In 2003, a writer named Jonathan Corum created a site devoted to answering the question scientifically, with the use of "alternate graphic presentations for kinematic ratios in winged flight."[3] He found the following conclusion:
Although a definitive answer would of course require further measurements, published species-wide averages of wing length and body mass, initial Strouhal estimates based on those averages and cross-species comparisons, the Lund wind tunnel study of birds flying at a range of speeds, and revised Strouhal numbers based on that study all lead me to estimate that the average cruising airspeed velocity of an unladen European Swallow is roughly 11 meters per second, or 24 miles an hour.
Anyone know the answer to the question?