when you move your hand up and down while holding on a jump rope, you make a wave. What is the medium

The medium of the wave is the Rope.

Explanation:

This is because it is transverse wave and transverse wave need a medium to transfer energy and this is because in transverse wave,particles of the medium move in a direction that is perpendicular to the direction of the transverse wave. From the question, when you move you hand up and down, you make a wave and the direction of the wave is also up and down which is perpendicular to the direction of the wave.

they are known to reach a speed of 21 miles per hour

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.