I'll build on the ideas of others (perhaps this means I'll have to share
the $$...NOT!)

Put the scale in front of the feeding or watering station, as others have
suggested, but add a trap door so that if the chicken is at the right
weight, the all-important PIC processor opens the door and the chicken
gets dropped into a chute and sent to the processing plant.  This way,
EVERY chicken is at optimal weight when sent.  Talk about QUALITY!
REPEATIBILITY! STANDARDIZATION! Whew! (Any more buzzwords I can use?)

On the lighter side, you could put boiling oil (or freezer) beneath the
trapdoor.  Talk about FRESH!

If you don't have room to build trapdoors and chutes beneath your houses,
use the catapult to get chickens out of the place.  If you have a strong
enough catapult, you can throw them direct to the processing plant,
saving transportation costs.  Since the chickens are all the same weight,
trajectory won't be a problem.  High velocities may be able to de-feather
the chickens during transport.

Or you could use vacuum funnels to suck the chickens off the scale.  It
doesn't matter if you suck the guts out, because you had to do that
anyway!  Or use a vacuum transport system like some stores and banks use
to transport money canisters.  Unfortunately, this idea sucks.

If you really want to use trucks, but them beneath the trapdoor, or at
the end of the vacuum tube, or on the catapult target.  Use a scale to
determine when the truck is full and should be sent off to the plant.
The PIC's could be networked to pause sending chickens while the trucks
are being changed.  Come to think of it, they could count chickens being
put in the truck, and not need to weigh it.

If you use more than one of my ideas, I'll give you a 10% volume
discount.

Waiting for your check.
Don