Last spring when my chicks were still only 7 weeks old, I found a whole intact chicken egg in my yard near the compost pile.The intestine sound to big to be from a chicken. Do you have birds that color? Could be something had its dinner in your yard and a mouse passing by with loose feather is collected stopped by to dine on the leftovers and forgot its feathers. I would definitely be cautious of your birds though because there is something killing in your yard. I have a cleaning mouse... it goes into my coop at night and collects feathers and leaves them on the step outside the coop. It's a weird little thing.
I guess some animal had stolen the egg from some coop near our property, and left it behind for some reason!