Not all birds flap their wings at the same speed when they fly. Certain birds have more flaps per second than others. Different conditions can also make a bird change the speed of its wing flaps.
A bird that is being chased by another, or one that is doing the chasing, will flap more often than one that is just flying. The number of flaps per second is also dependent on air temperature and currents.
Of course, the bird that does the most flapping is the tiny hummingbird. Slow motion cameras reveal that these wee fliers flap their wings between 50 and 70 times a second.