My difficulty is not identifying the species of butterfly. I am confident it is a Mourning Cloak butterfly - Nymphalis antiopa. You can see the upper wing of one and the under wing of another behind it.

It is the behavior shown that puzzles me.

I came across a small cottonwood tree with several butterflies on it. This butterfly dominated the one spot and when other butterflies (Mourning Cloaks and Grey Commas) tried to occupy its space the subject of this photo would open its wings and drive the others away. I assume it was feeding on sap somehow. I was concentrating on getting a good shot of this butterfly with its wings open when all of a sudden it opened its wings, arched its abdomen and about 1-2 mls of clear liquid squirted out the bottom. In this photo you can see one droplet still at the tip of its abdomen.

I assume the butterfly is just "urinating"...It was quite interesting to observe and I was lucky enough to snap this photo. Over the course of my observation (about 20-30mins) this butterfly performed this action twice. Am I correct?