Mysterious spherical mass of feathers (Reddit)
This is a flap of feathered skin from a Mallard, birds do not shed their skin. The bird in question is likely dead or at the very least, severely injured. 

The skin and curved feathers has curled up around itself into a "ball". As it's flattened/separated you can see there is reddish skin with some tissue on it the maggots have gathered on. The feathers here are relatively small and curved and the pattern is SPOT ON for the breast/belly feathers of a Mallard. 

While it's not really obvious in the photo of the Mallard below, it is very common for them to have that little bit of pale edging on the dark reddish-brown feathers, as seen in the first photo. 



Photo by Jo Garbutt (Flickr: https://flic.kr/p/jVbHwP )