Some birds don't like monofilament line. I have seen it strung across decks at restaurants. It would be something to try and would not really be that visually obtrusive.

Wire a battery to the railing(jk). Honestly, I would do what von suggested. Or get something that is shiny like mylar strips that can blow in the wind to scare them off.

Sometimes windchimes will work. They may drive you nuts though :).

Mylar does work, but I reserve that for protecting the nests of native birds. I'm so afraid to desensitize them to the mylar, it is the only thing that keeps them away from the bluebirds and tree swallow nests.

What about putting a fake owl on the railing? They won't land there if they think they'll be eaten.

Rubber snakes work well if you can tolerate them on the rail. They need to be moved a little every few days. I have effectively used them in fig trees to protect the figs. Stored them in a paper bag in the garden shed one year. Cleaning out the shed the next spring, I forgot what I had put in the bag. Dumped it in my lap to see what it was. Very exiting morning. BG