To Explain this we have to look at How animals and plants live in real life, For Example , a plant would hibernate in the winter , due to the cold , some plants go dormant ( dormant is when a animal or plants action slows down or stop. ) and the external stimulus would be the sun , warming the plant , the plant would still be alive, of course but it would be sort of in a sleep state , Now to answer your question on Migration and how it ties in with this , Lets look at another example for this one : Canadian geese Fly south for the winter, correct? well this is mostly due to the cold , Etc ... so when those geese get cold they leave , the External stimulus would be Wind ,temperature , etc ... :)
