we know equation of apperent height,
d/D = n(medium containing observer) / n(medium containing object)
(d - apparent height, D real height)
n(medium containing observer (here fish)) = μ
n(medium containing object (here bird)) = 1 (air is 1)
d/D = μ
d = μD
d/t = μD/t (divided both side with time ,t)
v' = μv (since distance/time is velocity)
(v' - apparent velocity, v = real velocity)
So answer is v' = μv
(Read twice if you can't understand)