They need a far bit of space to roam around as they can cover quite a large distance during the course of a day. If there is a way out of your garden - they'll find it. Some can climb a bit (yes, really) and manage to get over things you wouldn't think possible.
They also, oddly, have quite a lot of individual character and will have different feeding habits, times of day they like etc.
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Surely thats a Border Collie ?