The kinematics of the paddling stroke of ducklings swimming in formation were analyzed to detect differences in relation to swimming effort and position in the formation. Paddling motions of the feet were filmed as ducklings swam in a constant 0.3 m/s water current behind a decoy which could be in the water or suspended above the water. Ducklings were tested in clutches of one, two, and four ducklings at ages of 3, 7, and 14 days of age. Ducklings swam in organized formations with the lead duckling 0.25 body length posterior of the decoy and with an average inter-duckling distance of 0.1 body length. Stroke frequency was constant within any age and decreased with age. Age, decoy position, and clutch size affected the length of the arc traversed by the foot through the power stroke for the most posterior duckling in the formation. Trailing ducklings had an arc length 16.9% less than leading ducklings, and ducklings swimming in formation had an arc length as much as 29.8% less than a solitary duckling. These results suggest that mechanical energy is conserved when formation swimming is employed. © 1995 Wiley-Liss, Inc.