The heaviest flying birds (swans and bustards) have body masses of about 33-40 lbs/15-18 kg, or about 10,000 times that of the smallest bird (Bee Hummingbird). The largest bird alive today- the Ostrich- is 75,000 times heavier than the hummingbird. By comparison, in mammals, the biggest (Blue Whale) is about 75 million times heavier than the smallest (a shrew)! If you limit the discussion to land mammals the factor is 4 million.
What limits the maximum size of birds? Here is the latest on the subject:
That's an interesting article John. Before reading it I would not have guessed that feathers were the issue. I would instead have attributed bird size limitations to the same factors that limit the size of mammals, as discussed, e.g., at: http://www.sjsu.edu/faculty/watkins/scalelimits.htm.
Great read, simple and to the point. I too thought it was a physics argument. Remembers reading somewhere that nothing over 40 lbs could fly. The limits of feather reproduction is logical, nice article.