@jwolfanger - Thanks for attaching multiple pictures. I'll give it a go for you.
Assuming you have a Rachio Gen 2 16 zone controller. Also, assuming that all the wires (except the two attached to the SENS terminals) are in the same 12 conductor (or more) cable.
First, take tape and a marker and label the wires as you take them off the Rainbird. The pictures are great, but it looks like there are multiple wires with the same color.
Rainbird (RB) COM white wires goes to any Rachio C port
Green wire in RB 1 goes to Rachio zone 1
Red wire in RB 2 goes to Rachio zone 2
Brown wire in RB 3 goes to Rachio zone 3
Blue wire in RB 4 goes to Rachio zone 4
Green wire in RB 5 goes to Rachio zone 5
Yellow wire in RB 6 goes to Rachio zone 6
Red wire in RB 6 goes to Rachio zone 7
Brown wire in RB 6 goes to Rachio zone 8 (I'm splitting up the three wires in RB 6)
Blue wire in RB 7 goes to Rachio zone 9
Black wire in RB 8 goes to Rachio zone 10
And I think there is a yellow wire in RB 9 and I'd put that in Rachio zone 11
I'm assuming you have a rain sensor that is connected to the Rainbird SENS terminals. If not, then what is connected there? If so, do you want to reconnect the rain sensor to your Rachio or just use the selected weather station. If you want to connect the rains sensor, then put the RB SENS black wire in the Rachio S1 port and the RB SENS red wire in the Rachio SC port.
The reason I split out the wires on the RB zone 6 is because the Rachio can drive two solenoids at once, but I'm not sure about three. Plus, you can now control those two zones separately.
Good luck and let the forum know that it worked or didn't. If it didn't work, from what I understand Rachio has terrific wiring folks on their support staff.