Is there a predator nearby? Listen to the birds.

Here's a cool video showing the calls that birds make when a predator is present:

We had one instance where the resident ravens were squawking just before a bear came to visit us. We made the connection afterwards, and from then on when we heard the ravens sounding agitated we'd scan the area looking for the cause. It's hard to stay tuned in though when there are so many false alarms.

Squirrels seem to ALWAYS be worked up about something, but I've seen a lynx climbing a tree after a squirrel once, and recently a lynx hunkered down, waiting to ambush a rabbit, all while the squirrel was yelling like an idiot.

There's definitely a language there, but I haven't cracked the code.

In Wales I became very attuned to the blackbirds' calls. They have different ones for aerial threat and ground threat and I'd find myself checking the gate in case of visitors when I heard one call and checking the skies in case my chickens were at risk when I heard the other.

In Portugal the local blackbirds are even more territorial and complain incessantly when I go out to the garden, insisting that I am the threat and telling me to go away. I scold them back, but they never listen and we generally bicker the whole time I'm out there.