@DavidEduardo got this right six months ago.
"Chicken Rock" was a derisive term used for stations that tried to position themselves in between Top 40 and MOR. In L.A., that would have included KFWB after Crowell-Collier sold it to Group W in 1967 and until the flip to all-news in March of 1968 and KGBS from 1969 to 1974.
By that point, though, the term "Adult Contemporary" was in use...and in those days (prior to Jhani Kaye redefining it with the "Continuous Soft Hits" format he pioneered at KOST in 1982), it was whatever the Top 40 stations were playing, minus the six or seven hardest songs.
AC PDs filled that gap differently, or not at all. Some played softer songs that had no chance of being on Top 40 to flesh out their playlists to 30 or 40 songs. Some (me among them) went early on songs we thought would end up in the Top 40 anyway---beating the Top 40 stations to their own music and hastening the burnout once they were on them. And some (KFMB in San Diego was a good example), simply played a 22-24 song current list.
The other factor musically was a deeper oldies library than most Top 40s would go for. In the 70s, the bulk of Gold on a Top 40 was 3-5 years old. ACs in the 70s would start their Gold libraries either with the British Invasion (1964) or Elvis (1956).
And, as David notes, elements of Top 40 formatics (jingles, spot clusters, talk contained over intros, promotions) were a hallmark of the AC format, along with jocks who, in many cases, had done Top 40 and knew how to relax just enough to be more conversational without being too wordy.