A simple, relatively inexpensive technique would be a roaming robot scale.
It would have a weighing platform big enough to support just one chicken,
and a food dispenser, perhaps containing some type of food that's more
desirable than the normal feed (is there a chicken equivalent of catnip?)

When the robot detects a chicken on the platform, it would weigh it.  After
getting a solid reading, it would close the food dispenser, and tilt the
scale surface to tip the chicken off.  It would then move off to another
area of the barn, reset itself, open the food dispenser and wait for another
chicken.

It would be a heck of a lot cheaper and simpler than developing a
vision-based system or criss-crossing the barn with laser beams or
chicken-herding barriers.

But if you really like the vision-based approach, the idea above could be
complicated by adding a retinal scanner at the food dispenser.  That way,
you'd know if you were weighing the same chicken twice.

Anton