Is your rabbit hitting the bottle more than usual? Most rabbits drink anywhere from 50-150 ml of water per kilogram of body weight. This means, in most cases, a six-pound rabbit will drink about or under a cup and a half of water in 24 hours. If your rabbit has always been a heavy (or light) drinker, there is less cause for concern than if you're noticing a sudden change.
Hold the Intervention
Don't jump to conclusions just yet. Environmental changes can cause cause rabbits' drinking habits to vary. It's getting warmer out there. Even if your rabbit doesn't spend a lot of time outdoors, warmer temperatures can cause her to drink more than usual to stay cool. Conversely, with the air conditioning running, she may actually be a little chillier than she's used to indoors. Cold rabbits burn more calories to keep warm, meaning their bodies cue them to eat a little extra, and in turn drink a little extra, too.
Is she getting less fresh veggies than normal? Fresh foods, especially celery, cucumber, melons, and lettuces, are mostly composed of water. If she's not receiving as much hydration from food, she may feel a little more parched than usual. Experiment with her diet and see if her drinking habits vary when she's given various fresh snacks. As long as her poops don't become too soft and she's still eating enough hay (about the size of her body per day), it's no big deal for your rabbit to get a lot of her hydration from veggies.
Speaking of hay, a new type of hay and renewed enthusiasm for some high-fiber snacking can cause rabbits to drink a little extra. This is nothing to worry about either. A new type of pellets or changing the amount of pellets given daily can have a similar result.
Aside from temperature and food, are there any other changes in the household? Rabbits may throw back a few extra ounces a day if stressed. Anything from the loss of a pal to new smells or sounds in the home can cause anxiety in rabbits. If nothing else has changed, pain or illness may be the root cause of excessive thirst.
Is it time to dig deeper?
Once changes to the diet or environment are ruled out, it's time to look into alternative explanations. Rabbits may favor the water bottle when they are in pain. This could be a result of anything from an injury to a tooth issue, and should be investigated by an exotic vet. Be sure to tell them when you noticed the increase in thirst, as well as if you've noticed any other changes in behavior, eating habits, bathroom routine, or weight.
Just like us, rabbits' bodies change as they age. Organs might not work quite as well as they used to. Bladder stones, kidney failure, a pancreatic tumor, or liver disease can all cause the water dish to receive extra attention. Although rare, diabetes will make your rabbit drink more. Diabetes (or diabetes-like symptoms) is most common in rabbits that aren't fed a proper diet. An obese rabbit showing symptoms of diabetes can control or even reverse the condition with some lifestyle changes.
Excessive thirst isn't a problem in itself, but rather a possible symptom of a bigger issue. Never withhold water from a thirsty rabbit; their body is telling them they need the extra hydration to compensate.
References:
https://www.petmd.com/rabbit/conditions/urinary/c_rb_polyuria_polydipsia
https://www.vetstream.com/treat/lapis/freeform/polydipsia-polyuria
http://binkyabout.com/excessive-thirst-and-excessive-urine-production-in-rabbits/
http://veterinarycalendar.dvm360.com/urogenital-disorders-ferrets-and-rabbits-proceedings
