This number also lines up with the measured velocities of real birds, which means we're on the right track for the mathematics. So, answering the main question here, the "airspeed velocity" of an unladen swallow (European) is something like 20.1 miles per hour (32.4 kph) or 29.5 feet per second (9 meters per second).

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...