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.
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