they love them Squirrels are their favorite though. If it is a young cat, he may be feeling his oats, and want to go after something bigger (or didn't notice them, which would support him being captive raised). They don't leave their mothers till 9-12 months of age, and she teaches them that smaller prey equals less effort. Can you electrify your fence (even for a short period of time)? These guys are very smart, and have VERY long memories. One good zap, and he won't forget.
They also have rather large territories in rural settings, and tend to roam throughout their territory, rotating time spent in each area.