Outfield collisions can be very dangerous. Whoever on the field that wants to catch a fly should call "Ball, ball, ball". In the outfield, the center fielder is in charge. If a fly ball is hit between an infielder and an outfielder, the outfielder is in charge.
(The warning track [that goes around the field, between the grass and the wall] is there to "warn" fielders that they are running out of room.)
