Well, lets try to register this one:

Take an empty closed box (wood, metal, anything solid), 500 liter
volume, take a compressed air cylinder, 500 liter volume @ 10PSI,
connect both and empty the cylinder into the box. The box will end up
with 5PSI, right?  Simple formula where you use both volumes and both
pressures.

Now, insert any object into the empty box, say 250 liter volume, repeat
the experience. The box will end up with 7.5PSI, correct?

It means that any object (reducing the air box volume) will actually
increase the final pressure when you inject the 500 liter @ 10PSI air
into the box.

Chicken is also "meat (and bones) volume", but not feathers, since air
goes in between.

Install a low pressure sensor at the wall of an empty barn, provide a
somehow good windows automatic sealing during measurement, boom a "x"
liters of 2 to 5 PSI into the barn, measure the delta pressure.  You got
the first reference.

Weight a group of 1000 chicken and put them into the barn, boom again
and repeat the measurement.  You got the second reference.

The delta between references against the chicken count and weight, will
give you the programming parameter.

So, from now on, you have a reference table, use a PIC to convert and
display delta Pressure to Weight.

There is not this thing of a 10 lb small chicken or a giant chicken of
only 2lb, there is a steady relation between volume and weight, and
doesn't matter zones since volume will be the same anyplace into the
barn, so it works.

You just need to ensure "some" barn sealing during boom measurement,
like automatic electric closing windows or doors.  Any small pressure
leakage is automatic compensated by the delta references, if the leakage
is the same from the calibration.

Increase the pressure to 5000PSI and you will have a fast chicken
killing machine, can explode barn walls too... :)

Wagner