Here is an interesting article on parthenogenesis. I remembered this from several years back but it took a lot of Goggle to come up with that name.
http://www.thepoultrysite.com/artic...normal-fertilization-and-embryonic-mortality/
It appears to be more common in certain flocks of chickens and turkeys but it is possible for a female chicken or turkey to lay a fertile egg without a male around. It's rare and most of the time the embryo quickly dies if the egg is incubated. But it is possible to actually get a chick to hatch.
Perhaps something of this nature happened, rare though it is.