Usually a pasted vent is obvious but on occasion it can be subtle. This bird was on the bottom of the cage with its wings dropped and straining. On careful examination, I saw the vent was sealed shut even though not much fecal material was clinging to the vent.
Causes for pasted vent include a variety of infectious agents from bacterial, fungal, parasitic, and viruses to systemic disease and even toxins.
Cause separation, to some degree, can be made based on smell. Normal fecal material should be odorless while many bacterial such as E. coli and some fungal causes such as Candida produce odor.
In this case, the color was creamy and no odor was detected. I cleaned the vent and checked again in a couple hours and found it pasted again. This repeated most of the day.
Since I had Ronex in my bird medicine chest, I decided to try it and within a couple more hours the problem was gone. Ronex is effective against many parasitic causes for wet vent. Treatment is seven days in the water. (www.ladygloudianfinch.com)