It's been long enough at the skeleton that Strange's excitement and interest has morphed into a little bit of tense nervousness as well. This place is giving Bell the jitters which is giving him the jitters and the fact that he thought he saw that spectral horse is giving him even more jitters...he's starting to see why this place is haunted.

He's exploring the top of the skull as well, just to see if there's anything new he can discover. So Strange hears Rocket's frustration and can't help but laugh a little at it: it's a short, slightly tense sort of laugh.

"I think that's why she's acting odd. There's nothing here." No birds, no vermin, no animals like you would normally see in travels like this.