Well, a year has gone by now and I've tried all kinds bird scare tactics. Here are the winners and losers:
I started off with the rubber snake trick. This worked for about two days. I speculated that when the birds don't see the snake moving, they realize it can't harm them. I didn't want to re-position the snake every other day, so this was a bust.
I bought some plastic pinwheels from our local dollar store and clamped them to the top of the ladder. This initially worked well - the birds steered clear. Then, the winds picked up one day and obliterated the cheap pinwheels. Ugh!
Since the pinwheels worked well for a long time, I bought some beefier metal garden pinwheels and attached them in the same manner. This was a failure - the metal pinwheels need a lot more wind to move them, so no wind equaled a return of the birds.
I then tried some blank CD's. I put some zip ties through the CD holes and clamped the zip ties to the ladder in a manner that allowed the CD's to move in the wind. This worked well for about one week. Once again, the birds eventually realized the CD's were harmless and opened my ladder bathroom back open for business.
Time to get serious...I decided to try denlock's suggestion.
I went to Lowe's and picked up two 5 gallon size paint stirring sticks. They turned out to be the perfect size for the project - they needed no trimming. I ran a row of nails down the center of each stick. I sprayed the sticks with some anti-rust spray paint, to seal the wood and prevent rust stains from the nails. I then clamped the sticks to the top of my ladder with my good ole trusty clamps. Problem solved. The birds no longer use my ladder as a bathroom....they've moved on to my neighbor's RV!