Pilots should not take off or land in the presence of wildlife and should avoid migratory routes, wildlife reserves, estuaries and other sites where birds may congregate. When operating in the presence of bird flocks, pilots should seek to climb above as rapidly as possible as most bird strikes occur below . Additionally, pilots should slow down their aircraft when confronted with birds. The energy that must be dissipated in the collision is approximately the relative kinetic energy (formula_1) of the bird, defined by the equation formula_2 where formula_3 is the mass of the bird and formula_4 is the relative velocity (the difference of the velocities of the bird and the plane, resulting in a lower absolute value if they are flying in the same direction and higher absolute value if they are flying in opposite directions). Therefore, the speed of the aircraft is much more important than the size of the bird when it comes to reducing energy transfer in a collision. The same can be said for jet engines: the slower the rotation of the engine, the less energy which will be imparted onto the engine at collision.