It's a touch early but I suspect that is what is known as a "Clocker"
When a female bird is on the nest she obviously can't get up and roam around as she has to keep her eggs warm this leads to her becoming somewhat backed up as she can't just poop willy nilly through the day so, once a day she gets up, eats, finds a quiet spot, braces herself and... Lays a clocker. Then she wipes the tears from her eyes (presumably) and returns to her nest to keep her eggs warm before they catch a chill. Why is it referred to as a clocker you may well ask... Well, this happens so regularly that you could set your clock by it, or, because it's so large as she's stored it up by sitting round the clock.
If you have a river nearby these could be mallard clockers but they look to me like it might be a hen pheasant, the size is about right.
I can't find many references to it on the old Google machine but here is a link that mentions the behaviour in Sage Grouse.
https://conservationco.org/2015/04/03/blog-top-5-weird-sage-grouse-facts/
and greyhen.
https://gallowayfarm.wordpress.com/2014/06/01/greyhens-clocker/
I imagine all birds do it otherwise they'd cover their eggs in their own feaces.