We usually think of bacteria as germs that cause disease, but the body is actually full of bacteria both good and bad. Probiotics are live bacteria and yeasts that are reportedly good for the human digestive system because they "help keep the gut healthy." When the body loses good bacteria (like after taking a round of antibiotics) probiotics can help replace what was lost.
There are many general types of bacteria used as probiotics (the common ones are Lactobacillus and Bifidobacterium), and many different species as well as strains within species. They have different physiological effects so it's believed they provide different health benefits and also quite possibly different risks. Some yeasts, such as Saccharmyces, can also act as probiotics.
Lactobacillus is probably the most common probiotic. This is found in yogurt and other fermented foods. Different strains can help with diarrhea and may help people who can't digest lactose, the sugar in milk.
Bifidobacterium is found in some dairy products and may help ease the symptoms of irritable bowel syndrome (IBS) and some other conditions.
Saccharomyces boulardii is a yeast found in probiotics that appears to help fight diarrhea and other digestive problems.
Proponents claim that probiotics confer health benefits by rebalancing the normal microflora in the large intestine. Although probiotics have been around for several years, there is still some uncertainty about their use and a good deal of misinformation about them.
This is complicated by the fact that not all birds have the same or even similar populations of bowel bacteria. For example, birds with a caeca, like chickens, carry large numbers of gram-negative bacteria, which are capable of causing disease in other bird species. Parrots on the other hand do not have a caeca so the number of gram-negative bacteria is very low; however, normal parrots have large numbers of gram-positive bacteria. Lorikeets have virtually no bacteria, and many passerines such as canaries and finches have no permanent bowel bacteria so bacteria found in their droppings are considered transient bacteria. The bottom line is ... one probiotic doesn’t necessarily "fit all" birds and probiotics made for humans or other animals are probably not effective for birds.
While probiotics are not a miracle cure, the Melbourne Bird Vet site states, “Where animals are not stressed, have an appropriate diet, are not crowded, are not given drugs, do not contract infection or metabolic diseases and live in a clean environment, an ideal level of intestinal bacterial population may be maintained on a rather steady basis.”
Since most birds are not in the perfect environment all of the time, probiotics can be beneficial to help maintain the health of birds in certain situations such as:
Before or After any stress situations.
Stress disrupts the population of bacteria found in the bowel with the beneficial bacteria being the first lost. Once beneficial bacteria are gone this opens up the potential for an overgrowth of disease-causing bacteria or yeasts. During breeding and moulting.
Even with the best of care, birds can become rundown when their systems are over taxed. Molting and breeding use quite a bit of energy stores so using probiotics during these times would be beneficial.
Following purchase and transport.
Catching and confinement is extremely stressful, even more so is high-strung species. Not only is the bird stressed by the activities of a new home, their eating, drinking, and sleep patterns are disrupted which creates even more physiological strain on their systems. Following antibiotic use.
Most antibiotics will kill the beneficial bacteria of the bowel. When the course of antibiotic is complete probiotics can help protect your birds from disease while their gut naturally recovers from the affect of antibiotics. After fledging.
Less disease can be expected after weaning if birds are probiotic-supplemented until they are feeding properly and have established themselves in the aviary.
For more details on probiotics and birds: http://www.melbournebirdvet.com/use-of-probiotics.aspx