SEXING: 

Most 1cy birds should be left unsexed, while the vast majority of 2cy+ can be sexed according to the below.

1cy: According to Svenson (1992) birds without any black on head should be left without sex, while birds with rather much black in crown and nape should be males.

2cy+: Males have a rather clean white forhead and a lot of black showing in crown and nape. Females have a darker grey forhead and no (or little) black in crown and nape.