How to Prevent Small Worms in Birdbaths
To prevent small worms from getting into your birdbath, refill it regularly to keep the water fresh. When you fill the bath, limit the amount of water so that the birds can still flap their wings while standing in it. That way, you'll reduce the risk of the water stagnating because the birds will be able to move around easily. If you have branches or plants in the bath, make sure you replace them regularly to prevent parasite growth. You should also place the bath in the shade, which helps keep the water fresher. Since running water discourages parasite growth, make a water feature by punching a hole in a hanging bucket so water can drip into the birdbath.
Keep the setup natural. Attract birds and maintain cleanliness by setting up the birdbath in as natural a way as possible. Add features like sand, branches, or stones. This will allow birds to stand, drink, and bathe easily. Put a little sand or a few stones at the bottom of the bath so the birds can walk. Make sure your branches are fresh to minimize the risk of parasites or even worms. Make sure the bath has a shallow end or sloped sides. Avoid overloading the bath with too many features. Place the birdbath in the shade. Put your birdbath near trees or shrubs in your garden. The shade can keep the water fresher by slowing evaporation and heating. It will also protect your birds from predators because they can't fly when they're wet. Keeping the birdbath in a shady area may discourage the growth of little worms. Add a water feature. Moving or rippling water can attract birds and circulate water to discourage the growth of parasites. Buy or make a water feature to increase the birds'—and your enjoyment. Purchase a feature that ripples the water or drips it into the bath. Make a feature by punching a small hole in the bottom of a bucket or plastic container. Hang it above the birdbath so that it can drip into the basin. Change out the water in the feature whenever you clean or refill the birdbath. Refill the water as necessary. Birds will travel long distances for a stable and fresh source of water. Keep a steady and clean source water in your birdbath by refilling it regularly. Be sure to not overfill the birdbath. You want it to mimic a puddle. Put enough water in so that the birds can walk and flap their wings. Make sure the water is cool and fresh. Consider rinsing the birdbath before refilling it. Remove solid material. If you notice anything like algae or dead leaves in the birdbath, take them out. This can keep the water fresh and protects the health of your birds. Scoop out any solid material like leaves, food, or bird droppings. Consider a more thorough cleaning if you find bird droppings since feces can carry parasites. Wipe away any algae you see. Take out ice in the winter or use a heater to keep water from freezing. Clean the bath weekly. It's important to birds' health to have access to clean water. At least once a week, dump out the water and clean the birdbath. This can prevent parasites like small worms. Dump the water in a gutter so that it doesn't attract mosquitoes or other insects. Mix one part chlorine bleach and nine parts water to make a cleaning solution. Scrub the birdbath with the mixture to loosen any debris and kill parasites. Rinse the birdbath completely. Consider drying it in the sun before refilling it. Pay attention to the water. Check your birdbath every day. This can help you recognize any small worms, which are often mosquito larvae, before they become a problem. Check the worms' color. Small worms or larvae can be dark brown, grey, or black. They may be up to ½ inch in length. Changing the water weekly can disrupt the insect breeding cycle. Scrub down the birdbath. If you detect any worms, empty the water into a gutter or someplace where it can't sit. Use the chlorine mixture to scrub down every surface of the birdbath. Use hot water. This may help kill the worms. Consider scrubbing the birdbath twice to ensure that you kill and remove the worms. Allow the birdbath to dry completely in the sun before refilling it. Reinspect the bath before you refill it to make sure that you've removed all of the worms. Use a larvicide. Treatments called larvicides can kill small worms or other parasites. Place one of these products in your birdbath if you detect any worms. Buy larvicides at home or pet stores. Use either Bacillus thuringiensis israelensis or methoprene to kill the worms. These are often sold under the names Mosquito Dunks, Mosquito Quick Kill Granules, or PreStrike Granules. Follow package instructions on how to use larvicides. Use no more than 12 granules of larvicide in the birdbath. Catch worms with a dipper. If you can see the worms, remove them with a long-handled dipper. This can get rid of and prevent further worms without chemical treatments. Avoid leaning over the birdbath when using the dipper. Changes in light make the worms dive deeper into the water. Make sure to use a dipper or net that has small openings so that the worms can't escape. 
Keep the setup natural. Place the birdbath in the shade. Add a water feature. Refill the water as necessary. Remove solid material. Clean the bath weekly. Pay attention to the water. Scrub down the birdbath. Use a larvicide. Catch worms with a dipper. 