Hmm...

 Seeing as I have just returned to the list after a
fair break, I figure I should contribute something
(well attempt to anyway).

  How's this..

 If you provide a narrow(ish) passageway, that has a
low inertia "teeter-totter" arrangement, load sensor,
and micro-switch setup.

 So the chickens pass through the passageway, flip the
t-t which also acts as a counter, so you have not only
a weight measurement, but also a count of the number of
um..  er.. "victims" that have passed through, so you can
derive an average from the collected data - into a PIC of course !

 Regards,

         Glenville T. Sawyer

VK5ZCF