Has anyone any experience with using a motion detector to alert to wildlife. I am filming Caribou and will be set up near a known crossing but want a notification of when they are in the area. I am thinking of a silent alarm that will signal me when they cross a highway and enter a specific trail. That way I can camp nearby - ie within a half mile or so, and be alerted when they are in the area. I have found a couple of units under $100 but wonder about their effectiveness. Any suggestions appreciated.