Perhaps I could expand a little.
Tripod heads, having relatively compact dimensions, are less likely to have a negative effect on stability than the tripod itself. Compatibility between different manufacturers of head and tripod is a non-issue as 3/8"x16 threads are almost universal.
There are three possible movements, pan, tilt and lateral tilt, although some specific video heads also incorporate a levelling function.
Simple tilt heads such as the Manfrotto 234 and Sirui L-10 are the ideal solution for a monopod as the panorama function is not required and lateral tilt would just be an annoyance.
Three-way heads are only justified for photography where one might need the lateral tilt function, and are less useful for scopes and binoculars as the smoothness of motions is often inferior to video heads.
Ball heads can be lightweight with a high load capacity but here again have the unwanted lateral tilt. There are some ball heads where this can be locked out but users here on Birdforum were not that satisfied.
Gimbal heads offer the possibility of placing the centre of gravity in the tilt axis but are potentially less stable than other heads and would be cumbersome in the field.
That leaves us with two-way video heads. The extremely expensive offerings from the likes of Sachtler and Vinten with high load capacity and precise counterbalance are not necessary for birding scopes, where the maximum load would be aroung 3,5 kg for a BTX 115. In addition to those mentioned by Joachim there are suitable heads from Berlebach, Leofoto and Sirui. It's an advantage if the head has a counterbalance function and if it will accept long quick-release plates to achieve good balance in the horizontal without too much tilt friction.
If you want to take an occasional look at the stars with your scope, then the head should allow a 90° tilt.
Lastly, although I prefer the Arca Swiss QR system, it's advisable to stick with one manufacturer as there can be issues with compatibility and safety retention.
John