Yikes! I just put 15 Giant Blue Cochin eggs in the incubator today. They were shipped a long way, and we had a record high of 99 degrees today, but I can let you know in about three weeks what happens.
I hatched 11 out of 13 fertile twice this spring from standard cochin. Actualy 12 out of 13 in the second hatch but one baby was born with a nerv problem and had to be put down. If it is your own hens you need to cut the butt-duns and feathers around the opening very short to allow proper mating. I also learned from other breaders that they tend to hatch late. My first hatch hatched during day 22 and 23 butpartially because we had an electricity outage in the middle of the incubation. The second time all my own eggs hatched on day 21 but the eight eggs I bought with blue standard cochin hatched on day 22.
I have only tried Cochins one time, I had 10 eggs shipped to me and I only ended up with 2 making it to day 18 and hatching. Mine didn't hatch late, in fact they were the first 2 chicks that hatched out early in the day of day 21, followed by 3 silkies. Mine were shipped, and I really think that it takes a toll on them. Good Luck!