According to this ATTRA publication (pg. 17) large poultry plants average about 8 gallons a bird.
Small-scale operations are generally more water efficient than larger operations.(Data from an in-home situation - no equipment, just doing a few birds for home use - shows that a single chicken can generally be processed with less than 4 gallons.) The ATTRA publication linked above is a highly recommend resource regarding small-scale poultry processing how to and technical questions.
Small-scale poultry producers and processors might be further interested in this webinar on poultry processing exemptions:
http://www.extension.org/pages/Poultry_Processing_Exemptions_Webinar
