I think a fact like:
almost certainly cannot be copyrighted, see https://www.copyright.gov/help/faq/faq-protect.html:
A fact could be protected by things like non-disclosure agreement, trade secrets, export controls under arms trafficking regulations, or classified, but not copyrighted. However, as that FAQ says, the way the fact is expressed maybe can be copyrighted.
So in this case, what (debatably) could be copyrighted is the manner of expressing the fact, that is to say in this case you possibly could make a legal demand that a different inaturalist observation of the fact that “nancyinsunnyvale saw a great egret on X date and in Y location” be taken down. Obviously the duplication would also be against site policy. While demanding it be taken down is one thing, for an actual lawsuit the challenge you would run into is proving any specific harm to you from the alleged infringement. Maybe if, say, an infringing observation somehow made it into an ‘observation of the day’, you would have an argument that you had been deprived of accolades that could have had some kind of tangible value to you.
On the other hand, the actual verbatim text of your caption/commments/field notes almost definitely can be copyrighted, just like the pictures can. However, it is again hard to see in most cases how a caption would be used in an infringing manner that actually harmed you without also being fair use. The most plausible scenario I can imagine is if you had created your own personal ID key for a species and put that in the caption, and then your key was reprinted verbatim in a commercial field guide without permission/attribution (depending on your license terms).