Many species of birds like statues for the same reason they like telephone poles. That is, they (statues or poles) often sit in an open area with few if any higher perches or visual obstructions in the immediate area. Thusly they can do two of their favorite things at the same time:

Perch and look out for whatever whirling biting bit of death from above is coming along to eat them.

As to why bird shit sticks to statues more readily than benches I have to say I have no clue. Cautious speculation suggests however that perhaps since birds like statues more than other places to hang out and they shit on statues more often than other things as a result there is more shit that has to be washed off by the same volume of rain.