Save some rainwater
The first thing you need to do is collect some rainwater. There are a couple of easy ways of doing so and one of them is to collect it in a tank. If you don’t like that, you can gather some rainwater in a bottle cut from the top. Since you might need rainwater every time you clean the birdbath, it is ideal to store it whenever there is rainfall.
Remember you have to retrieve it as soon as the rain stops otherwise it will grow algae.
Drain the birdbath
Drain the contaminated water from the birdbath in order to clean it properly. If you see leaves or any other solid waste, remove it with your hands. Make sure to wear rubber gloves, so that your hands don’t get dirty.
Add rainwater in the birdbath
Pour some fresh rainwater in the birdbath and leave it for a while, so that all the bird droppings and other stains become a bit soft.
Clean with a scrubbing brush
Use a scrubbing brush to clean the birdbath. If you don’t have that, you can use a toothbrush too, but that will not be much effective, as some of the stains don’t go that easily.
Empty the birdbath again
After cleaning the birdbath properly, drain the rainwater from it and see if any more stains are left.
Fill clean water in the birdbath
Finally, fill clean water in the birdbath and enjoy looking at the birds.