Here's another pic

Mice foraging on the roots?

BEWARE CLEANING THE SHED! Hanta virus is always a possibility.

http://www.cdc.gov/rodents/cleaning/

Thank you for replying. I was just out in the yard and noticed movement in the grass, which turned out to be a little mouse scurrying back towards my house. The little guy ducked under my deck! Anyways, I'm thinking you're right. There's a good chance there's a family of them and they've been feeding off the lawn and everything inside the shed.

First step, get rid of the mice;
Second step, start cleaning up their droppings;
Third step, figure out a way to fix my poor lawn.

They are not mice. They are Voles (which are much like mice). That is classic vole track stuff. They had tracks under the snow during Winter.

Voles often have colonies under bedding plants, and you may find damage to the roots of hosta, tulips, daylilies, etc.

If you have voles (I'm certain you do), you will find that they will resort to trails again in your lawn if it is mowed above 3" tall, even in Summer.

Trapping them is very difficult, as they multiply faster than you can trap them. I've had some luck using Castor Oil on the lawn (they hate it and move elsewhere), but it doesn't work well during rainier times of the year.