Well after conjecturing and postulating, it turns out that the pigeons are here for a reason - Peregrine Falcon food!
George Colley told me of the story that there was at one time some kind of "save the Peregrine Falcon" society and they imported 100 pigeons to SPI. This was back in maybe the 80s or so.
He said it was a good deal the the Peregrine Falcons were only here three months a year and got to eat, but the rest of the year the pigeons could live unmolested, a win-win situation.
No wonder why my pigeons have been spooky lately! A few seem to be MIA as well. But as for a story - which seems right to me now that I think about it - it sure was something.