It is important to catch a pasted (caked) vent early because as the excrement accumulates the vent will become totally blocked preventing defecation and in time the bird unable to defecate will die.
The signs may be as subtle as seeing a bird stay on the cage floor or dropping its wings or noticing a small mass in the vent area or the bird may be straining while moving its rump up and down trying to relieve itself. Taking care of it promptly is important but before removing the mass note its color and whether or not there is an odor.
To prevent vent injury carefully rinse the area with water and remove the mass. When the dried excrement and associated feathers have formed a solid mass that partially or totally prevents defecation, once removed, the bird will promptly produce a voluminous stool because of all the obstruction.
Numerous infectious agents including bacteria, yeast, fungi or parasites as well as dietary issues can cause a pasted vent. If the color is dark green the starving bird may have a yeast (Candida) infection or if the mass is hard and black colored it may have a low grade megabacteria infection. A creamy color indicates urates (urine).
If the caked dropping reveals a sweet or pungent smell, bacterial infections such as E. coli, Clostridial, Streptococcus or yeast infection may be the cause. If there is no unusual color or odor, as was the case with this bird, I looks for dietary causes. Photo colors are off a bit as it was cream colored and not yellowish.
Is the bird losing weight? Have you changed the amount or kinds of extras? Are you feeding too many rich seeds especially Flax or possibly Sunflower chips?
I have put it on Dr. Rob Marshall of Australia's KD Powder in its water for a couple of weeks and added soft bread twice a day, unlimited oatmeal or steel cut oats, dry Biodecken novafood (nestling food) and eliminated flax and greatly reduced sunflower chips and offering couscous with quinoa and extra poppy seeds which helps naturally stop diarrhea.
Update October 5, 2013
After starting on KD Powder and dietary changes, no more caked vent and the bird seems in perfect health!