Thirsty sparrows

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We are having an unexpected February cold-snap (yes, even in Victoria...), and there is a group of sparrows hopping up and down on the bird bath. I suspect their water has frozen, and shall momently have to go and fill it up. In the rather posh shop catering to baby boom birders where I occasionally purchase seed, you can buy bird-bath warmers... seeing as we only need them about three days a year I think that may be an unecessary luxury, but I would hate the birds to go thirsty.