wwbrown:
There are image processing algorithms that can discriminate between bi- quadra-ped animals that would work for detecting animals within the field of view. Processing video streams is not within the capabilities of pre DUE Arduinos and DUE may need some kinks worked out of it as many first issue systems do.
Talking of image processing has anyone seen this AForge.NET :: Framework for .net
Don't know if and how well it works (I try to avoid .net) but was discussing this with a friend a couple of days ago and he suggested this site.