Hello and thanks for researching this very important question!
Rabbits do have more concentrated urine that dogs or cats do, and the color may naturally vary, depending on what a pet eats. Dark yellow, pale green and even a bit orange (for pets that eat food dyes or carrots) can all be considered "normal".
But it may be that your pet is drinking less than is optimal. Without knowing for sure how much he takes in, it is hard to tell. Standard U.S. bottles labeled for single Rabbits contain 16 ounces in total. If he's drinking 2/3 of 8 ounces, that would be OK for this size bunny.
Most bottles are meant to be filled completely (even if you have to pour out and refill it daily for freshness)...this has to do with how the sipper mechanism and bottle vacuum works. Make sure you test the sipper ball or valve each morning after refilling to make sure it works properly. Sometimes an air bubble will interfere with the dripping.
You can safely start to introduce some other greens that will be safe for his digestion...this link contains many suggestions, but I like asparagus a lot since it is a type of grass. Start by offering 1/4 of a stalk of raw asparagus daily, and increase a second 1/4 stalk every 3 days until he eats the entire stalk in 2 portions daily (1/2 stalk morning and evening). Other greens are safe, nutritious and will bring his kidneys some more water:See complete Rabbit Nutrition info by clicking here
Rabbits can develop bladder infections, and males can even develop plugs of gunk in their urethras, so if you have any doubt that he is able to pass urine, he should be examined by a veterinarian in person. If his urine feels grainy, this may be happening.Read more about Rabbit bladder health here
Not all dog-and-cat handling techniques and medicines are safe for Rabbits, so you would need to consult with someone who understands what to do and what NOT to do:Look for rabbit veterinarian here
.Look for exotic mammal specialist here
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