For example , in house sparrows , melanins are produced only in the presence of testosterone ; castrated male house sparrows -- like female house sparrows -- have brown beaks .
Typically , such a color difference is due to the presence of androgens .
In house sparrows , melanins are produced only in the presence of testosterone ; castrated male house sparrows -- like female house sparrows -- have brown beaks .
Typically , such a color difference is due to the presence of androgens .