I'm a believer in probiotics for not only people but also for my birds, my dogs, and my bearded dragon. I used probiotics extensively while breeding and hand-raising budgies, the probiotics help to save many a very young baby budgie, as well as my bearded dragon when he became impacted.
Antibiotics kill off bacteria, of course the bad bacteria that is making them sick, but the problem is that the antibiotics also often kill off the naturally occurring, good bacteria that lives in the digestive tract, and this also includes a bird's crop. This good bacteria does a lot of important things in the digestive system, I won't let them all, but the two most important things the good, naturally occurring bacteria does is #1 aids in natural, healthy, consistent digestion and excretion of food and waste products, respectively, and #2 it keeps other microorganisms in check, most importantly in the case of a bird, yeast. So the good, naturally occurring bacteria in the crop and digestive tract of a bird keeps yeast from overgrowing and causing an infection. This is why women often develop yeast infections after being on a course of antibiotics, specifically broad-spectrum antibiotics. The probiotics replace this healthy, much needed bacteria that the antibiotics have killed off, allowing the bird's digestive tract to function normally, and preventing yeast overgrowth in the crop, mouth, throat, etc.
As stated there are tons of different probiotics, and if you read the ingredients of most bottles of bird probiotics they consist of mainly Lactobacillus. There is a whole microbiology lesson here and I won't bore you, but Lactobacillus and acidophilus are the two things you need to see one of on a bottle of bird probiotics. Most probiotics made for specific species contain the most commonly occuring healthy bacterium for that species. Most avian vets will prescribe a prescription avian probiotic, or they will tell you to just go and buy a bottle of probiotics made for birds. They are all generally "good", Qwiko makes a very good avian probiotic blend at a good price available at Petco. If you are offered a prescription one by your avian vet then better to use it. Either way I highly recommend using probiotics on a regular basis, but definitely after a course of antibiotics. A good rule of thumb is use the probiotics for the same number of days you used the antibiotics.
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