Chestnuts are one of the more difficult birds to sex. There are a few guidelines though that may assist in sexing the birds:
Sexing Birds
These articles will help you tell the difference between mail (cocks) and female (hens) of various bird species.
Walter Burgess: Sexing 101 - species# 2 - Five-coloured Mannikin or - Lonchura Quinticolor:
Walter Burgess: Sexing 101 - species # 3 - Java Sparrow - Padda oryzivora :
Walter Burgess: Sexing 101 - species # 4 - Timor Dusky Sparrow - Padda fuscata :
Walter Burgess: Sexing 101 - species # 5 - Moluccan Munia - Lonchura molucca :
Walter Burgess: Sexing 101 - species # 6 - Javan Munia - Lonchura leucogastroides :
Walter Burgess: Sexing 101 - species # 7 - Lonchura cucullata scutata - Bronze Mannikin :
Walter Burgess: Sexing 101 - species # 8 - Lonchura fringilloides - Magpie Mannikin (incorrectly called the Pied Mannikin) :
Walter Burgess: Sexing 101 - species # 09 - Lonchura cantans - African Silverbi
Walter Burgess: Sexing 101 - species # 10 - Lonchura malabarica - Indian Silverbill :