There are many times in our daily travels through time and space when we may encounter things flying above our heads, things that we hope are merely kestrels. But when we are on a ship with ancient creatures from the past, and strange things from the future... we can't be quite sure what it is above us. We can only hope for the best.
Sometimes it happens that those black spots in the sky are kestrels, and we can breath a sigh of relief. Things are normal, we are safe, we don't have to worry. But then, sometimes, we realize that those aren't kestrels. They are bigger, dangerous, and they are flying straight at us. So then we must ask...
What are we going to do about the things that aren't kestrels?
Do we do as the Doctor suggests and run? That may be the most prudent course of action. After all, those things that aren't kestrels are very big and very dangerous, and if we stuck around we might end up very small and in very many pieces.
Or should we stand and fight? We might lose... but we might not. We might win the fight. Sometimes that may be the right choice... we can't know though until we figure out what exactly those things flying around are.
We could run. We could stay. Life is full of adventures and dangers and choices on what to do when faced with a situation we did not expect. All we can do is be prepared, have our wits about us, and be ready to figure out an answer when we ask, "Now what are we going to do about the things that aren't kestrels?"