Wow that's tough one.. Apart from the well known size / shape factors of the European swallow as apposed to the South African swallow. We must also take into account the Age of said swallow, as as the swallow ages the feathers on the wings start to soften and change shape.. thereby disrupting the laminar airflow and reducing a average top speed of 12 M/s for a young swallow to below 9 M/s for a elderly swallow. Aww Darn I forgot to calculate the Ballistic Coefficient of the Beak.. Well then I will have to say... 42. 😋 God Bless you and Yours Jimmy!