NBC News ran this item about a ghost tripping a motion activated camera in a work out gym.

http://video.msn.com/?mkt=en-us&vid=6296ab27-4812-40c3-a861-ef8d029d310a&playlist=videoByTag:tag:source_nbc%20news%20channel:ns:MSNVideo_Top_Cat:mk:us:vs:0&from=MSNHP&tab=m24&GT1=42003

Supposedly this model is not tripped by things like bugs or dust.

I'm thinking a bat or small bird maybe. Thoughts?

It certainly looks like an insect crawling over the camera glass in front of the camera. I've come across several similar cases in the past. The big question is, how did the motion sensors get tripped? Obviously, you'd need to test the sensors to see how sensitive they are. However, I have personal experience of spiders tripping PIRs in the past.

It would have to depend on where the motion sensor that activates the camera is located relative to the camera, and what type it is. One that's thermal sensitive wouldn't be triggered by a spider, per e, but a small mammal would set it off.

I worked on a job once where we were getting spurious false alarms on a fire system, after a long drawn out troubleshooting session it was discovered that a spider had built a web over the lens of a rather expensive flame detector.