It might not be that the chickens are good escape artists, rather that you may have overestimated the size of hole through which they'll fit.  
I had to build an impromptu fence around my veggie garden to keep my chickens out of it.  I mean, it's made of chairs, screens, wire fence sections, planters... etc.  (And yes, it's very attractive.)  It's incredible what size gap the chickens will fit through.  They find every gap, not because they're smart (BELIEVE me!) but because they're curious, food-driven, and opportunistic.
Get down to their level to see what they're seeing, then fix all holes and gaps.  If the chickens can get out, predatory critters will be able to get IN.