1 horsepower (HP) was defined ( reference: ) by taking a horse that could pull with a force of 180 lbs, and the calculation comes out to ~32,572 ft*lbf / min.
To estimate the pulling power of a duck, I will consider that they have enough force to propel themselves up into the air, so that is at least a force matching their weight (1.6-3.5 lbs). Since birds of prey can sometimes take off with a fish (perhaps 1 lb), I'll estimate that a duck or other similarly-sized bird can generate 4 lbs of force. HP formula from wikipedia gives:
P = W/t = F * d / t = 4 lbf * 2.4 * 2pi * 12 ft / 1 min = 723 ft*lbf/min
But this simplifies to a comparison of 4 lbs of force in the duck to 180 lbs of force from the horse..
180/4 = 45.
So perhaps 45 ducks, if they could all apply maximal force in the same direction, at the same time, might be able to generate the horsepower of 1 horse.