My candidate is Iberomesornis, which in my humble opinion is the earliest creature which manifests practically all clear avian characteristics...The problem is, however, that the only known specimen of Iberomesornis lacks its cranium, and thus we do not know if this animal had a true beak...although this seems extremely likely. Slightly later than Iberomesornis is Confuciusornis, which was definitely beaked, although rather more archaic in anatomical structure than Iberomesornis.But... Because this can run and run.
Here we have Xiaotingia, hailed as the new urvögel a few years back. Also described as a basal dromaeosaur after having being a troodontid for a short time, but may still be a troodontid depending on which cladogram you look at, or want to believe. But whether it is a troodontid, basal dromaeosaur, avialan or even a Romulan, it comes after Aurornis, which may or may not actually be Anchiornis, work still progresses on this.
So, while we may never know who basal paraves was, I'll reiterate my previous statement that the so far, and to my knowledge, earliest known, within paraves, bird ancestor, is Aurornis, while it is still it's own species and not Anchiornis, and no matter what, birds start with basal troodonts, which is of course contentious. And I still go with Austinornis lentus as the first, so far known, modern bird.
The OP's research has concluded that the first bird is ?
The controversy continues...