The black bib marking of the male gender

The male common house sparrow has a black throat ("bib"). It becomes most visible during winter and spring after the moulding season of autumn.

Males with larger bibs are preferred by females even though they are more likely to cheat on the females in an otherwise monogamous relationship.

Researchers have shown that the males with larger bibs tend to have larger testes (meaning larger sperm count), and in feathers less ectoparasites and fewer fault bars (a perpendicular line in feathers which indicates a period of stunted growth, sort of like a dent or white spots on peoples' nails). These sparrows are more likely to preen often to display their larger bib, which is another plus for health, while males with smaller bibs may not do so as frequently in order to avoid hazing. Females choose males with a larger bib to benefit a higher chance of fertilization and a lower chance of catching parasites during courtship.