I'm not sure how they came to the agreement.
Mama blue jay will hang out in the trees near the barn until I let the chickens out in the morning. Once they're all out and eating she'll leave. If the baby starts chirping one of the RR or an Australorp will climb up and sit on the nest.
If one of them is in the nest when she comes home they just step aside while mama feeds the baby. If she sticks around they leave, if not, back on the nest.
We have hawks, bald eagles, and buzzards. If one comes into the neighborhood the jays sound the alarm. I've seen them look right at the chickens and squawk. Once the chickens are hiding the jays fly up and harass the offender.
It's a good system, and I don't intend to screw it up.