An airplane flies into air after attaining a speed of 275 kmph speed on the runway. The wings of fan in the engine rotate at a speed of 4000 rpm. When a bird flying at some speed hits the plane, a lot of exchange of kinetic energy occurs.
Kinetic energy of the bird is 1/2 * mass of bird* square of its speed and though the mass of the bird is smaller, the kinetic energy is more because of square of the speed. Similar is with the airplane. Though after hitting the plane, the bird dies, a lot of damage occurs to the plane like shattered windscreens, hydraulic engines may get ruptured, propellers and turbine blades may get destroyed.A pilot may have to declare emergency.