To start, apologies for the dumbness
The user who deleted the tags was me, and I'm a rather unexperienced SE noob doing potentially dumb things without thinking too far into the future on the community consequences - apologies!. (In the meanwhile the tags have been restored).
I remember reading (link) that it's important to not introduce new tags and so was attempting to practice this (and clearly leaping before looking!).
The broad motivation behind the original deletion was to keep the number of unique tags low and general (as pointed out night-flight-call is a type of flight-call).
On making new tags
The point on the community size is a great one to consider going ahead.
Another point (in the opposite direction) is to make a new tag only if it isn't somehow covered by a pre-existing tag. Essentially to always use 'common-denominator' tags, rather than making more and more specialised tags.
On synonymous tags
About synonymous tags like
night-flight-calls, I'm still on the fence about if it can be a good thing to grow the list so early on. Background reading (will try to find the link later) says SE is capable of finding very specific things without the need for a pre-existing tag because searches also look at the actual text content.
Also, each question only gets 5 tags. Not sure if degeneracy in tags (full-word and acronym) increases 'searchability' of the question. In the interest of getting more users to a question, it may be better to include unique tags that convey slightly different aspects of the question.
When and if more users do introduce synonymous tags - it can be handled however. Having read up more on synonymous tags - degeneracy in tags can be handled at a later point to make sure users searching for tags will automatically find all synonymous ones.
Edit: now am on the fence about synonymous tags.