You have to be careful with digitals, sometimes they take a second or two to come out of 'sleep' mode and snap a pic.
I had some custom ones ordered up for an application once, and i had to be a little fussy picking the right one. Many i looked at had a two second delay. They can't stay powered on the whole time or they die pretty quick. Some manufacturers try to 'get around' that by widening the sensor beam, but that CAN lead to more 'false' game, like rabbits and such. Which is fine, if you want ALL the animals in the area.