Observed some Ibis laying down on the grass (flat) with the wings stretched. I know that darker color birds will open the wings to the sun for removing parasites .... but white don't. btw there were several birds doing the same behavior in the flock.
A couple of years ago I had three Mourning Doves do this in my backyard too. I was able to go out with my gear and creep up to document the behaviour...here is one of the three individuals. After a good while (about 20 mins) they flew up on the fence, preened, scratched, strecthed, and then flew back down and proceeded to feed. They are not white, of course, but maybe they do this for the same reason as your Ibises did. I'll be interested to hear the theories...
Sometimes the reason will be, that they want to cool down, some birds do it, because they take an "Ant bath". They want the ants to fight against the parasites with the acid the ants produce, when they are "excited" and "disturbed"