Smaller than normal poops can be the first symptom of a gastric slowdown or gas pain, so it’s something to pay attention to. (welcome to the world of being a bunny parent and being obsessed with poop!)
Is he eating normally? Behaving normally?
If he is eating normally and there are no other symptoms, I would monitor his poops closely to make sure they return to normal within a few hours. You can offer lots of fresh wet leafy greens to make sure he’s hydrated and give him a tummy rub.
If he isn’t eating the way he normally does, you should call your vet, and do the above things in the meantime. You can also give infant gas drops if you suspect gas. Sometimes bun’s get smaller poops after a stressful experience, or because they are eating less hay, but they should return to normal within a few hours. If they don’t, then that’s cause for concern.
Symptoms of pain in rabbits can be subtle. They include sitting hunched, pressing their tummy into the ground, lethargy, teeth grinding (sometimes you just see their ears trembling), and squinting. Some rabbits also have worse litter box habits when they are not feeling well.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.