Stability?
 
 
1 horsepower (HP) was defined by taking a horse that could pull with a force of 180 lbs, and the calculation comes out to ~32,572 ft*lbf / min. Historically they were comparing a horse walking in a circle, turning a mill wheel, to replace steam engines.

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. Using this value in the 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.
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