Dig, Wait, Listen: A Desert Toad's Tale

The desert Southwest is full of sounds.

Skitter, skitter, scratch.
A scorpion is crawling.
Pop, pop, pop.
kangaroo rat is hopping.
Tsk, tsk, tsk.
A rattlesnake is shaking its tail.

But the spadefoot toad is quiet in her underground burrow,
listening for one sound in particular...
What is that sound?
And what will the spadefoot toad do when she hears it?