In this case, give the chickens some nice juicy watermelon.
Step 2: Lie in wait. Do not look at the newfie. Pointedly talk about other things, like the weather.
Step 3: Catch the newfie trying to crawl under the fence:
Foiled! Now, whether this is the usual way, we don't know. This time she pushed up the fence at the bottom, leaving a hole big enough for the chickens to escape from. Previously there was no hole. So we'll be keeping an eye out for any other sneaky manuevers.
Don't feel too badly for Thora though. We baked homemade doggie biscuits today. I think she'll manage without watermelon just fine.