Poultry Talk Ontario (pto.ca)

My guess as well!

"Triplets are growing in leaps and bounds and 5 weeks later one more pathetic singleton hatched by another broody (MUST STOP doing that)
and since the last single chick grew up to be a neurotic mess, I am trying to socialize him with the triplets.
So far so good, he/she seems to accidentally wonder in to the triplets domain on regular bases and they seem to be ignoring it completely, so will wait few more days before I lift the divider and see what happens. Just need to find a day that I can spend babysitting just in case things go sideways.