Maybe a puffball or similar mushroom that crusted over? They get that color when they crust. Maybe the presence of DNA from the chicken in the soil
steered it's shape to look more like an egg. If it's a puffball or other mushroom involved, there should be an attachment point. The crust that
forms on mushrooms could obscure that though.
The theory is that the mitrochondria in our cells are descendants of ancient aerobic bacteria that integrated with bodies of animals and possibly some
mushrooms.to help power their systems. If the fungus can integrate some of the mitichondria of animals into it's structure it may incorporate some
of the shape configuration of the organism into the shape of it's fruit. Although it is doubtful that this could create a new animal it could sort
of steer it's shape. After all, where did animals come from? I don't believe that god created us, just steered our growth. God being structured
energy or truth in the universe and the devil being Chaos and deception.
Could also be a hoax or could be a part of some easter display that got caught up in wind currents somewhere and dropped in their field. I don't
think there are any elephant birds or Moa around anymore