Last summer, we had our air conditioner on, and there was a big robin outside that was constantly making noise. Before the bird showed up, my rabbit was always very calm, but after he showed up, she was very nervous and restless. Even though rabbits have great hearing, I'm not sure if she was capable of hearing the bird because all the windows were closed. How did she know that the bird was outside?
Disclaimer: I know precious little about rabbits. There's a couple explanations here:
The bird was emitting something supersonic that your rabbit could hear, and it was either irritating them, or making them nervous for some reason. Rabbits can hear sounds up to 42,000hz, which is significantly higher then what you can hear (the average person can only hear up to 18,000hz).
You could make the case that every living entity has some kind of detectable energy around them, and your rabbit was picking up on it. I doubt this is the case though, as I doubt a Robin is a typical enemy of the rabbit.
My money's on the bird just singing really high and irritating him. You could try buying a dog whistle and see if they react similarly to that.
Here are some non-supernatural explanations:
The rabbit may have been hearing the robin even though you couldn't. Rabbits have better hearing than humans.
The rabbit might have been restless for some unrelated reason. Just because two events happen around the same time does not mean that one caused the other.
The second explanation is more likely in my opinion.