v^2= v0^2 + 2a*dx
v - instantaneous velocity (velocity at the moment)
v0 - initial velocity
dx - total displacement
a - acceleration
We should find v from this formula
As the bird is falling from the tree, initially it doesn’t have any velocity, so v0=0
When the bird will hit the ground, its total displacement will be 15m, so dx = 0
The acceleration here is equal to the gravitational acceleration (g) = 9.8
So if we will plug in our values to this formula:
v^2=0+2*9.8*15=294
v=17.15 m/s (this answer depends on whether you take g=9.8 or g=10)
Btw, mass doesn’t affect the acceleration and the velocity