chicks with curly tails

[CovenantCreek]

Is there any rule of thumb regarding which chicks develop curly tails first? Among my week old chicks, 2 of the BRs and maybe 5 or 6 of the Dellies have the cutest little tails. The others have nothing to speak of. I'm about 98% sure one of the BRs is a cockerel and I'm hoping maybe this is a clue about the Dellies since the tails don't seem to coincide with age.

 

[speckledhen]

Usually the BR girls get tails before the boys, but the boys catch up fairly quickly. My Delaware cockerels got very pink combs at about four weeks old, but I dont recall a tail difference.