The sentinel scrub-jay is tasked with the duty of keeping a lookout for predators while other birds forage for food or perform other daily tasks. There is no one bird slated for this task. They take turns doing it.
It is an understood duty among the group that one bird must be on point in order for the entire group to survive. This makes sense for a bird species that acts communally. They live as family units with the older offspring staying around to help raise new fledglings. Sometimes stray or solo birds will be allowed into the family unit.
The sentinel bird will give one loud urgent sounding squeak if he or she notices something on the horizon. This is an alert squeak and is meant to get the other bird’s attention. The rest of the birds will often stop what they are doing and instantly become much more aware of their surroundings.
If no danger is present the sentinel bird will not make any follow-up squeaks. But if danger does present itself then many multiple squeaks may follow. The more squeaks and the faster the speaks amounts to the level of danger at hand. The sentinel will continue to squeak before it too dives into the scrub for cover. At this point the sentinel and all other birds remain quiet and still.
Very young juvenile birds are not expected to perform sentinel duty. Only when the blue head and neck feathers have grown in do you see a bird regularly perform this task.
It is not uncommon for scrub-jays to alert and dive into the bushes a dozen times or more in a single day. Predators are on the prowl. When the other birds have done what they needed to do the sentinel will then come down to secure food for itself often times being replaced by another bird.