Greetings, I am Dr. Pat. I have worked with birds exclusively for many years.
What exactly were the initial signs of illness?
You said underweight, was that determined by weighing him or palpation? Both are important as many canaries are obese to start with.
Were the droppings more liquid with small amount of feces? Or can you describe in detail the relative amounts of the components, and what parts were stained yellow?
Was the bruising in the skin or underneath? A large liver is very easy to see through the skin and can be mistaken for a bruise. Has he fallen from perches?
How has the baytril been given? It doesn't work, put into water. If baytril is going to work, it works pretty fast and he should be improved. If he has not, then either a bacterial infection is resistant and the baytril is not effective, or the disease is not bacterial. Unfortunately a majority of bacteria are now resistant to baytril, because of extensive misuse. That is why I almost never use it, and I do not recommend its use. Alternative antibiotics are very likely going to be needed if this is a bacterial infection.
Can you tell me more about the bird?
How long has this been going on?
How long have you had him?
Where is he from?
How is he housed? indoors, outdoors, cage, aviary, etc?
Any accidents or trauma?
Interactions with other birds/pets/children/guests?
What is the usual diet? has it changed recently?
Has the bird gotten into anything? Chewed electrical wires?
What is your geographic location and local weather?
These signs are of a very sick bird, and not specific to any one disease. And that means it is not fair to you or the bird to guess, there are so many possibilities.You are going to need local help on this, and a scientific and solid diagnosis to find safe and effective treatment.
The challenge is to find out exactly what is going on, since treatment will depend on careful and accurate diagnosis.
What you describe could be consistent with a blood-cell protozoan, similar to malaria in people, that canaries can have.
It could be from nutritional disease/obesity if he is on a seed diet. Liver disease, infections, clotting disorders all can result. Also they can have "food poisoning" from contaminated feed, which can harbor things like Salmonella, fungus spores, aflatoxins, rodent droppins etc etc.
It could be viral or fungal.
Canaries can have GI protozoans and fungal infections, can be very common depending on your locale.
Even a regular vet can send out a fecal sample for analysis, and you are definitely going to need at least diagnostics help from a vet. A simple blood smear requiring only a tiny drop of blood, properly prepared and sent to avian lab, can give a huge amount of information and also check for the blood parasites.
If you feel comfortable with it, examine the bird thoroughly, using gentle restraint via washcloth or hand towel: do not restrict the chest or hold around the body. Check the eyes, nostrils, mouth and beak if possible, having a good look in there for mucus, redness, masses or anything else unusual. Palpate the tummy for pain, fluid, lumps or anything else (eggs, if female or unknown). Check all the joints for swelling, pain, and mobility. The feathers should be parted to view the skin, muscles and skeleton below; this can be done using a q-tip with isopropyl alcohol or KY gel. Look for bruising, lacerations, injured feathers.
Your job is to keep the bird warm, safe, quiet, and confined; and to provide adequate hydration and calories.
Move the bird to a box or carrier with soft towels in the bottom, no perch, and food and water in low bowls that can be reached easily. Put the whole thing on a heating pad on low or medium. Check it frequently, no overheating allowed! Keep the unit partially covered, warm and quiet. White paper towels or white cloth towels will show the true color of the droppings. Small animal/reptile boxes are great for this purpose.
The bird, bowls and unit must be kept very clean.
Here are some helpful links:
Do not try to force food or water. Pedialyte or electrolyte replacer can help but many birds do not like them; when in doubt, plain warm water is best. They can hydrate from oral fluids almost as quickly as IV if the GI is functioning properly. You can offer warm cooked rice, pancakes, cornbread, grapes, melon, greens in addition to normal food.
I know how hard it is to find a vet to help, but you may not have many home options, because the first thing you need a vet for is to find out what is going on. Treatment is only as good as the diagnosis. If you call around, you may find a vet to work with a distance bird vet or vet school referral. There are good bird vets that are not specialists.
https://aav.site-ym.com/?page=basiccare click on "find a vet"
for members of AAV in your area or call your regular vet and see who they recommend; ask if they really have worked with birds a lot. Unfortunately, this list does not rate competency or experience, but is only a starting place; the vets at least take the avian medicine journal and hopefully see a bird or two a year. The best referrals are word-of-mouth, so check with several non-bird vets, the humane society, parrot rescue groups, bird clubs, etc. for their input. As you might guess there may be controversy and varying opinions even with this. Even board-certified avian specialists may not have a lot of practical bird experience. Unfortunately there are few resources available to refer you to really good, clinically-experienced bird vets.
If this were my patient, and money no object, I would start with complete fecal analysis and direct smear, stained with Sedi-stain and unstained for multiple parasites, fungi, spirals; direct smear stained with Sedi-stain and unstained of the oral cavity; bacterial culture and sensitivity of the feces and choana. Depending on the case I might do a fungal culture. Routine blood work is necessary to rule out other issues. There are MANY DNA/RNA tests for canary diseases. Ultrasound is often more informative than radiographs and does not require anesthesia (ask your vet about this option). Generally I start them out on medications as indicated by the tests.
AAV recommended lab work