Legit reason for barking

We have two dogs. One used to be a great watchdog, but now she is getting old and misses a lot. The other is and always has been too lazy to care (but she is sweet). The old timer decides to randomly bark at things she must think she hears through the day. Most of the time I ignore her because I know it is nothing. Today, BOTH dogs were in the back yard and I could hear them barking nonstop. I took a little peak out the window to see what had them both in such an uproar and to my surprise there was actually a reason! There was a fox! He wasn't very cute, not very Fox and Houndish at all. I tried to get some pictures, but it was hard due to the fact that I was trying to balance on a wobbly fence and not scare it, and the blurry two are through the side window of my house.