Why Are These Chicks Black If The Father Is A White Plymouth Rock?

[The Moonshiner]

I'd always heard it was 80% to 85% but I'd believe 90%.
But again that's barred rocks not SF barred males.

 

[nicalandia]

I'd always heard it was 80% to 85% but I'd believe 90%.

Yes, most of the time is about that percentage, only a few researchers have reported about 90%

The Canadian Department of Agriculture (Circa 1941) issues an excellently illustrated bulletin describing the method: https://archive.org/details/sightsexingbarre00cana I found this bulletin and thought it was an exceptional piece to be shared, it claims that since there is a tendency to classify males as females (an not vice-versa) so when in doubt, the chick tend to be a male.

But again that's barred rocks not SF barred males.

I have only mentioned Barred Rocks due to the OP White Rock rooster that is clearly hiding Barring(basically a Barred Rock painted in White due to recessive white) and the Sex linked video of the chicks was to ilustrate how a Barred Rock single barred males may look like.

 

[NatJ]

After staring for a while at both videos, I think the single-barred sexlink rooster in one video has a larger head spot than any barred rock in the other video (single-barred hens or double-barred roosters.) So I'm going to assume there's some kind of modifier genes at work, in addition to one vs. two copies of barring being present, possibly also with the addition of gender-specific hormones as suggested by nicalandia.

Nothing like a bit of confusion about something that started out seeming simple...

 

[Pretty Birds]

For the OP's chicks:
If the White Rock father is barred (hidden by the white), then all chicks would carry one copy of barring. The chicks who have a barred mother (Marans, Delaware, Barred Rock) will have a larger yellow spot on the head IF they are male (because a male can have two copies of the barring gene, B.) Any female chicks, and any male chicks whose mother was not barred, should have a smaller or not-present head spot (because they'd only have one copy of B.) The chick from the Easter Egger, for example, will not be sexable by head-spot.

Thank you! This cleared up most of my confusion!