Hi. Not without checking the cistern first. The water reg require cisterns to be fitted with screen overflow, lids insulation and so on. This is exactly the same spec. as distribution reservoirs, which can hold millions of gallons of water. Sorry to black dog the pigeon story, but heard of a situation that disrupted the supply to part of a town. The problem being an otter sucked down an outlet from a reservoir and blocking the supply. It caused an hour or two of head scratching, before the fault was discovered. I think the answer might be boil it first. Good luck