Where are you located? You might naturally have a higher population of bobcats than other places. Or maybe just the odd one or two. Either way, suggest you should secure your chickens, either with ample fenced runs and/or electric fencing, and/or aprons around the bottom of your runs. The bobcats I've happened to see on our ranch were always quickly trotting past the barnyard fencing, late afternoon, obviously on their way to another appointment somewhere.