Complex sentence: The original use of "pecking order" referred to the expression of dominance in chickens, asserted by various behaviors, including pecking, which was used by Schjelderup-Ebbe as a measure of dominance and leadership order.
Split into simpler sentences: The original use of "pecking order" referred to the expression of dominance in chickens. Dominance in chickens is asserted by various behaviours, including pecking, which was used by Schjelderup-Ebbe as a measure of dominance and leadership order.