You could use a laser break beam like the one shown.
Depending on your target, might be better experiment with Passive Infra Red (PIR) modules to detect the wildlife, there are lots of these about, range and sensitivity varies, so you would need to figure out what the best type for your needs was.
Most commercial and DIY wildlife cams operate using PIRs.
PIRs have the advantage of being relatively low powered, so you can leave them running for extended periods on battery power (a well designed system can sit for weeks waiting for its target to appear, they only power on the camera when something interesting happens).
Running a laser trigger and arduino for weeks from a battery would take a relatively large battery.
Ebay has lots of PIR modules to choose from, and a quick hunt round the internet will get you the spec and some suitable circuit layouts.
http://www.ebay.co.uk/sch/i.html?_trksid=p5197.m570.l1313&_nkw=pir+module&_sacat=0&_from=R40When I get the time (dont hold your breath) I intend to post a How To on interfacing PIRs with CHDK.
