Wood pigeon

The bird bath had all but dried up while we were away and I was preparing to clean and refill it but this pigeon was in my way, sitting on the rim, watching as I approached with the hosepipe. I expected it to flee, but was only when I brandished the dustpan (used to empty the water) that it jumped down onto the the lawn beneath and remained, picking at the grass. I might add that it did an enormous poo into the bath before doing so, adding the the accumulation already in there...why is that birds don't think to turn round to prevent polluting their bathing and drinking water? 
Several rinses later, the bath is pristine and not a single bird has come to bathe.