Unusual behavior of long-tailed parakeets

I first observed the unusual behavior of long-tailed parakeets feeding on the old roof tiles in my neighborhood last year after a heavy downpour.

On the evening of 8 January 2015, I again observed the same behavior with a a flock of long-tailed parakeets that made a stop over in the evening before they head to their roosting site. They were up in the rain trees as usual but a couple of birds started to descend onto a neighbor's roof then more started to follow. Soon there were over fifty birds on the roof peeling off material from the roof tiles to eat.

I managed to capture this behavior as it was relatively quiet and the birds didn't mind the couple of people walking around below them. They fed for almost 10 minutes before a neighbor's car horn scared the flock into flight.
After some speculation, it was assumed that like macaws and parakeets of South America, our long-tailed parakeets were harvesting clay. Upon checking with SK Khew, I found out that old roof tiles in the late 50s were made of concrete.

Upon closer examination of my photos, I could see the birds harvesting pieces of moss that grew on soil deposits accumulated from run offs. My guess is after the rain the moss and underlying soil is damp making it easier for the parakeets to pry the moss and soil off the concrete. It would seem that they are consuming minerals from the soil.