | Hailman, 1962 |
| Black Headed Gull | Plain red bill | Bill point √ Head shape × Bill length √ | Weidmann, 1961 |
| Sandwich Tern | Plain black bill | Bill blackness √ | Weidmann, 1961 |
| Mallard Duck | Yellow bill | Greenness √ (Grass?) |
Oppenheim, 1966 |