I found only one official research article which included coffee as feed for chickens.
The Inclusion of Coffee in Commercial Layer Diets, published in 2011, included 125 laying hens which were split into groups: a control, one fed 1.2% of their diet with caffeinated coffee (roughly 9 mg of caffeine per bird, considered a “moderate” amount), and the other fed a diet with 1.2% decaffeinated coffee. The study lasted between 21 and 35 weeks. The authors were assessing the affects coffee might have on the feed intake, egg production, and egg quality of commercial laying hens. What they found was there was no significant difference in feed intake, egg production, egg weight, egg mass, or feed conversion rate; however, egg shells were slightly thinner. They conclude by saying, “No scientific articles on feeding coffee to poultry were found, and therefore, further studies using coffee dregs, because it is a cheap byproduct and with economic potential, are recommended.” The unspoken conclusion here would be coffee does not appear toxic to chickens when consumed continuously in moderate amounts for a period of five to six months.