Green water is not a mystery. Bird bath algae needs three things to bloom: standing water, direct sun, and nutrients from droppings and seed hulls. Take away any one of them and the basin stays clear.
The fix is a two-day flush plus a weekly scrub. No chemicals, no tablets.
Change the water every two days
Algae needs still water to establish. Dumping and refilling every 48 hours interrupts the cycle before a film can anchor to the basin.
In hot weather make it daily. You are also removing mosquito larvae, which need roughly a week of undisturbed water to hatch.
Scrub weekly with a stiff brush
Rinsing does nothing once algae has gripped the surface. You need mechanical scrubbing.
Keep a dedicated stiff-bristle brush next to the bath. Thirty seconds of scrubbing on refill day beats a monthly deep clean, and it keeps the biofilm from ever getting a foothold in the texture of the stone or concrete.
For a stubborn basin, use one part white vinegar to nine parts water, scrub, then rinse hard until you cannot smell vinegar. Skip bleach, skip pool algaecides, and skip copper pennies. Birds drink this water.
Move the bath into dappled shade
Six hours of afternoon sun on a shallow basin is an algae incubator. Shade under an open tree canopy cuts the light and keeps the water cooler.
Keep the bath eight to ten feet from feeders. Seed hulls and droppings landing in the water are the nutrient half of the problem, and that spacing also gives birds a clear escape route. Placement principles carry over from where to hang a bird feeder.
Add movement
Moving water resists algae and attracts far more birds than a still basin. A solar fountain, a small dripper, or a battery agitator all work.
A dripper is the low-maintenance option. It disturbs the surface constantly and the sound carries, which pulls in birds that would fly past a quiet bath.
Choose a basin that is easy to clean
Rough concrete grips algae. Glazed ceramic, sealed stone, and smooth resin wipe clean in seconds.
Depth matters too. Two inches at the centre is enough for most songbirds and shallow water warms and greens more slowly than a deep bowl left to sit. If you are shopping, our roundup of the best bird baths flags which surfaces clean easily, and a DIY bird bath lets you pick a smooth saucer from the start.
