It's so hard to keep clean.
Brush or use roller to apply concrete sealer
Here's a good article with instructions: https://www.hunker.com/12003101/what-kind-of-paint-do-you-use-on-concrete-bird-baths
birds die so easy just because it doesn't directly kill them it could affect reproduction,egg shell comprising,etc.. this is what some people use AgraLife 1 gal. VOC Free Non Toxic Clear Satin Conkrete-Seal