(3-6)

so at 2 ms-1, we get m1  =  2.5 / √(1 -  22 / (3 x 108)2)   =  2.50000000000000006 kg.

So the effective mass of Cormorant is only increased by an exceedingly tiny amount at a normal flight speed.

At 2.95 x 108 ms-1, we get  m1   =   2.5 / √(1 -  (2.95 x 108)2 / (3 x 108)2)          =  13.8 kg.                  

At this speed – which is admittedly ambitious for your average seabird – Cormorant’s apparent mass (based on his momentum) is more than 5 times bigger than his rest mass.