I'm finding this scat under a tree. I see no critters but there must be some.
Unfortunately, I can't make any conclusions based on that picture. Please send a clearer picture, with the scat next to a ruler, and we'll see what we can figure out. Also, what kind of a tree are you finding this scat under? Is there a lot of scat? It is in a pile or dispersed around the base of the tree?
The little barrel-shaped fecal pellet that you have in your hand would be called "frass" instead of scat. Frass is a nice way of referring to insect poop. When we find barrel-shaped, ribbed frass pellets like the one you have, we should be looking for a large caterpillar in the tree. At this time of the year, some of the giant silk worm caterpillars are present in trees such as the cecropia moth or the polyphemus moth or the hickory horn devil. These caterpillars are BIG and green with various other characteristics that distinguish one species from another. But because they are mostly green in color, it may be very difficult to spot them amongst the foliage that they are consuming. Here is a link to a web page that covers the giant silk worms including a picture of frass: http://www.naturenorth.com/spring/bug/silkmoth/Fsilk2.html