I noticed today that this tag exists and is beginning to spread around the site. This was of personal interest to me since I'm a such a big cloaca fan, to the point where I've read books on avian anatomy.

The tag only makes a small degree of sense. There isn't much "inside" for a cloaca. There it is, but it's a pretty small area. To give a brief anatomy lesson, the cloaca only refers to the point at which the intestines and reproductive organs combine near the exterior of the body and it can be compared to the vulva for mammals. Birds still have an anal, and if female, vaginal track beyond the cloaca. Here's a simple cross section: http://fivelinedskinkresource.weebly.com/uploads/1/7/0/9/17092394/945945391.gif?378

This tag should probably just be changed to cum_inside, unless there's a cross section in the image that might suggest it's the vaginal or anal track.

As my mother always said, a hole's a hole

Terraraptor said:
The tag only makes a small degree of sense.

nipple_penetration

But you can't tell whether it's cum_in_ass or cum_in_pussy if there's no cross-section view. You can (usually) clearly tell it's a cloaca though, and I think that's good enough for cum_in_cloaca.

People searching cum_in_cloaca are looking for a specific thing that these images do represent, regardless of what's really happening inside. I doubt people searching for cum_in_pussy would want to find results with single-hole avians either.