A SMALL BIRD, that is by implication, A SMALL PECKER. Asian artists represent lewd and vulgar behavior using small birds with elements of「young and in mating stance」that describes a SPARROW:
「House Sparrows can breed in the breeding season immediately following their hatching, and sometimes attempt to do so. Some birds breeding for the first time in tropical areas are only a few months old and still have juvenile plumage. Birds breeding for the first time are rarely successful in raising young, and reproductive success increases with age, as older birds breed earlier in the breeding season, and fledge more young. As the breeding season approaches, hormone releases trigger enormous increases in the size of the sexual organs and changes in day length lead males to start calling by nesting sites. …Birds of a pair copulate frequently until the female is laying eggs, and the male mounts the female repeatedly each time a pair mates.」
[ https://en.wikipedia.org/wiki/House_Sparrow 2013-11-07, emphasis ours ]
KNOWLEDGEABLE PERSON results from knowing it all and telling everyone else about it,「something that ought to be stopped when being a jerk.」Or, performing sexual functions instinctively and while extremely young, and/or due to having bones in its tongue, as sparrows do…with which to hold seeds, although recent tongue piercing by humans in order to emulate tongue bones is normally performed for reasons of improving fellation technique, they say.
A LENTIGO is a skin mole or dark spot that is potentially malignant and a precursor to melanoma, skin cancer. Seems related to the freckled coloration of the various SPARROW species. Furthermore, due to the bird's association with vulgarity, lewdness and juvenile copulation, traditionally in Asia many folks have disliked seeing or having freckles…but other folks seek them out as a good omen for the same reasons others seek young females with hairy forearms as one indication of blood testosterone levels (young females normally acquire higher testosterone levels only by ingesting the hormone, that is to say, by swallowing as it was ejaculated or as part of one's regular diet). In modern times, attitudes about freckles are changing, somewhat, since human freckles actually indicate nothing in regard to vulgarity, lewdness, and juvenile copulation with small peckers or chicks, or the closeness of any genetic relationship to sparrows with or without boners on tongues.

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: small bird; sparrow; also pr. ; a freckle; lentigo
Japanese EDRDG: sparrow
Unihan extended: sparrow

Used within names (nanori)
ざく