I agree with Grumpy.Heat and Birds[:@] I've had same issue with my Leafriver 35mm w/sensitivity adjustment.I set up around alot of Marsh Grass and Frag areas.Darn stuff heats up during day and false triggers a roll of film(expensive). I've had this problem occur mostly in late spring when setting up in "cool temp" wooded area w/camera facing marsh.Big temp variation sets off false triggers.I simply turn camera away from heat source,trun down sensitivity.

Birds - My only clue on birds was catching a few faint blurs of bird(s) on occasion in corner of pic.

I found when you set corn out in open field setting its hard to control false flashes from birds without sensitivity adjustment.