Hi,
Losing a toenail or a toe isn’t usually serious, but it also isn’t really common. For you to have this happen to a few birds is an indication of a problem with something in their environment. You didn’t mention the species but I’m going to guess these are finches based on your email. If you breed them and provide nesting material, most likely this is the problem. Some of the nesting materials that are sold are not safe because they have small threads, and these can get wrapped around a nail or toe, cutting off the circulation and causing it to fall off. Any type of thread, straw, cotton or synthetic materials can have fine threads or fibers, so check what you are using to see if this may be what is happening. It’s actually quite painful for them, so you do need to find the source. If you cover the cages, likewise they can get fine threads from many fabrics that again, wrap around the nail or toe causing amputation. If you keep your birds in a flock and are breeding them, this can result in a lot of fighting and this can cause foot injuries. Nests should never be provided to flocks. For breeding, each pair needs to be in a separate cage. Lastly, there could be something causing infections in their feet, and in this case you need to consult with an avian vet.
Thank you for asking Lafeber,
Brenda