The mystery of the painted on bird on some 68's glass has been discussed in detail on this site if you search.
Rocky Rotella, in his "The Definitive Firebird and Trans Am Guide 1967-1969" book offered new insights into the mystery that I have not seen before. He said that the glass bird was planned for all '68s but when a federal vehicle standard prompted them to discontinue the baked on ceramic paint bird because it might obstruct the view in the side mirror . He states that most are on 11/67 and 12/67 Birds but showed up sporadically throughout the rest of the production year as they depleted inventory.
That pretty much matches the research by the Camaro Research Group statement about their Astro-Ventilization side windows: "The Astro-Ventilation logo was dropped from the 1968 door glass about Nov or Dec '67 production. However, one pane of apparently original May '68 door glass with the Astro-Ventilation logo has been observed in a 05C build. The mating glass on the opposite side of the vehicle is Sep '67 Astro-Ventilation glass. We believe that the May '68 pane had the Astro-Ventilation logo added by LOF to create a matching set, presumably to use up left-overpanes. At least two other late '68 cars have been observed with late 1967 Astro-Vent glass."
Jim is correct on this and there is a copy of that dot memo from 67 that I have seen floating around on the web somewhere. The only thing I would add is that some cars came through with only bird glass on one side. I have personally verified this on 2 original unrestored cars where all the glass was date coded the same. Both cars had the bird on the passengers side leading me to conjecture that because there was no right hand mirror on these 2 cars to obstruct they felt safer depleting that unused inventory on some of the lines this way. But it also could have just as well been because they were out of plain right hand glass at the time and did not want to slow down production. Food for thought. I have owned 4 sets of bird glass over the years and to me there is nothing sexier than factory green tinted bird glass.