There was only one loose supplement in the years you listed. The March 2018 issue contained a two-sided map entitled "How Birds Migrate".
No loose supplements in 2017. The last one prior to March 2018 was December 2016.
My brother is a subscriber to National Geographic and strangely didn't receive a map supplement to the 2018 March issue (the spine and magazine does show a bird migration story) but we didn't see a map anywhere. I wonder if the maps were US only, because a similar thing happened to the December 2016 issue (FAITH cover) where we got no map but people overseas seem to have received a map.
Question, is there any mention on the issue anywhere in the magazine that a map is included? I don't see anything in both the March issue of 2018 and the December issue of 2016 of the copies we have.
Back in the day, you could tell if there was a supplement by the red lettering on the spine. Not anymore. Now, all of the main articles get red lettering even without a supplement. The Society might have a problem with quality control in regards to inserting maps. I know one of the more recent maps was missing in Phil's issue. He wrote to Cathy Hunter and she mailed him a copy of the map. You could try that.
Thank you for that. I will give it a shot, because my brother reads the issues as the magazines come in and he feels incomplete not having read and admired the two maps!